296 - PLEISTOCENE OF INDIANA AND MICHIGAN. 



feet a mile or two north of Black River. The relief on the outer border continues about the 

 same, except that Black River cuts deeper as it flows south. 



From Wadhams to St. Clair the water-laid moraine declines from about 680 to about 625 

 feet; its relief on the outer slope is only 5 or 10 feet near Black River and 20 or 25 feet near 

 St. Clair; on the inner slope its relief is slightly greater. 



CHARACTER. 



The front ridge of the main moraine of the Port Huron system in Ogemaw and Iosco counties 

 is strong and has mainly swell and sag topography with scattered basins. The fainter branches 

 east of the front ridge have comparatively low knolls and are not conspicuous as ridges; among 

 them basins are numerous and more prominent than knolls. 



Maple Ridge, in southern Iosco and northern Arenac counties, is a strong moraine with 

 mostly swell and sag topography. It runs southwest as a high and prominent feature and ter- 

 minates abruptly at the edge of the 200-foot deep ravine of Rifle River. From its end a command- 

 ing view is obtained over the lower sandy plain to the north and west. 



Southwest from Rifle River Maple Ridge is not so strongly and evenly developed, but has 

 a swell and sag topography. Where it turns south near the county line it becomes broken and 

 discontinuous. The fainter ridge which runs southwest west of Sterling has mainly a swell 

 and sag topography. On entering Bay County the main ridge at first has a moderately undu- 

 lating topography, but as it begins to be water-laid it becomes smooth and broad and without 

 conspicuous surface features. Toward Saginaw it becomes so broad and flat that it ceases to be 

 a visible ridge; shortly, however, it resumes the form of a broad, smooth ridge, and a little north 

 of Vassar it becomes a rugged land-laid moraine with a mainly swell and sag topography of 

 rather pronounced type, carrying some basins and a number of high, rather sharp knoUs. 

 Toward Gagetown the inner ridge is broken and irregular with mainly a swell and sag topography. 

 Near Cass City the front ridge is still more broken, and much of the area between Cass City and 

 Gagetown is a flat till plain bearing only scattered low knolls. 



In Huron County west of the interlobate angle the front ridge is of the sweU and sag type ; 

 and the area between this and the Gagetown ridge, which is much weaker, is largely a plain with 

 only scattered low knolls. 



The interlobate area east of Bad Axe is mainly composed of rather high knolls and swampy 

 troughs. There are many small basins, but the knolls are much more prominent. 



Nearly all of the moraine from the interlobate angle to Black River, 7 miles west of Port 

 Huron, has a moderate swell and sag topography with comparatively few basins. This part 

 of the moraine, however, is composed of three or four parallel subordinate ridges whose crests 

 are generally quite clearly defined and have long longitudinal sags between them. Thus, though 

 the bulky ridge is compound, its separate elements are related in the simplest possible way. 

 There are very few basins in this part. The surface forms are excellent types of swell and sag 

 topography. 



South of Wadhams, on Black River, the water-laid continuation of the moraine is a broad, 

 low ridge largely covered with sand. 



STRUCTURE OF THE DRIFT. 



THICKNESS. 



In eastern Ogemaw and western Iosco counties the drift has a thickness of 100 to 200 feet. 

 Near Tawas and southwestward to northern Bay County the drift within 5 or 10 miles of the 

 shore is thin, varying from nothing to 50 or 75 feet. In western Arenac the thickness is about 

 200 feet, and along the course of the main ridge in Gladwin, Midland, and Saginaw counties it 

 is generally 200 feet. From Saginaw eastward, however, its depth is about 100 feet to the 

 north edge of Tuscola County, except along the high crest of the moraine northeast of Vassar, 

 where it is nearly 200 feet. In Huron County outside of the interlobate area the drift is in few 

 places more than 50 feet thick and at many places near the shore is very thin or absent. Near 



