152 H. L. FAIRCHILD PLEISTOCENE GEOLOGY OF XEAV YORK STATE 



As the ice advanced and closed the outlets the waters were lifted to 

 higher levels and pushed southward. The last lakes of the ice-advance 

 being in the heads of the valleys were the smallest, the highest, the most 

 detached and most southerly. The lakes of ice-front recession had pre- 

 cisely the opposite history. 



ErosionaJ work. — The lake features that are preserved for our study 

 may be discriminated as erosional and constructional. The erosion phe- 

 nomena are the wave-cut cliffs. The glacial lakes were commonly too 

 ephemeral or too unsteady in their levels to produce conspicuous erosion 

 features. However, the larger and longer-lived lakes, as Xewberry, War- 

 ren. Dana, and specially Iroquois, have left many cliffs. 



Constructional work — Beach ridges. — Embankments of sand and 

 gravel, the bars and spits of wave and shore current construction are the 

 complement of the erosion work, but are much more common and are 

 frequently very prominent features. They have long been recognized by 

 the people as the work of mysterious waters at high altitudes. For long 

 stretches the beach ridges have been utilized for "ridge roads.** while the 

 level stretches of wave-base along the beaches have afforded graded paths 

 for railroads and canals. The strongest ridges are those of Whitt. 

 and Warren in the Erie Basin and of Iroquois in the Ontario Basin. 



Deltas. — Of the several shore phenomena deltas are the most useful in 

 proving the former presence and determining the altitudes of the extinct 

 lakes. The production and size of the delta deposits are not wholly con- 

 ditioned by the size of the receiving water body, but by the volume of the 

 stream detritus relative to the distributing work of the receiving waters. 

 Hence deltas may be built in small lakes, and these hung-up mounds and 

 terraces of gravel on the valley sides serve well to mark the shores of 

 lakes that were too ephemeral or too small to produce either cliffs or 

 bars. Xaturally the deltas occur in the courses of land streams, and a 

 vertical succession of bisected delta terraces commonly indicate the fall- 

 ing levels of the lake. Fine examples of these gravel terraces are found 

 on the slopes of the Finger Lake valleys, and some of them are con- 

 ;ous features, like the terraces by Coy Glen, visible from the Cornell 

 University campus. 



Delta p;ains. — Genetically related to deltas are the plains of gravel, 

 sand, or clay, which may be extended in area and indefinite in limits. 

 Such plains usually represent wave-base, perhaps 20 feet or less beneath 

 the water surface. When partially eroded the remnants present extended 

 horizontal lines, excellent examples of which may be seen throughout the 

 Mohawk Tallev and about the Irondequoit Valley east of Roche- 

 clearly visible from the trains on the Xew York Central Railroad. Some 





