192 GEOLOGY OF ILLINOIS. 



Keokuk, Iowa. The figures indicate distances below the established "datum 

 of six feet below the lowest registered water of Lake Michigan :" 



FEET. 



Bluffs at Morris, north side (level of town) 55.938 



" south side 59.48 



" " lower terrace 78.00 



Level of river, at head of the Illinois 87.809 



" mouth of AuSable creek 92.664 



" Morris, under road bridge 95.13 



" Marseilles, LaSalle county, above dam 99.808 



" " " below dam 103.256 



" Goose Lake, about 60 



" Minooka, as per railroad survey, above datum 35 



These levels show that the elevation of the first terrace above the river, op- 

 posite Morris, is a little over seventeen feet, and that the elevation of the sec- 

 ond bluff or gravel ridge above the first terrace, is about eighteen and a half 

 feet. The present floods reach nearly up to the first terrace, and it is probable 

 that when the lake poured its waters through here, even the much wider val- 

 ley of the old river did not so accommodate the floods as to prevent their 

 nearly or quite overflowing the gravel ridge, and covering large portions of 

 the upper terrace, both north and south of the river. 



The coarser portion of the beds of river gravel consists mostly of fragments 

 of the Niagara group limestone, which forms so heavy beds, from below Joliet 

 to Chicago and beyond. Much of the sand is probably due to the disintegra- 

 tion of the Coal Measure sandstones, while some of it may have come from the 

 northward. There is, however, in these beds, but a very small proportion of 

 the mctamorphic material from Canada, which forms so large a part of the true 

 Drift, but, upon the surface of the soil, and often partially buried, are great 

 numbers of small boulders of quartzite, gneiss, granite and trap, unquestionably 

 of northern origin. These are especially abundant south of Goose Lake, over 

 the surface of the valley which starts from the Kankakee, near the county 

 line, includes Goose Lake, and joins the Illinois valley near where the Mazon 

 first strikes the bottoms. This was probably a shallow channel, in which float- 

 ing fields of ice lodged, melted and dropped the loads of stone which they had 

 brought from the northward. Similar aggregations of boulders occur in the 

 adjacent parts of Will county, at points where eddies would have been likely 

 to detain the ice floes. I have suspected that this Goose Lake channel was 

 formerly the main channel of the Kankakee, which thus met the DesPlaines 

 only four miles above Morris; but I have not collected sufficient data to decide 

 the point. 



The bed of " potter's clay," worked near the southwest bank of Goose Lake, 

 and lying " near the level of the fire clay," (see vol. i., p. 58) owes its origin 



