﻿14 NEW YORK STATE MUSEUM 



Close of the long period of erosion 



Eventually this long period of surface wear on a land area drew 

 to a close, and for a time the history of the region became of very 

 different nature, in other words instead of loss of surface material 

 it began to gain it in the shape of deposits on the old, worn land 

 surface. These deposits blanketed and preserved the old erosion 

 surface, and since the wear of today has come down to that precise 

 horizon over parts of the district, and the overlying deposits are 

 being peeled away from it, it is returning to daylight with precisely 

 the characters it possessed when it was buried and preserved ages 

 ago. Seldom does a district reveal so abundant and clear evidence 

 of the nature of an old fossil land surface. It is clear from its 

 study that long wear had reduced it to a surface of comparatively 

 slight relief, showing that no considerable elevation of the region 

 occurred during the latter portion of the long erosion interval. 

 Nevertheless it is very far from being a plane surface, but is of 

 considerable minor relief, of low ridges and shallow valleys, or of 

 low knobs and basins, the depressions eaten out on the weaker rocks, 

 chiefly the Grenville limestones and some of the schists, while the 

 more elevated ridges and knobs are due to the resisting qualities of 

 the Grenville quartzites and of many of the igneous rocks. The 

 knob structure is practically confined to the igneous rock areas, 

 chiefly in the Laurentian gneiss. 



While the region therefore is quite rugged in a mild fashion, the 

 extreme differences in altitude are but slight. One hundred feet is 

 about the measure of difference. Seldom does the difference in level 

 between valley bottom and ridge crest reach that figure, and rarely 

 does it exceed it. This is a small difference, considering the wide 

 variation in resisting power to wear which the various rocks present 

 and is indicative of a long period of wear under comparatively stable 

 conditions of level. 



Paleozoic sediments 

 Potsdam sandstone. A change in conditions followed and de- 

 position of sand commenced upon this old land surface. It natur- 

 ally began on the valley bottoms and encroached on the ridges only 

 as the valleys filled. The old limestone surfaces were pitted by 

 small depressions, and were somewhat intersected with widened 

 joint cracks also, and in these the first materials collected, some- 

 times full of coarse fragments of resistant thin quartzite bands or 



