MAIN MORAINIC SYSTEM IN THE GRAND RIVER LOBE. 447 



fall of 75 feet. As in Oil Creek Valley, tins change in rate of fall is prolD- 

 ably due to the relationship to the ice margin, the rapid fall being outside 

 of, and in the outer part of, the moraine, while the lesser rate occurs above 

 the moraine and in its inner portion. 



On Big Sandy Creek no examinations were made below Raymilton; 

 but just west of that village a gravel terrace standing about 50 feet above 

 the creek has its head in a series of drift knolls whose summits scarcely 

 rise above the terrace. For several miles above this point knolls and ridges 

 abound, but their height seldom exceeds 20 feet. 



In Little Shenango Valley strongly morainic topography is entered 

 near Clarks Station, the valley being nearly filled with knolls and ridges 

 for a distance of 3 or 4 miles. Their height ranges from 1 to 40 feet or 

 more, and their slopes are very abrupt. On the uplands between Big 

 Sandy and Slippery Rock creeks the general description previously given 

 will apply, there being in the outer portion of the morainic system a knob- 

 and-basin and in the inner a swell-and-sag tract. 



On the southeast side of Slippery Rock Creek, in northwestern Butler 

 County, is a tract of sharply ridged drift, which Lewis and Wright con- 

 sidered extramorainic, as they did the hummocky drift of some of the 

 valleys of Warren and Crawford counties.. The following description is 

 taken from Lewis's report:^ 



In front of the moraine at this point and extending for a mile or more 

 southeastward is a magnificent kame-hke accmiiulation of sandj^ stratified drift. A 

 remarkable series of hummocks and interlacing hills inclosing basin-like dejiressions 

 rises above the surface of the surrounding country at least 100 feet in height. The 

 sides of the sandy ridges are verjr steep and the whole kame, if such it be, is as fine 

 as any along the whole line of the moraine. * * * This kame-like accumulation 

 is of special interest, in that it lies in front of the moraine and that it is not in the 

 immediate valley of a modern creek. South of the kame another one of almost 

 equal interest starts from a portion of the moraine a mile farther west and forms a 

 steep and straight ridge 3 miles long. It runs along a valle}^ and in part along a 

 small stream in a direction south of east, and can be traced continuously from 

 Mechanicsburg to within one-half mile of West Libertj^ in Brady Township. * * * 

 It lies at the foot of the moraine and is a steep ridge of sandy stratified gravel, in 

 which are no large bowlders, and all the pebbles of which are waterworn. The 

 ridge is narrow and straight at first, but in Brady Township it seems to consist of 

 several reticulated ridges. It evidently represents an ancient water course and is 

 worthy of more extended study. 



^ Second Geol. Survey Pennsylvania, Eept. Z, p. 185. 



