304 PLEISTOCENE OP INDIANA AND MICHIGAN. 



sandy plains. It is probable that these great depressions were occupied by tongues of ice 

 during the decadence of the ice sheet. (See p. 315.) 



In northeastern Manistee County the outer moraine is split into several members which 

 become successively lower from east to west and which with the intervening plains give a step- 

 like appearance, the morainic slope forming the riser and the gravel plain the tread. 



The broken ridges leading from Manistee to Muskegon County show great variations in 

 strength, their relief ranging from about 150 feet above the bordering plains down to swells 10 

 to 20 feet high. The most prominent are south of Ludington and in southern Manistee County. 



On the outer border the moraine which marks the limit of glacial readvance has (somewhat 

 surprisingly) a general relief of only 20 to 40 feet above the table-land outside. The crest of 

 the moraine is close to its outer edge and the knolls along it are only 15 to 30 feet high. Similar 

 small knolls are distributed over the surface of the great spurs on the inner slope. 



The second ridge on the Lake Michigan slope is also inconspicuous on its outer border, 

 in most places rising only 10 to 20 feet above the narrow gravel plain that lies between it and 

 the outer ridge. In this respect it contrasts strikingly with the inner border relief of the outer 

 ridge, which is 150 to 200 feet along much of its course from northwestern Otsego County to the 

 crossing of Manistee River east of Manistee. The gravel plain is like. a terrace built up along 

 the slope at a level 150 to 200 feet below the crest of the outer moraine. 



On the inner border the relief is generally great. The spurs and ridges stand 150 to 300 

 feet or more above the intervening depressions. 



STRUCTURE OF THE DRIFT. 



The surface portion of the drift, as exposed in ravines, road gradings, and other shallow 

 excavations, is largely till with a liberal admixture of stony material. Bowlders are rather 

 numerous both on the surface and in the upper part of the till. The wells and deep ravines 

 indicate that the stony till changes to sand at moderate depths along a considerable part of the 

 morainic system, and, in the inner border district, especially on the Lake Michigan slope, to a 

 fine silt or clay, which bears some resemblance to a lake deposit. (See p. 314.) The glacial 

 material referable to this readvance of the ice forms therefore but a thin coating over water-laid 

 deposits of great depth. 



The color of the till shows considerable variation, some of it being distinctly red and other 

 parts brown or yellow in the oxidized portion and blue in the unoxidized. The red color appears 

 to be due to the incorporation of material from red rock formations in the Lake Superior basin 

 and not to high oxidation in postglacial time. The red portion is about as calcareous as the 

 brown and yellow. Where the underlying rock formations are of limestone or blue shale, as 

 in the district between Little Traverse and Grand Traverse bays, there is less red drift than in 

 neighboring districts to the north and south, where these rock formations are so deeply buried 

 as to have made little or no contribution to the material in the till. It is not improbable, there- 

 fore, that local limestone and shale have served to give the brown and blue tills a distinctive color 

 and to disguise the ingredients derived from the red rock formations. 



On the Wisconsin side of Lake Michigan the correlative drift material is generally red. 

 It also contains a much larger amount of fine silt or clay than is present in the surface portion 

 of the drift on the east side of the lake or in ordinary bowlder clay. It seems to be largely 

 derived from lacustrine silts, which were gathered up and redeposited by the ice with the addi- 

 tion of a little stony material very irregularly distributed. 1 Chamberlin made reference to this 

 deposit in his work on the geology of Wisconsin and interpreted it to be the product of a lacus- 

 trine submergence that followed the retreat of the ice. Alden, however, has determined that it 



i Two mechanical analyses of samples collected by Alden near Fond du Lac, Wis., made by the Bureau of Soils, United States Department of 

 Agriculture, in 1910 show 41.3 and 51.5 per cent, respectively, of material below 0.005 millimeter. A lacustrine clay from Sauk County showed 

 46.9 per cent, and two samples of the buff-colored ordinary Wisconsin till near Oshkosh show only 8.2 and 7.6 per cent. It thus appears that the 

 red drift is largely made up of fine lacustrine material. 



