78 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



the border of the southern portion of the Appalachian Mountain system. 

 The slowness of the breaking down of the hilly country, compared with that 

 of the bordering- plain country, as above noted, seems due to the more 

 resistant nature of its rocks. Had its strata been as soluble and friable as 

 those of the plain country its surface would probably have been reduced 

 to a level correspondingly low. Had there been greater uniformity in tex- 

 ture these formations, like the limestone formations, might present a near 

 approach to a jjlain. But their variability has caused an uneven degradation 

 which has resulted in the development of a hilly country. The uplands, 

 the slopes, and the valley bottoms have each developed types of topogi-aphy 

 consistent with the varying resistance of the rock strata which underlie or 

 border them. 



The amount of drift is so great on the border of this hilly country as 

 to obscure to a considerable extent the topog'raphy of the rock surface. 

 But toward the glacial boundary the concealment becomes less, and oppor- 

 tunit)' is afforded for comparing the rock contours of the glaciated with 

 those of the unglaciated tracts. This comparison has shown the modifica- 

 tion by glacial erosion to be surprisingly small in the vicinity of the 

 glacial boundary. It will be very difficult to determine the extent of gla- 

 ciation by a study of topographic maps. Take, for example, the Olean 

 quadrangle, New York (PL IV), which embraces both glaciated and ungla- 

 ciated tracts. The changes produced by glacial erosion are so few and 

 so slight that neither a map nor a study on the ground makes the extent 

 of glaciation clear. It is determined only by a careful search for glacial 

 deposits. 



The hills occasionally present abrupt slopes, or even mural faces, where 

 the hard ledges outcrop, but usuall}" they rise with gentle grade and give 

 generally the impression of a moderately rolling country. A considerable 

 part of the region may be easily brought under cultivation, and it is already 

 largely an agricultural district. 



The breadth of the dividing ridges is subject to much variation on 

 account of diff'erences in resistance of the rock strata. If hard strata lie 

 near the surface a broader divide may be expected than where the rocks 

 are soft. It is not necessary, however, that the uppermost rock should be 

 liard. Often a hard ledge extending from the brow of the bluff back beneath 

 the divide serves as a table to support softer strata which cover it. Similarly, 



