358 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



20 to 40 feet high occur out to distances of a mile or more in the gravel 

 plain. They appear to have been built up at the time the gravel plain was 

 occupied by glacial waters, having been no evidence found that they were 

 cut into by glacial streams. However, the materials comprising the knolls 

 are such as are subject to landslides, creeping, and rapid erosion, and it has 

 been so long since the gravel plains were occupied by streams that erosion 

 marks may have been obliterated. . 



The several members of the morainic system are sufficiently distinct 

 around the southern end of the loop to make it advisable to discuss them 

 separately. The portion of the outer member lying south of Mad River, 

 along the east bluff of the Great Miami, has greater strength than is 

 displayed elsewhere by this member. There is in places a well-defined 

 ridging in a north-northeast to south-southwest direction, i. e., in line with 

 the trend of the moraine, the ridges succeeding each other at intervals 

 of one-half mile or more and standing 15 to 30 feet above the intervening 

 tracts. One ridge was noted, however, in which the trend was nearly east 

 to west. It lies 2^ miles north of Centerville, is about a mile in length 

 and one-fourth mile in width. Its highest points reach an altitude of 50 

 to 60 feet above the bordering district. From this ridge northward to Mad 

 River Valley the moraine has sharper knolls than it has toward the south, 

 there being many whose height is 40 to 50 feet and whose slopes are but 

 20 to 30 rods in length. The outer border of the moraine in this portion 

 of the belt is somewhat irregular, and patchy developments of morainic 

 topography occur for a mile or more east of the main ridges. 



In the Great Miami Valley this moraine is very feebly develo^Ded, 

 but in the vicinity of Carlisle low ridges occur, among which are basins 

 .and irregular depressions. 



On the uplands between the Great Miami and Sevenmile Creek the 

 moraine consists of a broad basement ridge standing 20 to 40 feet above the 

 outer border tract and having a width of 1^ to 3 miles. On its crest and 

 slopes are minor ridges and knolls 10 or 15 feet in height, among which sags 

 and irregular dejDressiims are inclosed. 



In Sevenmile Creek Valley the moraine is only feebly developed, but 

 where the moraine crosses the valley a decided change in structure occurs, 

 there being a well-defined gravel plain south of the moraine, while north of it 

 there is scarcely any gravel, the valley of the creek being cut in till depos- 



