ROCK FORMATIONS BENEATH THE DRIFT. 185 



the bluffs of the river valley. The first 30 feet of the Trenton series is 

 the mostly heavy-bedded and richly fossiliferous limestone which is 

 worked by many quarries both in Saint Paul and Minneapolis. Shales, 

 also containing abundant fossils, with occasional layers of limestone, 

 from a few inches to a few feet thick, form the upward continuation of 

 this series, and are exposed to an observed height of about 75 feet on 

 each side of the river at a distance of one to two miles up the valley 

 from the union depot. The maximum thickness of these shales known 

 in the city of Saint Paul is revealed by their outcrop at the intersection 

 of Carroll street with Dale and Kent streets, where they rise in a low 

 ridge above the surrounding drift gravel and sand of the Summit Ave- 

 nue plateau. At the house of Mr J. B. Chaney, on Rondo street, the 

 highest part of this outcrop of shales, overlain by only one to two feet of 

 glacial drift, reaches a height of 945 feet above the sea, or about 145 feet 

 above their base in the neighboring valley bluffs. 



The Trenton limestone extends with almost perfect horizontality from 

 Saint Paul ten miles west to Minneapolis, and also southward to Fort 

 Snelling, but toward the northeast, and also farther southward, this for- 

 mation and the Saint Peter sandstone rise slightly, not more than a few 

 feet to the mile, so that the underlying Shakopee or Lower Magnesian 

 limestone comes into view in the bluffs of the Saint Croix, Mississippi, 

 and Minnesota rivers, with still lower formations, as described by Pro- 

 fessor N. H. Winch ell, the state geologist, and by the present writer in 

 the reports of the Minnesota Geological Survey. 



Modified Drift of the Mississippi Valley. 



Plains of drift gravel and sand border the Mississippi river, varying 

 in height from 25 to 75 feet above the river and its scanty alluvial bot- 

 tomlands, along nearly all its course from Brainerd southerly for more 

 than a hundred miles to Minneapolis and Fort Snelling, often expand- 

 ing on one or both sides of the river to a width of a mile or more. At 

 Brainerd the plains are 1,200 to 1,210 feet above the sea; at Saint Cloud, 

 1,030 to 1,040 feet, and at Minneapolis 825 to 840 feet. They are thus 

 seen to descend at nearly the same rate as the present river. Within the 

 city limits of. Saint Paul, however, the valley has only a very slight de- 

 velopment of this declining glacial flood deposit. 



Farther down the valley, at South Saint Paul, Newport, and Lang- 

 don, and onward to the south, similar beds again appear, with an ele- 

 vation of 800 to 825 feet, in terraces on each side of the river. These 

 modified drift deposits south of Saint Paul were doubtless formed as a 

 flood-plain continuous across the valley ; but they may have been par- 



