198 PLEISTOCENE OF INDIANA AND MICHIGAN. 



Gravel plains, which are extensive from Commerce Township northeastward to Oxford, are 

 crossed or interrupted by two morainic ridges. One ridge, shown on the Pontiac topographic 

 sheet, leads northwestward from Orchard Lake to White Lake and separates Commerce Plain 

 from Drayton Plain. Another ridge runs east from Clarkson village and separates Drayton 

 Plain from Oxford Plain. These ridges connect at either end with prominent morainic belts 

 trending with the gravel plain, the moraine at the north being the Saginaw and that at the 

 south the Huron-Erie. East of Drayton Plain, in the district immediately north and west of 

 Pontiac, a till plain covers about 15 square miles. This is the only conspicuous till plain along 

 the junction of the two lobes, except that in Jackson County, already noted. 



From the slopes of the different portions of the gravel plain some inferences have been 

 drawn as to the ice lobes that contributed the principal outwash. The Commerce Plain seems to 

 have been formed by the Huron-Erie lobe, for, as shown on the Pontiac topographic sheet, it 

 has a northwestward slope from the Huron-Erie moraine to Huron River, which follows its 

 northwest edge. Drayton Plain, on the other hand, seems to have been built up largely by out- 

 wash from the Saginaw lobe, for it has a southeastward slope from the moraine of that lobe to 

 Clinton River, which follows its southeast border. Oxford Plain seems to have been filled by 

 outwash from the east and the west aud the head of the reentrant on the north. 



As the ice shrunk back from the north end of Oxford Plain into southern Lapeer County 

 outwash from along its edge filled the low places among the morainic knolls to altitudes nearly 

 100 feet above Oxford Plain, without, however, covering the principal knolls. 



Several prominent kames lie along the line of the moraines on each side of the gravel plain 

 from Milford up to Oxford Township. The most prominent are fully 200 feet in height. Most 

 of them rise above the 1,100-foot and several above the 1,200-foot contour, and one about 6 

 miles northeast of Oxford appears by aneroid to reach an altitude of 1,300 feet. Oxford Plain 

 is about 1,060 to 1,075 feet at its northern edge, Drayton Plain about 1,000 feet at its northern 

 edge, and Commerce Plain about 940 feet at its eastern edge. The most prominent knoll between 

 Commerce and Drayton plains rises to 1,111 feet. The Pontiac and Rochester topographic 

 sheets indicate that a knoll about 2 miles west of Clarkston reaches 1,201 feet; Pine Knob, 1\ 

 miles east of Clarkston, 1,221 feet; Mount Judah, 6 miles north of Pontiac, about 1,180 feet; 

 and Bald Mountain, 8 miles northeast of Pontiac, 1,195 feet. 



TOPOGRAPHY. 



The present relief of the divide above the lowlands on either side is only in small part due to 

 drift, for the rock surface of the divide stands fully as high above the rock surface of the low- 

 lands as the present drift surface stands above the lowland drift surface. The drift deposits are 

 generally as thick in the lowlands as on the divide; indeed, in small areas along or near the 

 divide in southwestern Jackson and neighboring parts of Calhoun, Hillsdale, and Branch coun- 

 ties, the drift deposits are insufficient to conceal the rock hills, despite the fact that the hills have 

 but slight relief above the thick drift filling that covers the lower land. In some of these lulls the 

 rock attains an altitude of over 1,100 feet above sea level, higher than in any other part of the 

 southern peninsula, but along much of the divide it is only 800 to 900 feet on the hills and ridges 

 and much lower in the drift-filled interspaces. The altitude of the drift surface is highest in two 

 somewhat widely separated localities, one in Hillsdale County where the rock is exceptionally 

 high, and the other in northern Oakland County where the drift aggregation is very great. In 

 both places the maximum altitude is about 1,300 feet and in numerous knolls it is 1,200 feet or 

 more. The drift surface along this divide ordinarily is not far from 1,000 feet above sea level, 

 or about 400 feet above the surface of the neighboring Great Lakes. 



