TOPOGRAPHY. 79 



the hills and knobs may have soft beds at the top which are supported by 

 hard ones that outcrop along- the side. Near the north end of the hilly 

 country in western New York the hard sandstones of the Portage group 

 have withstood denuding action to a remarkable degree, as shown in the 

 following description by Hall : ^ 



These often extend northward on the elevated grounds between the deep north- 

 and-south valleys, presenting a gentle northern slope to the shales of the Hamilton 

 group, while on the sides of the same hills the slope is abrupt, and the surface bemg 

 but little covered with northern drift, the valleys are bounded on either side bj' 

 steep hills. 



This character is well illustrated along the southern part of the Genesee Vallev 

 toward Dansville, and in the valleys of Allen Creek, the Tonawanda, and the different 

 branches of the Seneca and Ca3'uga creeks. The valleys just spoken of, in their 

 course through the Hamilton group, present gentlj' sloping sides, and the country 

 rarely rises above the level of the valley bottom or bed of the stream. On approach- 

 ing the northern margin of the Portage group the observer finds gradually increas- 

 ing elevation of the hills on either side and an abruptness of their slope, and in a 

 short time he finds himself in a deep vallej', bounded on either side by hills rising 

 400 or 500 feet, and in some instances even 800 feet above the level of the stream. 



- The upland surface presents numerous low passes which cross the 

 divides and have cols or saddles in some cases standing several hundred 

 feet below the highest points on the bordering uplands. The low altitude 

 of the passes strongly supports the interpretation that the hill}- region would 

 have been reduced to a level in harmony with that of the liordering plain 

 country if the strata had been correspondingly weak. Several of these 

 passes are represented on the map of the Olean quadrangle (PI. IV). The}' 

 may be seen in all parts of the hilly country both outside and inside the 

 glacial boundary. They are therefore not dependent upon glacial erosion, 

 though some within the glacial boundary (including those of the Olean 

 q-uadraugle) may to a slight degree be modified by glacial action. 



There are within the glaciated region several low tracts leading across 

 the continental divide in western New York, northwestern Pennsylvania, 

 and northeastern Ohio. A few occur also in the district north of Cattaraugus 

 Creek, in western New York, and they are not rare between French Creek 

 and the Allegheny in western Pennsylvania, as well as in the districts 

 farther south and west. They may in some cases be old lines of drainage, 

 but in most cases they probably pass over old water partings. The great 

 majority are so heavily filled with drift, the filling not infrequently being 



'New York Geol. Survey, Fourth Geol. District, 1843, p. 225. 



