GRUNDT COUNTY. 191 



Post-tertiary Formations. 



Alluvium. The beds of this formation are very largely developed in the ter- 

 races of the river valley and the beds of the smaller streams. From the west 

 line of the county nearly to AuSable creek, the Illinois and Michigan canal fol- 

 lows the north bank of the present river valley pretty closely, while the second 

 terrace varies from a half mile to two miles to the northward. On the south 

 side of the river, the high, gravelly banks of the second terrace hug the river 

 banks very closely, as far at the Waupecan creek. Here the^ lose much of 

 their elevation, and have as their continuation a low ridge, about a mile distant 

 from the present bank. East of Mazon creek this declines still more, and be- 

 comes the heavy sand ridge which bears still farther southward, and then east- 

 ward, south of Wilmington, into Kankakee county. This sand ridge forms the 

 watershed between Mazon creek and Kankakee river, so that, where it strikes 

 the bank of the latter stream, to the southward of Wilmington, the water flows 

 from within two hundred yards of the river, through swamps and sloughs, and 

 finds its way, through the Mazon, into the Illinois, opposite Morris. 



The flats of the old river valley, back of the present banks, show, in many 

 places, plain evidence of the comparatively recent date of their formation. At 

 Gen. Birney's place, on section 11, town 33 north, range 6 east, my attention 

 was called to the fact, that at a short distance beneath the surface, at a pretty 

 uniform depth through that neighborhood, there is a layer of thin slabs of the 

 fissile sandstones of the Coal Measures, still tolerably solid. They were evi- 

 dently distributed here by the current of the river, not long before it became 

 so contracted as to leave this level dry. When this old channel was the outlet 

 of Lake Michigan, a large body of water must have flowed through here, and 

 appearances seem to indicate that its diversion toward Niagara must have been 

 sudden, rather than gradual; otherwise, the present valley would probably have 

 been wider, and the descent to it less abrupt. 



A topographer would take peculiar pleasure in studying the various islands 

 of the old valley, especially at the confluences with the Illinois of AuSable and 

 Nettle creeks, both of which streams, apparently, were much larger than at 

 present. Upon one of these islands stands Morris, the county seat. 



Another, and far the largest in the county, is the high land lying between 

 the head of the Illinois, the lower part of the Kankakee, and the slough which 

 contains Goose Lake, and runs thence to Pine Bluff, near the embouchure of 

 the Mazon, upon the Illinois valley. 



The following levels of points within this county have been furnished to me, 

 mostly from the notes of the Illinois .River Survey, from the office of its chief, 

 Gen. J. H. Wilson, U. S. A., now in charge of the river improvements at 



