POET HURON MORAINIC SYSTEM AND PBOBABLE CORRELATIVES. 297 



the shore in Bay County the thickness is about 100 feet. In Sanilac County the thickness along 

 the moraine to the vicinity of Carsonville is about 150 feet and along the shore 20 to 50 feet. 

 From the vicinity of Port Sanilac to Port Huron and St. Clair the thickness is 200 feet or more. 



COMPOSITION. 



In Iosco and eastern Ogemaw counties the main moraine of the Port Huron system is 

 bowldery and contains notable amounts of gravel and sand. The land-laid parts on the 

 "thumb" are also bowldery. But aside from these bowldery parts and the surficial sands and 

 gravels of the beaches, the drift of this whole district is notable for the large predominance 

 of clay in its composition. 



At Tawas, Lexington, and other places near the lake shore, there are flowing wells which 

 seem to issue from porous beds beneath the till sheet of the moraine. The beds are inclined 

 upward inland and have their intake areas at higher levels some miles back from the shore, 

 and the water percolates down the slope under the inclined surficial till sheet. 



TILL PLAINS. 



In eastern Ogemaw and western Iosco counties the area east of the front ridge of the 

 main moraine of the Port Huron system is a till plain of considerable extent. It carries only 

 low, undulating ridges but is characterized by a great many basins, many of them deep and a 

 considerable number containing lakes. The lands surrounding the area carry mainly sand and 

 gravel of low fertility, but the area itself has a fertile clay soil that promises excellent results 

 under cultivation. 



A small till plain above the level of the lake waters lies along the south side of the older 

 moraine fragment in northwestern Arenac County. Small till plains are included between the 

 outer and inner ridges of the moraine between Cass City and Gagetown and southwest of Bad 

 Axe. A narrow till plain lies between the Warren beach and the inner slope of the moraine in 

 southeastern Huron County and extends for about 10 mdes southward into Sanilac County. 



The lake bottom below the level of the highest beach commonly has more or less of the 

 character of a till plain, but wave work along the lines of the several beaches has modified 

 the surface considerably both by erosion and deposition. Considerable parts of the drift have 

 been removed, leaving bowlders on the eroded surface, and sands and gravels have been de- 

 posited in extensive beach ridges and sand belts. The finer sediments were carried away and 

 deposited in deep water. Where the basins were deepest, immediately in front of the ice, 

 lake clays of considerable depth were deposited. A large area in the low part of the Saginaw 

 Valley outside of the main moraine of the Port Huron system contains 50 to 100 feet of lake 

 clay without stones or sand. A thickness of 90 feet was penetrated at St. Charles in a coal 

 shaft; and 150 feet or more is reported to he in the Lake St. Clair basin south of the moraine. 1 



OUTWASH. 



While the ice front was resting on the front ridge of the Port Huron system in Ogemaw and 

 Iosco counties, a large river followed the course of the Au Sable to the bend in northwestern 

 Iosco County and thence went west and south along the front of the moraine into southern 

 Ogemaw and northern Gladwin counties. 



The descent of this stream was rapid, and although it probably obtained a considerable 

 amount of sandy outwash directly from the ice front, it must also have gathered much from 

 the erosion of its bed, for almost its whole course was through older sandy outwash deposits. 

 It carried enormous quantities of sand into southern Ogemaw and northern Gladwin counties 

 and spread them in a relatively thin sheet over the flat areas of that region, covering up what 

 would otherwise doubtless have remained a fertile till plain suitable for cultivation, and covering 

 also in all probability the Arkona beaches northward from a point 7 miles northeast of Gladwin 



1 Cole, L. J., Delta of the St. Clair River: Michigan Geol. Survey, vol. 9, pt. 1, 1903, p. 14. 



