300 II E. Gregory — Note on the Shape of Pebbles. 



Art. XX. — Note on the Shape of Pebbles / by Herbert 



E. Gregory. 



Attempts to establish criteria for distinguishing beach 

 pebbles from those formed by rivers, glaciers, or wind, or 

 resulting from weathering in place, have led to unsatisfactory 

 results. The summary classification of Mansfield : * marine 

 pebbles, " fairly uniform in size, well rounded " ; fluviatile 

 pebbles, " all sizes, generally subangular " ; glacial pebbles, 

 " faceted, rounded edges, snubbed ends, polished and stri- 

 ated " ; is better adapted for use in the classroom than in the 

 field. With respect to shape and distribution of pebbles, the 

 same may be said of the criteria established by Trowbridge. f 

 The distinctions drawn by Suess and Hoernes4 that marine 

 and lacustrine pebbles are round and oval or roller-shaped, but 

 not wedge-shaped ; that fluviatile pebbles are flat and wedge- 

 shaped ; are not applicable to shores and streams which have 

 come under my observation. 



The assumption that flat pebbles are characteristic of river 

 deposits is usually accompanied by the explanation emphasized 

 by Liburnau,§ that horizontal rotation rather than rolling is 

 the normal method of attrition for river pebbles. Field obser- 

 vations show that this process of shaping is of local significance. 

 Vertical and horizontal rotation, saltation, rolling and sliding, 

 are effective at the same time, or at different times, in all 

 moving water. Snubbed slabs, blunt wedges and disks may 

 be collected from lake and ocean shores, and where conditions 

 are favorable — as along shale cliffs — the beach gravel may 

 consist in large part of these. Strong waves overturn and vig- 

 orously roll pebbles, while weak waves may shove pebbles of 

 suitable shape and size or may not disturb them at all. On a 

 shore fretted by waves of slight power the only modification 

 in shape may be that due to the wearing accomplished by 

 streams of sand which pass to and fro. Along a coastal belt 

 whose shore line is rapidly migrating landward, and especially 

 if the coast is low and the waves are weak, pebbles may be 

 preserved by burial or drowning without much change in orig- 



* The characteristics of various types of conglomerates, Jour. Geol. , xv, 

 pp. 550-555, 1907. 



f A classification of common sediments and some criteria for identification 

 of the various classes, Jour. Geol., xxii, pp. 420-436, 1914. 



\ Suess : DerBoden der Stadt Wien, 1882 ; quoted by Grabau, Stratigraphy, 

 p. 595. 



Hoernes : Gerolle und Geschiebe, Verhandl K.-K. geol. Reichsanstalt, No. 

 12, 1911. 



§Die geologischen Verhaltnisse von Grund und Boden, 1888. 



