328 F. Carney — Wave-cut Terraces in Keuka Valley. 



valleys trending with the direction of the moving ice. Hence 

 in a series of terraces along a valley wall, the lowest one would 

 be the most modified by glacier ice. 



The beach structures of these former lakes have suffered 

 further from wave work of more recent water bodies, espec- 

 ially of the high-level lakes. The degree of effacement through 

 this agency depends upon the coincidence of the surface-planes 

 of the two bodies of water, or upon their approximation 

 to coincidence ; if these planes intersected at a very slight 

 angle, the vertical range of beach agents would at least par- 

 tially overlap for a considerable horizontal distance ; if the 

 planes were actually coincident, then the extent of the deface- 

 ment would depend largely upon the relative duration of the 

 two bodies of water. 



Probably the most effective agency in the obliteration of 

 these shore structures is the deposit of drift made by an 

 ice sheet. Within the belts of thickened drift the burial must 

 be quite complete, the chances of survival being greater with 

 the higher beaches. But at all levels the mantle of ground 

 moraine would in any event partially cover the weaker expres- 

 sions of wave and current work. And even the pronounced 

 cliffs and terraces might be covered in places. 



Furthermore, normal subaerial weathering has tended to 

 render less obvious such remnants of these old beaches as have 

 survived the factors above described ; the least changed would 

 be the forms cut in the more resistent rocks. 



Forms which Simulate Wave-cut Terraces. 



1. Variation in the texture of rocks is manifest in differen- 

 tial weathering;* sharp slopes simulating cliffs maybe thus 

 produced. The resemblance, however, leads to confusion only 

 when the plane of the lake surface coincides with, or is par- 

 allel to and vertically within a few feet of the hard layer or 

 horizon of rock which marks the bench ; such a ledge, in the 

 absence of a terrace or other evidence of a beach, cannot be . 

 defined finally as a wave-cut cliff. The attitude of a bench 

 resulting from weathering, in reference to the horizon, depends 

 upon the dip and strike of the hard layers ; because of this 

 fact, it is not difficult to distinguish the wave-cut cliff, except 

 when the bench is discontinuous, showing only in short seg- 

 ments, a condition not unusual in the coarse sandstone hori- 

 zons because of the horizontal variations in texture. 



2. Streams held against a slope, or against a rock salient, by 

 ice, often form a bench somewhat simulating a wave-cut ter- 

 race and cliff, f Such benches have been investigated by Fair- 



*T. L. Watson, N. Y. State Mus., 51st Ann. Rep., vol. i, p. r76, 1897. 

 fG. K, Gilbert, Bulletin Geol. Soc. Am., vol. viii, p. 285, 1897. 



