THE LAKE-BORDER MORAINIC SYSTEM. 39 I 



removed by the stream which formed the valley. The same stream also 

 passed through the gap south of Zeeland and perhaps has widened that gap. 



This ridge, like Covert Ridge, is composed of a clayey till strikingly 

 in contrast with the more porous till of the Valparaiso system. It is com- 

 monly spoken of as a "clay ridge," and it forms the southeast border of an 

 extensive sand-covered plain, with which it is in sharp contrast. 



The ridge carries only gentle swells 5 to 15 feet in height, but its relief 

 above the lower tracts on either side is sufficient to make it a prominent 

 feature. It rises about 30 feet above the general level of the sand-covered 

 districts which border it on the west and about an equal amount above a 

 narrow plain on the east which separates it from Covert Ridge. 



The extent of this ridge toward the north remains to be determined, as 

 no investigation has been made north of Grand River. An inspection of 

 railway profiles suggests that its course may be directly north, past Coopers- 

 ville into the southeast part of Muskegon County. The relation of this 

 ridge to certain sand deposits found on its eastern border is discussed on a 

 later page. 



RELIEF. 



The outer ridge and Covert Ridge each show slight relief above the 

 districts on their outer border. The greatest relief scarcely exceeds 50 

 feet and the usual relief is but 25 or 30 feet. The abruptness of the outer 

 slope, however, in each of the ridges renders this slight relief a somewhat 

 conspicuous feature, and the ridges may be seen distinctly for a distance of 

 several miles when viewed from their outer border. On the inner border 

 the descent is gradual from the crest of each ridge, usually amounting to 

 but 20 or 30 feet per mile. 



RANGE IN ALTITUDE. 



Each of the ridges show comparatively slight range in altitude. The 

 lowest parts are nearly 75 feet above Lake Michigan and the highest 

 scarcely 150 feet. Throughout much of its course each ridge stands about 

 100 feet above the lake. The crests of the ridges are usually but 40 or 50 

 feet above the level of the highest beach of Lake Chicago, and in places 

 they come down nearly to the level of the beach. The plains bordering the 

 ridges were quite extensively submerged by the lake or stood so little above 

 the lake level as to be poorly drained until the lake level had been lowered. 



