of the Hudson River Valley. 463 



formation suggests that two distinct conditions prevailed dur- 

 ing the time of its deposition. The clay represents a period of 

 still water deposition when little or no siliceous material was 

 washed into the basin, while the overlying stratified sand was 

 evidently deposited at a time when much siliceous matter was 

 carried in by the tributaries. 



The causes of this differentiation are not clearly manifest. 

 If there were good evidence to show that, at the close of the 

 ice-period, there had been a greater submergence of the conti- 

 nental margin than that proven by the delta deposits above 

 mentioned, the hypothesis might be advanced that during this 

 greater subsidence there was but little of the land surface ex- 

 posed above sea-level in the vicinity of this estuary and conse- 

 quently but little surface drainage. The larger tributaries fed 

 by waters from the melting glacier would then bear into the 

 estuary a large quantity of rock flour which would be held in 

 suspension for a time and would finally be deposited in the 

 deeper water as clay. As the land rose from its submergence, 

 however, a larger area would be exposed to surface drainage 

 and would yield in immediate proximity to the basin, an 

 increasing amount of siliceous matter which would be deposited 

 over the clay and constitute the upper member of the estuary 

 formation. 



It remains for future investigation to determine the total 

 amount of submergence in this region cotemporary with the 

 last advance of the ice sheet and subsequent to its retreat. 



The records of ocean wave action are in many cases different 

 from those of the extinct Quaternary lakes and not so easy to 

 recognize. It is not always possible to decide a question of 

 submergence by the presence or absence of a distinct shore- 

 line. On a lake shore wave action tends to cut in an horizontal 

 plane and the result is a series of terraces or a beach plane 

 associated with shore drift and littoral deposits in various 

 phases. When ocean waves act upon a shore there may be two 

 cases : 



1. The land may be at rest. In this case the result will be 

 the same as on the shore of a lake which maintains its level for 

 a comparatively long time. 



2. The land may be rising or subsiding with respect to sea- 

 level. In this case the plane of erosion will be a resultant of 

 two planar forces : a, the wave force which operates in an 

 horizontal plane ; &, the force of elevation or depression which 

 acts in a vertical plane and subjects to the former successively 

 lower or higher portions of the land margin. According to 

 the relation of these forces or the relation of the rate of land 

 movement to the rate of wave cutting the plane of erosion will 

 vary in its inclination. As the cutting rate relatively increases 



