654 PRINCIPLES OF STRATIGRAPHY 



chance of sorting than does material in the sea. Moreover, material 

 dropped by one air current may be picked up again by another, 

 while sands dropping in deeper water below the reach of the cur- 

 rents are more likely to be left undisturbed by them. 



If the amount of supply of detritus is great, sorting will be im- 

 peded. Flocculation, or the gathering together of particles, will oc- 

 cur, the coarser carrying down with them the finer. Flocculation is 

 less marked in wind-transported material, where the load is always 

 much less per unit of bulk of the carrier than in most waters, and 

 for this reason also the sorting by wind is more pronounced than 

 that by water. Udden thinks that under ordinary circumstances 

 this difference is nearer i to 100,000 than i to 1,000. (gg-.^^S.) 

 The slope of the sea bottom (Willis-105 : 484) is also a determining 

 factor in the transportation of material by marine currents. Where 

 the slope is a gentle one, sand may be carried for 200 miles or 

 more, as on the Atlantic coast of the United States, where the conti- 

 nental plateau is covered with sands to its outer rim. The trans- 

 porting current here is the undertow, assisted by tides. Since, 

 however, the force of the undertow is largely determined by the 

 strength of the waves, it follows that in circumscribed and very 

 shallow seas no such extensive transportation is possible. Where 

 the slope is a steep one, as en the west coast of South America, the 

 force of the undertow is dissipated, though pebbles and sand will 

 more readily move down the slope. As a rule, the distance to which 

 sands are transported in such a case is limited. 



Organic remains in marine and lacustrine sands. These are gen- 

 erally common, especially in the sea, though areas free from such 

 remains are known, as in the case of the Alaskan coast, already 

 cited. Such absence is, however, due to purely local causes. Even 

 in the beach sands organic remains abound. Everywhere along our 

 coasts shells in numbers, and crustacean and echinoderm tests to a 

 lesser degree are buried in the sands of the beach, and in those 

 just below the low-water line. In the neritic zone animal life of all 

 kinds abounds on the sandy bottoms (see Chapter XXVIII). 

 Abandoned shores of lakes and of the sea also are rich in organic 

 remains. The higher beaches of the late Pleistocenic stages of the 

 Great Lakes contain numerous shells of fresh- water mollusca, and 

 so do the sands of the old shore lines of Niagara and other rivers. 

 So abundant are shells and other organisms in the sands of the 

 modern sea coast that their entire absence from older sandstones 

 must be looked upon as indicative of conditions of deposition other 

 than normal. It is begging the question to assume the subsequent 

 removal of such remains either by solution or otherwise, for even 



