WISCONSIN EXPOSURE 355 



graded pink Cambrian sandstone which underlies the surface and later 

 deposits of the entire upper Saint Croix River valley, and for a consider- 

 able portion of the valle}^ overlies the Keeweenawan eruptives. A good 

 exposure of eoliansand was seen last autumn at Grantsburg, Wisconsin, 

 where in recent road improvements a cut several feet deep had been made 

 into the modified till ; overlying this material was a bed of dune sand 3 

 feet and more in thickness. 



EAST MINNEAPOLIS 



At the corner of Tenth avenue southeast and Como avenue, Minne- 

 apolis, is an exposure of special interest. The section of Glacial and 

 post-Glacial succession is as follows, in descending order : 



1. A bed of Minnesota slough peat with a thin layer of soil and vegetation cov- 

 ering it. The peat is fairly pure save in the local segregations of gray material 

 which consist of a mixture of peat and corroded and crumbling molluscan shells 

 chiefly of gasteropods. These segregations are due either to the occurrence of 

 colonies of molluscan forms which were located at these spots, or to the driving 

 together of large numbers of abandoned shells by wind action, as is often seen in 

 the lakes of Minnesota at the present time. The peat layer varies from a few 

 inches to 2^ feet in thickness. 



2. Lying directly beneath the peat and quite sharply separated from it is a layer 

 of dune sand. The color of this sand is a light gray with a somewhat pinkish tint ; 

 its texture is fine and even ; structurally it has no trace of stratification whatever, so 

 far as can be seen. It carries no fossils, while the peat above it has many. The 

 dune sand layer varies in thickness from 6 feet down to complete disappearance; 

 it is about 2 feet thick where the peat now lies heaviest on it. It contains, wher- 

 ever the exposures permit examination, many decayed roots or canals which once 

 were filled with roots. Decomposition has gone so far that it is impossible to 

 identify the plant forms. 



3. Next in downward succession is a layer of soil ; it is a sandy loam and varies 

 in thickness from 1 to 2 feet. The dark color, which characterizes it and plainly 

 marks it off from the overlying dune sand and the underlying modified till, plainly 

 is due to the intermingled vegetable debris. This soil, like the eolian sand above 

 it, is thickly penetrated by many small roots having a prevailingly vertical direc- 

 tion. Their presence indicates a subsequent vegetation. The contact of this soil 

 layer on the modified drift beneath it is decidedly irregular- Tubes of soil extend 

 downward from 1 to 2 feet as if the soil had become packed into rodent burrows ; 

 again, charges of sand seem thrust upward, almost or quite reaching the dune sand 

 layer. 



4. Below layer 3 lies the characteristic modified drift which, so far as known, 

 extends downward to the glaciated surface of the Trenton limestone, probably not 

 less than 30 feet. Locally it is beautifully cross-bedded and occasionally a large 

 boulder is seen. 



The locality just described is apparently a mound. Standing on its 

 top, other mounds can be seen in either direction, southeast or north- 

 west. In the latter direction the dune-like elevations extend for miles. 



