186 PLEISTOCENE OF INDIANA AND MICHIGAN. 



of the basins and the slope of the plains indicate the source of the outwash, which in some places 

 appears to have been the product of but one of the lobes. For instance, a plain north of Grand 

 Rapids on the east side of Grand River has conspicuous basins on its eastern edge next to the 

 moraine of the Saginaw lobe, and it slopes from that moraine toward Grand River. The north- 

 eastern portion of a plain in northwestern Montcalm County and neighboring parts of Mecosta 

 and Newaygo counties rises toward the moraine of the Saginaw lobe, but a narrow southern 

 extension rises toward the west to a moraine of the Lake Michigan lobe. The plain in north- 

 eastern Allegan County and the large plain outside the Kalamazoo morainic system in southern 

 Barry and neighboring parts of Kalamazoo counties both seem to have been joint products of 

 the Saginaw and Lake Michigan lobes, for they slope away from a moraine of each lobe. 



On the whole, gravel plains are rather poorly developed in this interlobate tract. The 

 most conspicuous is one that covers perhaps 100 square miles in northwestern Montcalm and 

 neighboring portions of Mecosta and Newaygo counties. Another in eastern Allegan County 

 is of similar size if the entire outwash from the Lake Michigan lobe in the district north of Kala- 

 mazoo River is included. The gravel plains in the vicinity of Grand Rapids and northward 

 from there into northern Kent County are all very small, amounting altogether to scarcely 

 100 square miles. 



In the very massive part of the interlobate moraine from southern Mecosta County north- 

 ward to Cadillac, the only gravel plains are along the valleys of the principal streams, Muskegon 

 and Hersey rivers. The one on the Muskegon is largely a line of glacial drainage from the head- 

 waters of Muskegon River and is therefore to but slight extent referable to drainage attending 

 the development of the interlobate tract. The plain along Hersey River appears to be an 

 outwash from the Lake Michigan side of the interlobate moraine. This massive part of the 

 moraine is made up of strips of gravelly or sandy moraine alternating with strips of somewhat 

 clayey moraine. These gravelly and sandy strips are thought to correspond in a certain degree 

 to outwash aprons, but owing to the nearly complete coalescence of the ice lobes the gravel 

 and sand was prevented from being spread out in a plain and has a topography similar to the 

 remainder of the moraine. 



The relation and trend of the morainic ridges of the Saginaw and Lake Michigan lobes in 

 the interlobate district indicate the northward shift of the reentrant angle. The moraines 

 of the Saginaw lobe lead up to the line of junction from the southeast, and those of 

 the Lake Michigan lobe lead up to it from the southwest or south, and the outwash aprons or 

 gravel plains just mentioned occur at the junction. The reentrant appears to have worked 

 back northward to the southwestern part of Mecosta County while the massive moraine which 

 extends northward from there to Cadillac was yet in process of formation. From this massive 

 moraine the lobes probably shrunk away, the Saginaw to the east and the Lake Michigan to 

 the west, uncovering it simultaneously along nearly the entire length, instead of retreating 

 along it from south to north. 



STRUCTURE OF THE DRIFT. 



THICKNESS. 



The rock surface beneath this interlobate tract probably in no place greatly exceeds 700 

 feet above sea level and may in places drop to as low as 200 feet above the sea. The drift surface 

 ranges from 700 to 1,700 feet. The thickness of the drift may, therefore, in places exceed 

 1,000 feet and it probably averages more than 500 feet. Yet there is a small area in the 

 vicinity of Grand Rapids where the rock is near the surface and where all but one of the borings 

 that have reached rock are located. The one exception is the Red Cross mineral well near 

 Big Rapids, which penetrated about 450 feet of drift and struck rock at 525 feet above sea 

 level. A boring made by the Grand Rapids & Indiana Railway at Big Rapids reached a depth 

 of 245 feet (670 feet above sea level) without striking rock. A waterworks boring at Big Rapids 

 200 feet in depth terminated at an altitude 680 feet above sea level without striking rock. At 

 Reed City a boring at the waterworks and another at the electric-light plant, each about 275 

 feet in depth, terminated at 750 feet above sea level without reaching rock. A well only a 



