134 Oeology of Upper Illinois, 



Art. XII. — Geology of Upper Illinois; by Charles Upham 

 Shepard, M. D., Professor of Chemistry in the Medical Col- 

 lege of the State of South Carolina. 



The remarks contained in this memoir are derived from obser- 

 vations made the past season, during a short residence at Rock- 

 well, in La Salle county. Having entered Illinois by the way 

 of Chicago from the northern Igikes, I shall commence my obser- 

 vations with some account of this place and its vicinity. 



Lake-shore near Chicago. 



The western shore of Lake Michigan, above Milwalky, pre- 

 sents no rocks as seen from the lake, being generally level, and 

 but little elevated above its surface. It consists either of a sandy 

 beach, or, as is more commonly the case, of an abrupt bank of 

 blue clay. Chicago is situated on a beach-shore ; but the low 

 ridge of sand which formerly intervened between its site and 

 the lake, has in a great measure been obliterated, in order to im- 

 prove the building lots contiguous to the water. The easterly 

 gales, however, silt up fresh deposits of sand and gravel, which 

 singularly enough for this secondary region, abound in grains of 

 garnet, magnetic iron and epidote, as well as in pebbles of granite, 

 gneiss, sienite and trap. These foreign materials are no doubt 

 derived from primitive boulders scattered over the bottom of the 

 lake. 



The city plat scarcely varies from a perfect level, and rises only 

 high enough above the surface of the lake to secure it a bare im- 

 munity from inundations during severe gales, and seasons of 

 unusually high water. In the rear of the town lies a broad level 

 tract of wet prairie, still lower than Chicago, being only about 

 ten feet above Lake Michigan. The width of this tract varies 

 from six to nine miles, while it extends as far down the lake as 

 the unobstructed view can reach ; and in an opposite direction, 

 follows quite round to the head of the lake, where however it ex- 

 periences an extraordinary modification from the presence of sand- 

 ridges, which we shall presently describe. 



The origin of so extensive a region of lagoon, which is almost 

 completely submerged during the spring freshets, is not easily 

 accounted for ; since it is not contiguous to a broad slope of coun- 



