RIVER PEBBLES 595 



Form of River Pebbles. Rudolf Hoernes (32) has recently 

 insisted upon the distinctive characters possessed by the pebbles 

 subject to prolonged river and wave transport. According to him, 

 river-borne pebbles are always flat, and more or less wedge-shaped, 

 owing to the fact that the current merely shoves the coarser mate- 

 rial along its bottom. Marine and lacustrine pebbles, on the other 

 hand, are rounded or roller-shaped, because the waves tend to roll 

 the material. 



Eduard Suess in 1862, in describing the fluviatile Belvedere 

 gravels of the A'ienna district, says, in efifect (53 :d^, dj) : "A 

 comparison of a considerable quantity of such pebbles shows that 

 they conform more or less to a single typical form, being almost 

 without exception sharpened to a wedge-shaped form on one side. 

 This form distinguishes shoved pebbles from rolled pebbles ; it is 

 produced by the pushing along, over the bottom, of the rock frag- 

 ments by the current of the stream. Rolled pebbles, such as are 

 moved to and fro by the surf on the shore, never have the wedge- 

 shaped form, but a uniformly oval or cylindrical ground form." 

 According to J. Lorenz von Liburnau (2>7'-95^ P^) the flat river 

 pebbles sufTer chiefly a horizontal rotation, so that the top and 

 iDottom of the pebbles are rubbed against the overlying and under- 

 lying ones, while at the same time the edges are worn ofl:". the re- 

 sult being a mass of flat, worn cakes with smooth, rounded edges. 



In contradistinction to these observations and deductions, Penck 

 emphatically insists upon the rolling of pebbles on the stream-bed 

 as the chief method of river transport. Such rolling may aflfect 

 scattered pebbles or the entire mass, in which case the entire sedi- 

 ment on the river bottom is in motion, the individual pebbles roll- 

 ing and striking against one another, with the result that rounded' 

 pebbles are produced. In portions of the Rhine bed, such a mass 

 three meters in depth is thus moved along. (Penck-41 -.284.) 



It is extremely doubtful if the distinctions made by Suess and 

 Hoernes can be considered of more than local applicability. The 

 character of the rock which has furnished the material is probably 

 of much greater significance, as pointed out by ^\'althe^. Thus, on 

 the shores of Lake Michigan, where the bed rock is a uniform 

 grained limestone, the pebbles are chiefly of a rounded or roller- 

 shaped character, while on Lake Erie, where the clififs are of shale, 

 flat gravel predominates, except where glacial deposits have formed 

 a local source of supply. Again, on a relatively steep shore, where 

 wave-work is pronounced, as on the northern Massachusetts coast, 

 the pebbles are well rounded through rolling, while on a shallow 

 coast, where the wash of the waves rushes up and down the beach 



