E. Blackwelder — Yahutat Coastal Plain of Alaska. 463 



sticky mud forms the bottom. Although we can not see what 

 is being deposited on the bed of the ocean, yet we need not 

 doubt that the ceaseless milling of the breakers and currents 

 is comminuting the sediment and distributing the different 

 kinds of detritus in belts varying with the depth of the water 

 and the exposure. This part of the formation should be well 

 assorted and the individual deposits of sand and mud tolerably 

 homogeneous in composition and texture. 



For a picture of the gross structure of the beds we can not 

 rely upon actual observation, for there are no sections which 

 expose more than a few feet of the deposits. But from the 

 nature of the agents engaged in building the plain we may 

 infer with confidence that the structure is complex. The 



Fig. 3. 



BRABAZOrf 

 /?AHGE 



Fig. 3. Ideal cross-section of the foreland, showing the relations of 

 the different kinds of sediments. Vertical scale exaggerated. (The 

 cross-lined pattern denotes moraines.) 



glaciers have probably been subject to advance and retreat, as 

 all well-known glaciers are, and, if this is true, they have left 

 sheets of till between beds of gravel. The streams are bor- 

 dered by gravel wastes through which old stumps protrude, — 

 remnants of forests invaded by the rivers. These and other 

 facts show that the rivers are constantly changing their courses 

 so that river gravels are interlaminated with peaty layers made 

 in bogs and forests. It has been said that the larger rivers 

 are subject to marked fluctuations in volume. In time of 

 flood gravel is spread far out over the sands, and at low-water 

 stages such sheets of gravel are again covered by sandy beds. 

 As these changes recur frequently, the sediments must consist 

 of rapidly alternating coarse and fine beds. The coast line 

 itself must be sub ject to shifting ; for even if there have been 

 no changes of level the plain must have been built out into 

 the ocean. If, on the other hand, this is a region of tectonic 

 disturbance, as shown by Tarr and Martin,"- then we should 

 expect to find yellow dune sands, gray estuarine clays and 



* R. S. Tarr and Lawrence Martin. Recent changes of level in the Yakut at 

 Bay region, Alaska: Bull. Geol. Soc. Amer., xvii, 29-64, 1906. 



