MAIN MORAINIC SYSTEM IN THE GRAND RIVER LOBE. 441 



Altitudes along outer moraine — Continued. 



TOPOGRAPHY. 



In Warren and Crawford counties, Pa., where the morainic system is 

 narrowest, there is a hummocky topography in its entire width of 3 to 7 

 miles. On the highlands the knolls are lower than in the valleys, but in 

 both situations they are sharp and inclose basins whose slopes are so abrupt 

 that they are often difficult to cultivate. The knolls in the A^alleys range 

 from 15 to 100 feet in height, while those on the uplands are but 10 to 25 

 feet. The individual knolls have areas ranging from a fraction of an acre 

 up to 10 acres or more. The valley slopes are dotted with drift knolls whose 

 contours are fresh and altered scarcely more by erosion than are those of 

 the upland knolls. The heavy forests which cover this region have served 

 to check erosion to a g'reat degree, as may be seen by a comparison of fields 

 which have been cleared and cultivated for a half century with those on 

 which the forest still stands. Cultivation of the soil, together with free 



