PHYSICAL FEATURES OF THE REGION. 13 



of drift (see PI. V). The first two profiles show a gradual slope from the 

 west side to points beyond the middle of the lake, followed by a somewhat 

 abrupt rise to the east shore. The amount of drift on the east shore is 

 somewhat greater than on the west, the rock floor at St. Joseph being but 

 456 feet above tide, or 124 feet below the lake, while on the west shore it 

 rises in many places 20 or 30 feet above the lake level and generally is not 

 far below it. The next profile leading eastward from Racine passes across 

 the deepest part of the basin in the southern end of Lake Michigan, and 

 shows no essential difference on opposite sides of the lake in the slope of 

 the lake bottom. The slopes and bottom are very smooth compared with 

 those shown in profiles farther north. The profile leading from Milwaukee 

 to Grand Haven shows a much shallower part of the basin than that east 

 from Racine, the altitude being nowhere below 200 feet above tide, while 

 opposite Racine it reaches sea level. This line between Milwaukee and 

 Grand Haven seems to mark nearly the summit of a ridge between two 

 basins, both of which, together with the dividing ridge, are covered by the 

 lake. In the profile a few miles to the north, leading from Port Washington, 

 Wisconsin, to Muskegon, Michigan, the lake bottom is shown to be singu- 

 larly irregular. The appearance presented is that of a series of escarp- 

 ments facing westward, similar to the escarpment of Lockport (commonly 

 known as Niagara) limestone a short distance west from Lake Michigan, 

 but it is not entirely certain that they are rock escarpments. Were the 

 drift to be removed from the eastern border of the basin the profile would 

 be quite different. A well at Muskegon, reported by the Michigan Geo- 

 logical Survey, has 235 feet of drift, which brings the rock floor down to 

 an altitude but 360 feet above tide, or only a few feet above the crests of 

 the ridges in the midst of the lake. The dotted line at the right of the 

 profile indicates the depth of the drift at that point. The profiles north 

 from this line show irregularities of lake bottom which give it the appear- 

 ance of being channeled longitudinally. Whether these irregularities are 

 due to drift accumulations or to rock ridges is not manifest from an inspec- 

 tion of the lake charts. The remarkable thickness of drift at Manistee (640 

 feet) is worthy of note, and indicates that the abrupt border of the lake 

 there is of drift. 



Evidence that the present smooth bottom of the Lake Michigan basin 

 in its southern end is due to the planeness of the drift surface instead of the 



