﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 29 



northward, we are in doubt as to the correctness of this view. 

 Certain it is, however, that the present course of Indian river is 

 a patchwork of various preglacial valleys, the modern character 

 of the course being most excellently shown at Theresa where 

 the river drops 80 feet, from a shallow valley into a much deeper 

 one, entering this on its east side 3 miles below its valley head, 

 with cutting of a short, postglacial gorge in the old valley 

 side. 



Plateaus, terraces, scarps 



With the streams cutting down valleys and exposing rock 

 formations of varying age and resistance in their valley walls, 

 and with the slow widening of the valleys, the stronger rock 

 beds of the region tend to outcrop in cliff form, the scarps run- 

 ning across country in the direction of strike, and curving up 

 the consequent valleys in the direction of dip. The stronger 

 cliffs result where a more resistant rock overlies a considerably 

 less resistant one, the more rapid wear of the underlying rock 

 tending to keep a tolerably steep and precipitous cliff front. 

 Where the differences in resistance are less, or where rapid 

 changes in resistance occur, involving no great thickness of 

 rock, low, subdued scarps are produced. 



Furthermore, where an overlying formation is weaker than that 

 beneath, rapid wear is checked at the upper surface of the lower 

 rock, the upper rock is stripped away from it and a flat bench of 

 varying breadth is produced, separating the cliff fronts of the upper 

 and lower formations. In the large way, ignoring minor complica- 

 ting factors, the general topography of our district is of this type : 

 flat platforms developed on the surfaces of the hard layers, and 

 cliff fronts which mark the descent from one rock platform to the 

 next, the cliff fronts facing toward the old land area, in this case to 

 the north, hence often called infaces. 



The most prominent cliffs, and the broadest platforms of the dis- 

 trict are those of the Potsdam sandstone, as it usually has consider- 

 able thickness, is the strongest or most resistant of the Paleozoic 

 rocks, and more enduring than much of the Precambric, on which it 

 rests. The Precambric topography has already been described, and 

 this does not need repetition. The Potsdam is thickest where the 

 underlying Precambric is weakest, the bulk of the remaining Pots- 

 dam rests on these weaker rocks, this being notably true in the case 

 of the outliers. Potsdam cliffs from 20 to 60 feet high are abund- 

 ant throughout the district, and are absent only where the under- 

 lying rock is granite and the Potsdam very thin. Broad Potsdam 



