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A Scale of Grade and Class Terms for Clastic Sediments

A Scale of Grade and Class Terms for Clastic Sediments SCALE OF AND CLASS TERMS FOR A GRADE CLASTIC SEDIMENTS' CHESTER K. WENTWORTH University of State Iowa CONTENTS INTRODUCTION THE GRADE TERMS Fragment Terms Aggregate Terms Rock Terms THE CLASS TERMS INTRODUCTION In no other science does the of terminology present problem many in geology. With of knowledge so difficulties as the growth in any field of devise or old investigation, men new terms redefine attempt and ones in the to convey more precise definite ideas. In all the branches of science much confusion has followed the redefinition of old terms because of the indiscriminate use of the old and new senses. But geology, dif- terms both in the the in of ficulties this kind are peculiarly great. Because geology is a field science and has followed in the footsteps of exploration, it has acquired terms from all parts of the world. Many of the names for the less common special features have come from the dialect or colloquial speech of that part of the world where they are best developed. With the of these use terms of geologists of other regions, much irregularity usage of and hence much confusion has arisen. Since '917, the writer had been engaged in the study of abrasion and of and pebbles action shaping cobbles by the of running water. In the course of this study the loose usage of cobble, pebble, and own practice was exception) has related terms (in which his no and impressed him with the need of greater uniformity of usage ' Published by permission of the Director of the United States Geological Survey. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER K. WENTWORTH definition terms. this in more careful of such With need mind, he sent to about sixty of his colleagues of the United States Geologi- cal Survey a questionnaire asking them to give the limiting dimen- sions in their conception or usage of the terms bowlder, cobble, pebble, sand grain, particle. Replies received from and clay were of the men. These were compared about thirty studied and and the composite results presented in preliminary unpublished form which was distributed to more one hundred geologists through- than out the in of receiving comment and country the hope additional very helpful criticism. A small number of replies were received and utilized in modifying, to some extent, the size limits and the terms used. Early 1921, mimeographed of modified scheme in copies this sent of terms were to about a dozen geologists in this country and who the of and England were known as workers in field sediments sedimentary rocks, and deemed competent to criticize the usages proposed. They were asked to reply to specific questions in regard to the terms which had been to the most criticism to subject and general comment in upon the plan. The replies from this smaller group were most gratifying, since nearly every geologist addressed in of sent a reply which the writer found useful the preparation presented. the classification here In addition to the studies above, writer mentioned the com- menced 1920 the collection in of definitions of sedimentary rock terms. These definitions are taken verbatim from textbooks, dictionaries, encyclopedias, and glossaries. They are typewritten on cards the proper references and filed under of with the name the term defined. Many of the definitions collected are from sources seventy-five to one hundred years old represent the and former usage of certain terms as understood by the compiler. The definitions collected in this way vary greatly and none in value be regarded absolute authority. is to as of They constitute, how- ever, part of the data of the problem. As will appear from the foregoing, the writer has compiled the present scheme in of classification part from a specific study of the terms presented in part from the of a here and results general consideration of terms in the field of sedimentary rocks. He is This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 379 indebted to a large number of geologists who have helped him by spoken and written criticism. Space will permit acknowledgment of only Dr. M. and B. gratitude to I. Goldman to Dr. J. Woodworth, interest whose and frank criticism have been especially helpful in the preparation of this paper. GRADE TERMS THE It purpose to suggest terms is the writer's here which are specific size piece and, at least for the as regards of larger pieces, as regards of apply shape of piece. The terms this scheme to rounded materials in so far as materials of the size in question become rounded by transportation. this regard will Strict uniformity in the as they occur nature. not fit sediments in Bowlders, cobbles, and rounded fragments, pebbles are rock whereas most clay particles are angular, yet geologists will recognize that they all natural series. Likewise, bowlders clay particles belong to a and commonly of the composition but spite are not same mineral in this extremes transported of fact they are the two of the series of rock fragments. By an excessive multiplication of terms it would be possible to make a classification in which each term was specific to of particle, shape particle, lithologic character, as size of and other characteristics. a scheme would be artificial in Such highly and writer the many of its categories seems to the impracticable in present state of knowledge. present scheme terms is, accordingly, what The of grade just its name implies-a series of names for of different clastic fragments sizes. They apply to fragments in the case only rounded except of sands, silts, in which prolonged transportation fine and clays even does always the The names applied the not round pieces. to different grades carry no lithologic, mineralogic, or chemical significance so far scheme Sands are as the present is concerned. dominantly quartzose, clays are made up of whereas largely kaolin, is in but this fact incidental and not necessary the use of the terms. FRAGMENT TERMS Bowlder.-This term is in common use in English-speaking countries for rounded smoothed larger and masses of rock than This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER K. WENTWORTH cobbles' resulting from abrasion in transportation. Angular masses of rock of the same size are commonly called blocks or The word bowlder is related English bellow; slabs. to the word compare Swedish to rattle or roar. Equivalent terms bullra, in languages ideas of rumbling several other carry similar or rolling in their derivation. Cobble.-Cobble or cobblestone is used generally, both by geologists, and in common speech, for a rounded stone smaller than larger The is a a bowlder and than a pebble. term diminutive the or knob, related of word cob, meaning a rounded hump and to the Kopf, German for head. term is a very ancient one is used commonly Pebble.-This which for rounded, transported rock fragments smaller than cobbles. past for In the it was more commonly used than it is at present rounded stones up to bowlders. The the size of tendency now is to use the term cobble in an intermediate sense, as stated above. Pebble is from the Anglo-Saxon papol, which meant something round, the papula, a small and perhaps akin to Latin pustule. is Granule.2-The term granule here proposed by the writer as a term for rounded rock fragments larger than very coarse sand grains but smaller than pebbles. Rounded pieces too small to be called been to pebbles have still too large be called sand grains the practice most geologists. Granule Latin in of is from the granulum, of granum, grain, grain, a diminutive meaning a little pellet. spite of In of apparent infelicity meaning (little grain), chosen this term was as best adapted for this grade of material. The term grit grain was considered for use in this sense, but was thought less satisfactory. Grit is used in another sense, as for fine sandstone of angular grain. It seemed undesirable to include these grains either with small with coarse sand pebbles or grains, and it is hoped that the term granule may fill an apparent gap in the series of terms heretofore used. grain.-The several the use of Sand terms made up by adjectives qualifying sand grain are self-explanatory. ' For explanation of the basis on which the sizes limiting the several grades were follows. chosen, see the text which 2 This was to writer by Dr. England. term suggested the Herbert A. Baker, of This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). AND TERMS CLASTIC 381 GRADE CLASS FOR SEDIMENTS term individual Silt particle.-The silt particle is here applied to particles smaller than very fine sand grains but larger than clay from objected particles. The term silt which it was derived was to by some geologists on grounds that are stated under the heading of silt. These grounds were not sustained even by a minor part of the data available to the writer and the term is here used as the most satisfactory one. TABLE I THE TERMS GRADE The Pieces The Aggregate The Indurated Rock Bowlder gravel conglomerate Bowlder Bowlder 256 mm. gravel Cobble Cobble Cobble conglomerate 64 mm. Pebble Pebble gravel Pebble conglomerate 4 mm. Granule Granule Granule gravel conglomerate 2 mm. Very coarse sand grain Very coarse sand Very coarse sandstone mm. Coarse sand grain Coarse sand Coarse sandstone I/2 mm. grain Medium sand Medium sandstone Medium sand I /4 mm. Fine grain Fine sand sand Fine sandstone I/8 mm. Very fine sand grain Very fine sand Very fine sandstone 1/16 mm. Silt particle Silt Siltstone I/256 mm. Clay particle Clay Claystone Clay particle.-After consideration of several other terms for materials finer than silt, the term clay was finally the adopted most to meet with general approval. Clay particle as likely is therefore for individual pieces. used the The size fixing of the limits.-In the limiting sizes several writer has grades of the scheme shown in the table, the been governed by two considerations. First, there is a growing accept- ance geologists and engineers of a series of sieves for the among of in classification natural clastic materials which the openings of consecutive size stand to one another in the ratio 2 or 1/2 starting with I as the standard. mm. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER 382 K. WENTWORTH It has long been recognized that the differences between two consecutive screen size openings should be greater large for the sizes small. is followed selection than for the This principle in the lo, millimeters, limits of such limits as I, 2, 5, 20 making the fall notation. This on convenient whole numbers in the decimal series, however, is a crude to geometrical series which approach a in each value a constant ratio to one. geo- bears the preceding A metrical the for a series is ideal such purpose, since a change of i" is of the same significance and importance in the size of io" cobbles as change of 1-" in the size of pebbles. Only by the a I" use similar of of logarithmic or some graphical scheme representa- tion can the size composition data be shown adequately for great size ranges. use of a geometrical the successive The series makes fall equal on the arrangement much grades into units graph-an other easier to read and interpret than any known to the writer. most ratio for the construction such is The convenient of a series the ratio 2, and the most convenient and logical starting-point, i mm. A large number of of mechanical analyses sediments made with screens microscopic and by measurement conforming to such a series been made.' a minute subdivision needed, have If more is the of or the ratio 1/ 2 can be used, giving twice number grades, in subdivisions fit exceptional cases 2. These extra in with and form further subdivisions of the fundamental series of the powers of 2. Conformity to this geometrical series is first consideration the which has the writer limits the several guided in fixing the between grade terms. The second consideration has been the desire to make each of the as to common practice majority limits close as possible the of the geologists. Figure i shows of the composite opinions of twenty- eight geologists of the United States Geological Survey, as reported by them in response to a questionnaire on the sizes limiting several of terms. The table a of schemes the below shows number different of classification which have been published. There is a close agree- some but, with exception of ment between of those shown, the that of of Udden, all lack, in the sizes successive grades, the uniformity x J. A. Udden, "Mechanical Composition of Clastic Sediments," Bull. Geol. Soc. Amer., Vol. XXV (1914), pp. 655-744. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 383 of the geometrical seems be of ratio series which to the writer to essential to any thorough quantitative study of the mechanical composition of sediments. data II, Using the shown in Figure I and Table the writer has selected the limits conforming to the power series of 2 conform also the concensus of opinions which most closely to the FIG. of authorities there. the quoted The names of several grades thus established were then chosen as described above. AGGREGATE TERMS size terms The assignment of definite limiting values to the for the aggregates as named below, as well as the rocks named in this on the that another part of paper, will be objected to ground aggregates are not made up in nature of one grade or even of a few grades, and that therefore the names are inapplicable. The author This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). by XXV - - sand Basin, sand sand gravel gravel sand gravel gravel mm. sand mm.- mm. Vol. mm. mm.- mm. mm.- mm. mm.- mm. Proposed fine Silt Clay Sediments coarse 256 1/2 I/4 1/8 London Fine 1/256 Coarse Classification - Cobble Pebble Medium Here Granule Very the Amer., Bowlder -I - -/16 Very of -64 -2 Soc. Arenaceous Side - - - - 18. - - Geol. - - - gravel silt silt sand clay sand silt gravel sand clay mm.- mm.- Loose bowlders silt 17, mm. mm. mm. gravel sand mm.- mm.- clay bowlders mm.- mm. mm. mm. mm.- mm. gravel mm.- mm. mm. fine of bowlders bowlders mm. mm. mm. mm.- Bull. Udden fine Southern 8 4 2 pp. 16 coarse 256 1/2 1/4 1/8 Fine A. small I/i6 I/32 1/64 Fine Fine Coarse - I/I28 1/256 1/512 the Fine Coarse Coarse Medium Very Medium 1/1024 1/2048 Coarse Medium J. Very Small Medium Large of Medium Very Very - -64 ----32 - - - --i - - --- - - (1906), Beds Constitution Sediments," Book - ___ - - sand -- sand silt sand Thanet sand Clastic mm. Field mm. mm. Bakertt mm.-- mm.l mm. mm.- Silt Clay the 2 coarse S of Gravel Mechanical A. .25 .05 .0O Fine -.1 to Coarse Coarse - -I -.5 Medium H. the Survey Very - - of Soil Reference Composition - -- Soils, Soils sand sand sand of sand of mm. States** mm. mm. Special Gravel mm.- mm.- mm. mm. Investigation fine Silt 1920. Clay 2 I .25 .05 the Fine with Coarse - Fine - -.5 -.1 Co., Medium Very Bureau Bureau United On "Mechanical - -- -.00oo5 & States Baker, Dulan Udden, Elutriation, - - - A. 655-744. sand of A. sand silt sand sand mm.- J. mm. United H. gravel mm. Diller DillerT mm.- mm.- pp. mm. fine Silt Clay ** tft $4 Gravel . London, S. S. .25 . .01 Fine Finest Method II Fine Coarse 2, J. J. -m Medium -i -.5 - Very the p. (1914), - - -.005 mm. Cie. TABLE - sand - - Enke. sand sand m. dust sand mm. mm. mm.- mm. mm. Dust Mineralogie, coarse Orth[ Gravel 3 i .25 .02 Fine Finest - Coarse Medium Chapelot fir Ferdinand - Very -.5 - - - R. Company. flour Paris, - - - - Jahrbuch sand Stuttgart, gravel sand sand 64, sand flour size Seiler gravel sand rock mm.- mm. gravel mm. mm. mm. mm. mm.- mm.- mm.- mm. p. Grabau§ mm. mm. G. Neues 5 coarse i .i Cobbles course 2.5 .25 .05 .o1 286. 150 Clay W. Bowlders Fine .oo5 Rock Auflage, A. Fine - Coarse -50 -- Coarse Medium p. Superfine A. - 7ery -25 Very -.5 -- - - uperfine 2te anciens, et York, Kornes. (1913), (1908), 380. des New actuel sand sand p. sand sand sand gravel mm. mm. mm. mm. mm.- fine 3 Grbsse 45 Geologie Thoulett (1913), .89 .26 .04 Fine (1902), Fine Coarse Stratigraphy J. Medium Very Fine-fine der - - - - of sous-marins iSo nach praktischen fonds Bull. - - Stratigraphy sand - de der sand sand Principles of sand dust sand Sandes mm. mm. Surv., mm.- mm.- mm.- mm. mm. Dust 2 coarse Gravel .1 Keilhackt .05 .0O F.2 des Fine Finest - -I Coarse d'analyse Geol. -.5 Grabau, Lehrbuch K. Medium Superfine Very - - Principles W. U.S. Prdcis 1875. A. flour by Keilhack, for - - Beseichnung - Grabau, City* sand Diller, sand sand size flour sand rock mm.- W. S. mm. mm. mm. Thoulet, mm.- mm.- mm. mm. Fine J. i Quoted Orth, York gravel Konrad J. A. Volume Coarse Gravel .i S5 .25 .05 .0O Clay | t $ § Fine * Aqueduct Rock Coarse Commission - Medium -.5 Superfine 551, New Superfine This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM - - - -.065 P. All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). FOR 385 GRADE AND CLASS TERMS CLASTIC SEDIMENTS grade are recognizes this difficulty and the terms here defined applicable strictly without modification to but few and natural sediments. are proposed the in They as foundation of, and the course of development of, the several class terms and adjectives proposed in the last of this paper. of grade part Photographs from mm. aggregates 64 to 3z mm. are shown in the accompanying Figures 2 and 3. Gravel.-Among some geologists the term gravel has been used only for material composed of small pebbles and but more granules, reference commonly and especially in America and in to glacial gravels, term material the has been used to include containing great up to a meter or more in diameter, has bowlders and recently been so J. Flett' who considers the term to be defined by S. the of equivalent conglomerate as applied to the consolidated rocks. It is in this latter and prevailing sense that the term is here used and with qualifying words in the terms gravel, combined bowlder and cobble gravel, pebble gravel, granule gravel. term sand is use all Sand.-The in common among English- speaking geologists for mineral grains smaller than i or 2 mm. than silt. some sand applied only to and larger By writers, is but others use rounded mineral grains, the term more generally. Sand is from the Anglo-Saxon word of the same spelling and meaning. Silt.-The silt is considered by geologists to term some apply properly to matter to deposits containing organic in addition the particles. writer was any mineral The unable to find considerable this past has support of view by either or preseAt authorities, and the here used the term for the grades designated in table on page 384. The word silt is probably akin to a number of Germanic roots meaning to sift or filter, compare German seihen, to strain. Clay.-After of a of terms, consideration number alternative term has been selected as most likely to the clay to be acceptable geologists for the finest clastic sediments. A few geologists to term ground that implied plasticity objected the on the it or that referred is the it to a definite chemical composition. It view S. (I911), 382, and 'J. Flett, Encyclopaedia Britannica, IIth ed. Vol. XII, p. Vol. VI, p. 913. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER 386 K. WENTWORTH 64-32 mm 32-16 mm mm 16-8 Pebble gravel Natural Size FIG. 2 This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). AND TERMS CLASTIC SEDIMENTS 387 GRADE CLASS FOR of the writer and of many other geologists that nearly all clastic materials grade consist largely of the of this hydrous aluminum silicates which make up the clay of the chemist and also that the material is always more or less plastic. There is, therefore, in his opinion a common for geologist without ground the and chemist the an insistence on the use of the term clay for pure chemical kaolin minerals this compounds or other of group. ROCK TERMS Conglomerate.-There are certain terms which are used with greater and less abuse than others. One of is uniformity these applied conglomerate. This term is very widely to rocks which are the consolidated equivalents of gravels. It is applied just as is the term gravel to rocks which vary widely in the sizes of their constituent particles. It seems desirable, therefore, to apply to it modifying adjectives as has been done with gravel, making the terms bowlder-conglomerate, cobble-conglomerate, pebble-conglomerate, and granule-conglomerate. The term granule-conglomerate is preferred grit has been used to the term grit because in England for both coarse- and fine-grained and angular-grained sandstones. The use of the term grit in the present sense seems therefore inadvisable. Sandstone.-This term is generally used by geologists and no great in its usage here proposed. It is qualified by the change is adjectives very coarse, coarse, and very fine, medium, fine, and used for consolidated equivalents of the various of sand. the grades The term grit used for the grade for the is not coarser reasons stated in considering the term granule-conglomerate. Siltstone.-After a a terms, siltstone consideration of number of the most acceptable consolidated was adopted by writer as for the of a silt. as some geologists, equivalent Shale, proposed by was in the a considered objectionable because, usage of majority of present, geologists at as well as etymologically, it is a structural term referring to the shelly structure of the rock rather than to the size of its grains. used Claystone.-The term argillite has already been in another sense and the term shale is for the reasons given objectionable This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). K. CHESTER WENTWORTH sand mm mm Silt I-3 g-A Coarse sand sand mm mm coarse fine 2-I l-iA Very Very Size 3 FIG. Natural grave sand mm mm 4-2 4-B Fine Granule sand gravel mm mm 8-4 .1 Medium Pebble This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 389 here satisfactory above. The term claystone is proposed as most for the indurated equivalent of clay, as defined above. THE CLASS TERMS The grade terms defined above for aggregates and for are rocks in of applicable, as the writer has pointed out another part this paper, very origin. Most to few sediments of natural such sedi- ments are composed of particles of several or many grades and the names suggested above with definite numerical limits cannot properly be applied them. This difficulty has been long to recog- sedimentary rocks for nized by students of and several schemes meeting the difficulty have been proposed. Notable among these, amount of accordance because of the large data published in with it, that of the United States Bureau which is below. is of Soils given MATERIAL CLASSIFICATION OF SOIL UNITED STATES BUREAU OF SOILS* 20% and clay: Soils containing silt Coarse sand 25% very and sand and more than coarse sand coarse grade less than 50% any other Sand 25% coarse coarse and more than very sand, medium sand sand, and less than 50% fine Fine more than fine sand, less than very sand 50% or 25% coarse sand, coarse and medium sand Very fine sand 50% very fine sand more than containing 20-50% silt and clay: Soils Sandy loam more than 25% very coarse sand, coarse and medium sand Fine sandy loam more than 50o% fine sand or less than 25% very coarse sand, coarse and medium sand Sandy clay less than 20% silt Soils containing more than 50o% silt and clay: Loam less than clay, less than 20% 50o% silt Silt loam. clay, more 50o% less than 20% than silt 20-30% Clay loam clay, less than 50o% silt clay loam 50% silt Silty 20-30% clay, more than Clay clay more than 30% * C. C. Fletcher and H. Bryan, "Modification of the Method of Mechanical Soil Analysis," U.S. Dept. of Agric., Bur. Soils, No. 84 (1912). Bull. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). K. CHESTER WENTWORTH 39° by the The scheme of class terms presented below was devised writer after a somewhat extensive consideration of several more complicated classifications. These were characterized by more use adjectives coarse extensive of the and fine and by the use of many combinations and permutations of several grade terms. These were all rejected in favor of the simpler plan here presented because of the futility of attempting to make seeming detailed discriminations by use of scheme. quantitative a verbal Such discrimination can be satisfactorily made only by graphic or tabular methods and the writer believes the simpler classification most acceptable and most the will be generally therefore useful in study of sediments. The table shows only class terms for aggregates; the proper rock terms will be and used in an analogous manner. chosen CLASS TERMS FOR SEDIMENTS Specifications by Grade Terms for Classes Gravel > 80% Gravel > > Others< gravel Gravel sand o10% 10% Sandy Sand > gravel > 10% Others< 10% Gravelly sand Sand > 80% Sand Sand >silt Others< Io% Silty sand > o10% Silt > o10% silt >sand Others< o10% Sandy Silt > 80% Silt Silt >clay > o10% Others< o10% Clayey silt Clay >silt > o10% Others< o10% Silty clay Clay > 80% Clay of sedimentary but highly variable Certain materials origin till, into of composition, notably glacial will not fall any the divi- sions of the table above. No attempt has been made to make an all-inclusive scheme. Till is known by its extreme range of mechanical composition and by definition, therefore, will not fit simple test of in into so a classification. The any classification is its to natural objects. The terms proposed applicability here were tested by reference a large number analyses made by to of inspection fifty of these random Udden.' By of about chosen at not it was found that only the analyses of till did accord with the given one terms, and that each of the other specification for of the ' J. A. Udden, loc. cit. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 391 sediments tested was assigned to a suitable name differing only in exactness from that original description. In added used in the the table below are given nine of these analyses showing the OF MECHANICAL ANALYSES BY J. THE TABLE A. UDDEN* SHOWING SEDIMENT NAMES APPROPRIATE THE CLASSIFICATION PAPER TO OF THIS No. No. 16 No. 75t No. 9 No. 18 68 Diameter in Millimeters Gravel t Sandy Gravel Gravelly Sand Sand Silty Sand 64-3 2 32-16 14.0 16-8 13.2 3.3 8-4 20.0 0.I 65 9 14.4 I.7 .2 4-2 26.0 7.4 2-I 3.0 II.4 24.4 .3 3.6 18.2 1-1/2 7.2 .3 7.2 49.2 1/2-1/4 . 2 36.8 8.3 5.6 17.9 2.6 1/4-1/8 .7 37.4 48.7 14.8 1.2 I.0 1/8-1/16 .3 .2 8.4 13.2 1/I6-1/32 .4 1/32-1/64 JrI 3.2 1/64-1/128 1/128-1/256 .I J. A. * Udden, loc. cit. Udden sediments. The are t The numbers are those used by in describing these names applied here by the author of this paper according to the terminology presented. No. 129 131 Io No. 70 No. No. Diameter Millimeters in Clay Sandy Silt Silt Clayey Silt Silty 2-I 1-1/2 2 tr. tr. tr. I/2-1/4 -9 tr. tr. 2.3 1/4-1/8 II.9 1/8-1/10 2.0 9.8 3-1 5.3 1/16-1/32 28.7 12.4 13.2 5.6 I/32-1/64 40.6 28.2 13.2 37. 2 1/64-1/128 24.6 31.1 26.4 5-9 I/I28-1/256 1.5 9.6 II.7 19.8 1/256-1/512 .2 17.8 6.4 3.5 1/512-1/1024. 10.0 2.4 2 .8 I/I024-I/204 . 2.3 number the composition, the in Udden's table, and name according classification. to the present pointed out to name a sediment, one It will be that in order first true must make a mechanical analysis. This is to the degree This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER K. WENTWORTH it that is true that microscopic study precedes the final naming of an igneous rock. However, as in the case of igneous rocks, field names on simple megascopic be extensively based inspection will before studies made. used detailed are It is believed that field names can be more accurately and expressively used if the principles and essential facts of a quantitative such that classification as here proposed are duly recognized. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of Geology Unpaywall

A Scale of Grade and Class Terms for Clastic Sediments

The Journal of GeologyJul 1, 1922

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0022-1376
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Abstract

SCALE OF AND CLASS TERMS FOR A GRADE CLASTIC SEDIMENTS' CHESTER K. WENTWORTH University of State Iowa CONTENTS INTRODUCTION THE GRADE TERMS Fragment Terms Aggregate Terms Rock Terms THE CLASS TERMS INTRODUCTION In no other science does the of terminology present problem many in geology. With of knowledge so difficulties as the growth in any field of devise or old investigation, men new terms redefine attempt and ones in the to convey more precise definite ideas. In all the branches of science much confusion has followed the redefinition of old terms because of the indiscriminate use of the old and new senses. But geology, dif- terms both in the the in of ficulties this kind are peculiarly great. Because geology is a field science and has followed in the footsteps of exploration, it has acquired terms from all parts of the world. Many of the names for the less common special features have come from the dialect or colloquial speech of that part of the world where they are best developed. With the of these use terms of geologists of other regions, much irregularity usage of and hence much confusion has arisen. Since '917, the writer had been engaged in the study of abrasion and of and pebbles action shaping cobbles by the of running water. In the course of this study the loose usage of cobble, pebble, and own practice was exception) has related terms (in which his no and impressed him with the need of greater uniformity of usage ' Published by permission of the Director of the United States Geological Survey. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER K. WENTWORTH definition terms. this in more careful of such With need mind, he sent to about sixty of his colleagues of the United States Geologi- cal Survey a questionnaire asking them to give the limiting dimen- sions in their conception or usage of the terms bowlder, cobble, pebble, sand grain, particle. Replies received from and clay were of the men. These were compared about thirty studied and and the composite results presented in preliminary unpublished form which was distributed to more one hundred geologists through- than out the in of receiving comment and country the hope additional very helpful criticism. A small number of replies were received and utilized in modifying, to some extent, the size limits and the terms used. Early 1921, mimeographed of modified scheme in copies this sent of terms were to about a dozen geologists in this country and who the of and England were known as workers in field sediments sedimentary rocks, and deemed competent to criticize the usages proposed. They were asked to reply to specific questions in regard to the terms which had been to the most criticism to subject and general comment in upon the plan. The replies from this smaller group were most gratifying, since nearly every geologist addressed in of sent a reply which the writer found useful the preparation presented. the classification here In addition to the studies above, writer mentioned the com- menced 1920 the collection in of definitions of sedimentary rock terms. These definitions are taken verbatim from textbooks, dictionaries, encyclopedias, and glossaries. They are typewritten on cards the proper references and filed under of with the name the term defined. Many of the definitions collected are from sources seventy-five to one hundred years old represent the and former usage of certain terms as understood by the compiler. The definitions collected in this way vary greatly and none in value be regarded absolute authority. is to as of They constitute, how- ever, part of the data of the problem. As will appear from the foregoing, the writer has compiled the present scheme in of classification part from a specific study of the terms presented in part from the of a here and results general consideration of terms in the field of sedimentary rocks. He is This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 379 indebted to a large number of geologists who have helped him by spoken and written criticism. Space will permit acknowledgment of only Dr. M. and B. gratitude to I. Goldman to Dr. J. Woodworth, interest whose and frank criticism have been especially helpful in the preparation of this paper. GRADE TERMS THE It purpose to suggest terms is the writer's here which are specific size piece and, at least for the as regards of larger pieces, as regards of apply shape of piece. The terms this scheme to rounded materials in so far as materials of the size in question become rounded by transportation. this regard will Strict uniformity in the as they occur nature. not fit sediments in Bowlders, cobbles, and rounded fragments, pebbles are rock whereas most clay particles are angular, yet geologists will recognize that they all natural series. Likewise, bowlders clay particles belong to a and commonly of the composition but spite are not same mineral in this extremes transported of fact they are the two of the series of rock fragments. By an excessive multiplication of terms it would be possible to make a classification in which each term was specific to of particle, shape particle, lithologic character, as size of and other characteristics. a scheme would be artificial in Such highly and writer the many of its categories seems to the impracticable in present state of knowledge. present scheme terms is, accordingly, what The of grade just its name implies-a series of names for of different clastic fragments sizes. They apply to fragments in the case only rounded except of sands, silts, in which prolonged transportation fine and clays even does always the The names applied the not round pieces. to different grades carry no lithologic, mineralogic, or chemical significance so far scheme Sands are as the present is concerned. dominantly quartzose, clays are made up of whereas largely kaolin, is in but this fact incidental and not necessary the use of the terms. FRAGMENT TERMS Bowlder.-This term is in common use in English-speaking countries for rounded smoothed larger and masses of rock than This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER K. WENTWORTH cobbles' resulting from abrasion in transportation. Angular masses of rock of the same size are commonly called blocks or The word bowlder is related English bellow; slabs. to the word compare Swedish to rattle or roar. Equivalent terms bullra, in languages ideas of rumbling several other carry similar or rolling in their derivation. Cobble.-Cobble or cobblestone is used generally, both by geologists, and in common speech, for a rounded stone smaller than larger The is a a bowlder and than a pebble. term diminutive the or knob, related of word cob, meaning a rounded hump and to the Kopf, German for head. term is a very ancient one is used commonly Pebble.-This which for rounded, transported rock fragments smaller than cobbles. past for In the it was more commonly used than it is at present rounded stones up to bowlders. The the size of tendency now is to use the term cobble in an intermediate sense, as stated above. Pebble is from the Anglo-Saxon papol, which meant something round, the papula, a small and perhaps akin to Latin pustule. is Granule.2-The term granule here proposed by the writer as a term for rounded rock fragments larger than very coarse sand grains but smaller than pebbles. Rounded pieces too small to be called been to pebbles have still too large be called sand grains the practice most geologists. Granule Latin in of is from the granulum, of granum, grain, grain, a diminutive meaning a little pellet. spite of In of apparent infelicity meaning (little grain), chosen this term was as best adapted for this grade of material. The term grit grain was considered for use in this sense, but was thought less satisfactory. Grit is used in another sense, as for fine sandstone of angular grain. It seemed undesirable to include these grains either with small with coarse sand pebbles or grains, and it is hoped that the term granule may fill an apparent gap in the series of terms heretofore used. grain.-The several the use of Sand terms made up by adjectives qualifying sand grain are self-explanatory. ' For explanation of the basis on which the sizes limiting the several grades were follows. chosen, see the text which 2 This was to writer by Dr. England. term suggested the Herbert A. Baker, of This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). AND TERMS CLASTIC 381 GRADE CLASS FOR SEDIMENTS term individual Silt particle.-The silt particle is here applied to particles smaller than very fine sand grains but larger than clay from objected particles. The term silt which it was derived was to by some geologists on grounds that are stated under the heading of silt. These grounds were not sustained even by a minor part of the data available to the writer and the term is here used as the most satisfactory one. TABLE I THE TERMS GRADE The Pieces The Aggregate The Indurated Rock Bowlder gravel conglomerate Bowlder Bowlder 256 mm. gravel Cobble Cobble Cobble conglomerate 64 mm. Pebble Pebble gravel Pebble conglomerate 4 mm. Granule Granule Granule gravel conglomerate 2 mm. Very coarse sand grain Very coarse sand Very coarse sandstone mm. Coarse sand grain Coarse sand Coarse sandstone I/2 mm. grain Medium sand Medium sandstone Medium sand I /4 mm. Fine grain Fine sand sand Fine sandstone I/8 mm. Very fine sand grain Very fine sand Very fine sandstone 1/16 mm. Silt particle Silt Siltstone I/256 mm. Clay particle Clay Claystone Clay particle.-After consideration of several other terms for materials finer than silt, the term clay was finally the adopted most to meet with general approval. Clay particle as likely is therefore for individual pieces. used the The size fixing of the limits.-In the limiting sizes several writer has grades of the scheme shown in the table, the been governed by two considerations. First, there is a growing accept- ance geologists and engineers of a series of sieves for the among of in classification natural clastic materials which the openings of consecutive size stand to one another in the ratio 2 or 1/2 starting with I as the standard. mm. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER 382 K. WENTWORTH It has long been recognized that the differences between two consecutive screen size openings should be greater large for the sizes small. is followed selection than for the This principle in the lo, millimeters, limits of such limits as I, 2, 5, 20 making the fall notation. This on convenient whole numbers in the decimal series, however, is a crude to geometrical series which approach a in each value a constant ratio to one. geo- bears the preceding A metrical the for a series is ideal such purpose, since a change of i" is of the same significance and importance in the size of io" cobbles as change of 1-" in the size of pebbles. Only by the a I" use similar of of logarithmic or some graphical scheme representa- tion can the size composition data be shown adequately for great size ranges. use of a geometrical the successive The series makes fall equal on the arrangement much grades into units graph-an other easier to read and interpret than any known to the writer. most ratio for the construction such is The convenient of a series the ratio 2, and the most convenient and logical starting-point, i mm. A large number of of mechanical analyses sediments made with screens microscopic and by measurement conforming to such a series been made.' a minute subdivision needed, have If more is the of or the ratio 1/ 2 can be used, giving twice number grades, in subdivisions fit exceptional cases 2. These extra in with and form further subdivisions of the fundamental series of the powers of 2. Conformity to this geometrical series is first consideration the which has the writer limits the several guided in fixing the between grade terms. The second consideration has been the desire to make each of the as to common practice majority limits close as possible the of the geologists. Figure i shows of the composite opinions of twenty- eight geologists of the United States Geological Survey, as reported by them in response to a questionnaire on the sizes limiting several of terms. The table a of schemes the below shows number different of classification which have been published. There is a close agree- some but, with exception of ment between of those shown, the that of of Udden, all lack, in the sizes successive grades, the uniformity x J. A. Udden, "Mechanical Composition of Clastic Sediments," Bull. Geol. Soc. Amer., Vol. XXV (1914), pp. 655-744. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 383 of the geometrical seems be of ratio series which to the writer to essential to any thorough quantitative study of the mechanical composition of sediments. data II, Using the shown in Figure I and Table the writer has selected the limits conforming to the power series of 2 conform also the concensus of opinions which most closely to the FIG. of authorities there. the quoted The names of several grades thus established were then chosen as described above. AGGREGATE TERMS size terms The assignment of definite limiting values to the for the aggregates as named below, as well as the rocks named in this on the that another part of paper, will be objected to ground aggregates are not made up in nature of one grade or even of a few grades, and that therefore the names are inapplicable. The author This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). by XXV - - sand Basin, sand sand gravel gravel sand gravel gravel mm. sand mm.- mm. Vol. mm. mm.- mm. mm.- mm. mm.- mm. Proposed fine Silt Clay Sediments coarse 256 1/2 I/4 1/8 London Fine 1/256 Coarse Classification - Cobble Pebble Medium Here Granule Very the Amer., Bowlder -I - -/16 Very of -64 -2 Soc. Arenaceous Side - - - - 18. - - Geol. - - - gravel silt silt sand clay sand silt gravel sand clay mm.- mm.- Loose bowlders silt 17, mm. mm. mm. gravel sand mm.- mm.- clay bowlders mm.- mm. mm. mm. mm.- mm. gravel mm.- mm. mm. fine of bowlders bowlders mm. mm. mm. mm.- Bull. Udden fine Southern 8 4 2 pp. 16 coarse 256 1/2 1/4 1/8 Fine A. small I/i6 I/32 1/64 Fine Fine Coarse - I/I28 1/256 1/512 the Fine Coarse Coarse Medium Very Medium 1/1024 1/2048 Coarse Medium J. Very Small Medium Large of Medium Very Very - -64 ----32 - - - --i - - --- - - (1906), Beds Constitution Sediments," Book - ___ - - sand -- sand silt sand Thanet sand Clastic mm. Field mm. mm. Bakertt mm.-- mm.l mm. mm.- Silt Clay the 2 coarse S of Gravel Mechanical A. .25 .05 .0O Fine -.1 to Coarse Coarse - -I -.5 Medium H. the Survey Very - - of Soil Reference Composition - -- Soils, Soils sand sand sand of sand of mm. States** mm. mm. Special Gravel mm.- mm.- mm. mm. Investigation fine Silt 1920. Clay 2 I .25 .05 the Fine with Coarse - Fine - -.5 -.1 Co., Medium Very Bureau Bureau United On "Mechanical - -- -.00oo5 & States Baker, Dulan Udden, Elutriation, - - - A. 655-744. sand of A. sand silt sand sand mm.- J. mm. United H. gravel mm. Diller DillerT mm.- mm.- pp. mm. fine Silt Clay ** tft $4 Gravel . London, S. S. .25 . .01 Fine Finest Method II Fine Coarse 2, J. J. -m Medium -i -.5 - Very the p. (1914), - - -.005 mm. Cie. TABLE - sand - - Enke. sand sand m. dust sand mm. mm. mm.- mm. mm. Dust Mineralogie, coarse Orth[ Gravel 3 i .25 .02 Fine Finest - Coarse Medium Chapelot fir Ferdinand - Very -.5 - - - R. Company. flour Paris, - - - - Jahrbuch sand Stuttgart, gravel sand sand 64, sand flour size Seiler gravel sand rock mm.- mm. gravel mm. mm. mm. mm. mm.- mm.- mm.- mm. p. Grabau§ mm. mm. G. Neues 5 coarse i .i Cobbles course 2.5 .25 .05 .o1 286. 150 Clay W. Bowlders Fine .oo5 Rock Auflage, A. Fine - Coarse -50 -- Coarse Medium p. Superfine A. - 7ery -25 Very -.5 -- - - uperfine 2te anciens, et York, Kornes. (1913), (1908), 380. des New actuel sand sand p. sand sand sand gravel mm. mm. mm. mm. mm.- fine 3 Grbsse 45 Geologie Thoulett (1913), .89 .26 .04 Fine (1902), Fine Coarse Stratigraphy J. Medium Very Fine-fine der - - - - of sous-marins iSo nach praktischen fonds Bull. - - Stratigraphy sand - de der sand sand Principles of sand dust sand Sandes mm. mm. Surv., mm.- mm.- mm.- mm. mm. Dust 2 coarse Gravel .1 Keilhackt .05 .0O F.2 des Fine Finest - -I Coarse d'analyse Geol. -.5 Grabau, Lehrbuch K. Medium Superfine Very - - Principles W. U.S. Prdcis 1875. A. flour by Keilhack, for - - Beseichnung - Grabau, City* sand Diller, sand sand size flour sand rock mm.- W. S. mm. mm. mm. Thoulet, mm.- mm.- mm. mm. Fine J. i Quoted Orth, York gravel Konrad J. A. Volume Coarse Gravel .i S5 .25 .05 .0O Clay | t $ § Fine * Aqueduct Rock Coarse Commission - Medium -.5 Superfine 551, New Superfine This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM - - - -.065 P. All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). FOR 385 GRADE AND CLASS TERMS CLASTIC SEDIMENTS grade are recognizes this difficulty and the terms here defined applicable strictly without modification to but few and natural sediments. are proposed the in They as foundation of, and the course of development of, the several class terms and adjectives proposed in the last of this paper. of grade part Photographs from mm. aggregates 64 to 3z mm. are shown in the accompanying Figures 2 and 3. Gravel.-Among some geologists the term gravel has been used only for material composed of small pebbles and but more granules, reference commonly and especially in America and in to glacial gravels, term material the has been used to include containing great up to a meter or more in diameter, has bowlders and recently been so J. Flett' who considers the term to be defined by S. the of equivalent conglomerate as applied to the consolidated rocks. It is in this latter and prevailing sense that the term is here used and with qualifying words in the terms gravel, combined bowlder and cobble gravel, pebble gravel, granule gravel. term sand is use all Sand.-The in common among English- speaking geologists for mineral grains smaller than i or 2 mm. than silt. some sand applied only to and larger By writers, is but others use rounded mineral grains, the term more generally. Sand is from the Anglo-Saxon word of the same spelling and meaning. Silt.-The silt is considered by geologists to term some apply properly to matter to deposits containing organic in addition the particles. writer was any mineral The unable to find considerable this past has support of view by either or preseAt authorities, and the here used the term for the grades designated in table on page 384. The word silt is probably akin to a number of Germanic roots meaning to sift or filter, compare German seihen, to strain. Clay.-After of a of terms, consideration number alternative term has been selected as most likely to the clay to be acceptable geologists for the finest clastic sediments. A few geologists to term ground that implied plasticity objected the on the it or that referred is the it to a definite chemical composition. It view S. (I911), 382, and 'J. Flett, Encyclopaedia Britannica, IIth ed. Vol. XII, p. Vol. VI, p. 913. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER 386 K. WENTWORTH 64-32 mm 32-16 mm mm 16-8 Pebble gravel Natural Size FIG. 2 This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). AND TERMS CLASTIC SEDIMENTS 387 GRADE CLASS FOR of the writer and of many other geologists that nearly all clastic materials grade consist largely of the of this hydrous aluminum silicates which make up the clay of the chemist and also that the material is always more or less plastic. There is, therefore, in his opinion a common for geologist without ground the and chemist the an insistence on the use of the term clay for pure chemical kaolin minerals this compounds or other of group. ROCK TERMS Conglomerate.-There are certain terms which are used with greater and less abuse than others. One of is uniformity these applied conglomerate. This term is very widely to rocks which are the consolidated equivalents of gravels. It is applied just as is the term gravel to rocks which vary widely in the sizes of their constituent particles. It seems desirable, therefore, to apply to it modifying adjectives as has been done with gravel, making the terms bowlder-conglomerate, cobble-conglomerate, pebble-conglomerate, and granule-conglomerate. The term granule-conglomerate is preferred grit has been used to the term grit because in England for both coarse- and fine-grained and angular-grained sandstones. The use of the term grit in the present sense seems therefore inadvisable. Sandstone.-This term is generally used by geologists and no great in its usage here proposed. It is qualified by the change is adjectives very coarse, coarse, and very fine, medium, fine, and used for consolidated equivalents of the various of sand. the grades The term grit used for the grade for the is not coarser reasons stated in considering the term granule-conglomerate. Siltstone.-After a a terms, siltstone consideration of number of the most acceptable consolidated was adopted by writer as for the of a silt. as some geologists, equivalent Shale, proposed by was in the a considered objectionable because, usage of majority of present, geologists at as well as etymologically, it is a structural term referring to the shelly structure of the rock rather than to the size of its grains. used Claystone.-The term argillite has already been in another sense and the term shale is for the reasons given objectionable This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). K. CHESTER WENTWORTH sand mm mm Silt I-3 g-A Coarse sand sand mm mm coarse fine 2-I l-iA Very Very Size 3 FIG. Natural grave sand mm mm 4-2 4-B Fine Granule sand gravel mm mm 8-4 .1 Medium Pebble This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 389 here satisfactory above. The term claystone is proposed as most for the indurated equivalent of clay, as defined above. THE CLASS TERMS The grade terms defined above for aggregates and for are rocks in of applicable, as the writer has pointed out another part this paper, very origin. Most to few sediments of natural such sedi- ments are composed of particles of several or many grades and the names suggested above with definite numerical limits cannot properly be applied them. This difficulty has been long to recog- sedimentary rocks for nized by students of and several schemes meeting the difficulty have been proposed. Notable among these, amount of accordance because of the large data published in with it, that of the United States Bureau which is below. is of Soils given MATERIAL CLASSIFICATION OF SOIL UNITED STATES BUREAU OF SOILS* 20% and clay: Soils containing silt Coarse sand 25% very and sand and more than coarse sand coarse grade less than 50% any other Sand 25% coarse coarse and more than very sand, medium sand sand, and less than 50% fine Fine more than fine sand, less than very sand 50% or 25% coarse sand, coarse and medium sand Very fine sand 50% very fine sand more than containing 20-50% silt and clay: Soils Sandy loam more than 25% very coarse sand, coarse and medium sand Fine sandy loam more than 50o% fine sand or less than 25% very coarse sand, coarse and medium sand Sandy clay less than 20% silt Soils containing more than 50o% silt and clay: Loam less than clay, less than 20% 50o% silt Silt loam. clay, more 50o% less than 20% than silt 20-30% Clay loam clay, less than 50o% silt clay loam 50% silt Silty 20-30% clay, more than Clay clay more than 30% * C. C. Fletcher and H. Bryan, "Modification of the Method of Mechanical Soil Analysis," U.S. Dept. of Agric., Bur. Soils, No. 84 (1912). Bull. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). K. CHESTER WENTWORTH 39° by the The scheme of class terms presented below was devised writer after a somewhat extensive consideration of several more complicated classifications. These were characterized by more use adjectives coarse extensive of the and fine and by the use of many combinations and permutations of several grade terms. These were all rejected in favor of the simpler plan here presented because of the futility of attempting to make seeming detailed discriminations by use of scheme. quantitative a verbal Such discrimination can be satisfactorily made only by graphic or tabular methods and the writer believes the simpler classification most acceptable and most the will be generally therefore useful in study of sediments. The table shows only class terms for aggregates; the proper rock terms will be and used in an analogous manner. chosen CLASS TERMS FOR SEDIMENTS Specifications by Grade Terms for Classes Gravel > 80% Gravel > > Others< gravel Gravel sand o10% 10% Sandy Sand > gravel > 10% Others< 10% Gravelly sand Sand > 80% Sand Sand >silt Others< Io% Silty sand > o10% Silt > o10% silt >sand Others< o10% Sandy Silt > 80% Silt Silt >clay > o10% Others< o10% Clayey silt Clay >silt > o10% Others< o10% Silty clay Clay > 80% Clay of sedimentary but highly variable Certain materials origin till, into of composition, notably glacial will not fall any the divi- sions of the table above. No attempt has been made to make an all-inclusive scheme. Till is known by its extreme range of mechanical composition and by definition, therefore, will not fit simple test of in into so a classification. The any classification is its to natural objects. The terms proposed applicability here were tested by reference a large number analyses made by to of inspection fifty of these random Udden.' By of about chosen at not it was found that only the analyses of till did accord with the given one terms, and that each of the other specification for of the ' J. A. Udden, loc. cit. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). GRADE AND CLASS TERMS FOR CLASTIC SEDIMENTS 391 sediments tested was assigned to a suitable name differing only in exactness from that original description. In added used in the the table below are given nine of these analyses showing the OF MECHANICAL ANALYSES BY J. THE TABLE A. UDDEN* SHOWING SEDIMENT NAMES APPROPRIATE THE CLASSIFICATION PAPER TO OF THIS No. No. 16 No. 75t No. 9 No. 18 68 Diameter in Millimeters Gravel t Sandy Gravel Gravelly Sand Sand Silty Sand 64-3 2 32-16 14.0 16-8 13.2 3.3 8-4 20.0 0.I 65 9 14.4 I.7 .2 4-2 26.0 7.4 2-I 3.0 II.4 24.4 .3 3.6 18.2 1-1/2 7.2 .3 7.2 49.2 1/2-1/4 . 2 36.8 8.3 5.6 17.9 2.6 1/4-1/8 .7 37.4 48.7 14.8 1.2 I.0 1/8-1/16 .3 .2 8.4 13.2 1/I6-1/32 .4 1/32-1/64 JrI 3.2 1/64-1/128 1/128-1/256 .I J. A. * Udden, loc. cit. Udden sediments. The are t The numbers are those used by in describing these names applied here by the author of this paper according to the terminology presented. No. 129 131 Io No. 70 No. No. Diameter Millimeters in Clay Sandy Silt Silt Clayey Silt Silty 2-I 1-1/2 2 tr. tr. tr. I/2-1/4 -9 tr. tr. 2.3 1/4-1/8 II.9 1/8-1/10 2.0 9.8 3-1 5.3 1/16-1/32 28.7 12.4 13.2 5.6 I/32-1/64 40.6 28.2 13.2 37. 2 1/64-1/128 24.6 31.1 26.4 5-9 I/I28-1/256 1.5 9.6 II.7 19.8 1/256-1/512 .2 17.8 6.4 3.5 1/512-1/1024. 10.0 2.4 2 .8 I/I024-I/204 . 2.3 number the composition, the in Udden's table, and name according classification. to the present pointed out to name a sediment, one It will be that in order first true must make a mechanical analysis. This is to the degree This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). CHESTER K. WENTWORTH it that is true that microscopic study precedes the final naming of an igneous rock. However, as in the case of igneous rocks, field names on simple megascopic be extensively based inspection will before studies made. used detailed are It is believed that field names can be more accurately and expressively used if the principles and essential facts of a quantitative such that classification as here proposed are duly recognized. This content downloaded from 141.218.001.105 on August 03, 2016 21:46:48 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).

Journal

The Journal of GeologyUnpaywall

Published: Jul 1, 1922

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