598 PRINCIPLES OF STRATIGRAPHY 



from increase in the amount of water by sudden melting, or from 

 a general uplift of the region, will cause the rivers to entrench 

 themselves in the old flood plains, leaving the remnants as terraces 

 on either side of the valley. Nearly all the streams coming from 

 the glaciated region have such terraces, which mark the greater 

 deposition in periods preceding the present one. Examples of 

 deposits of this type now forming are seen in the valleys of the 

 streams coming from the Alaskan and other glaciers. 



River and Flood Plain Deposits from Continental Ice-SJieets. 

 Several types of such deposits are recognized and are noteworthy 

 on account of their peculiarity; among these are: i. the torrential 

 moraine, or kame ; 2. the frontal, or apron plain ; 3. the esker ; and, 

 4. the glacial sand plain, or temporary lake delta. The essential 

 characters of each are as follows : 



1. Torrential moraine or kame deposits. These are irregular 

 hills of semi-stratified sands and gravels, all of the pebbles being 

 water worn. The material was derived from the ice and was 

 dumped by the rivers in front of the ice-sheet. These deposits are 

 commonly of irregular shape, being more or less confluent cones of 

 debris, and often complicated by the formation of kettle holes from 

 the melting of included ice blocks, as in normal moraines. The 

 topography of such a deposit, when extensive, is often a series of 

 knobs and basins. 



2. The frontal or apron plain. This is also a subaerial deposit 

 and consists of the material carried forward from the glacial mo- 

 raine by streams, and spread as a veneer or mantle over the older 

 formations in front of the ice. \\'here it is spread over a coastal 

 plain, it is generally of uniform thickness, this decreasing from the 

 source outward. The material also becomes finer away from the 

 source of supply. Kettle holes may occur, but they are less fre- 

 quent than in the kame moraine. Cross channels made by the 

 streams from the ice are not uncommon features. Typical examples 

 of such apron plains are found south of the moraine on Cape Cod, 

 and in similar relations to the moraines on Nantucket, Martha's 

 Vineyard, and Long Island. In the Cape Cod sections of this apron 

 plain facetted pebbles or dreikanter have been found in place, in- 

 dicating wind activity and further proving the subaerial origin of 

 this deposit. Such wind work may locally modify the deposit, sub- 

 stituting wind cross-bedding for the horizontal bedding given to 

 them by the water. 



Marine fossils may be locally embedded in such deposits if 

 they are formed near the shore, and when a partial subsidence is 

 succeeded by further deposition by streams from the ice. A fine 



