342 TIIK GEOLOGY OF MINNESOTA. 



[Upper Trenton and green shales. 







nesian composition, and becomes more nearly a pure limestone. It is more 

 compact, but also is sometimes arenaceous, particularly near the bottom. 

 These beds appear in the banks of Root river in Pleasant Grove. They are 

 observed on sees. 8 and 16. They are cut by the railroad grade in the north 

 part of Orion township. They consist, in general, of the following rocks, in 

 descending order, seen in Pleasant Grove. 



Section on sec. 16, Pleasant Gron . 



1. Perpendicular escarpment, showing generally a thin-bedded and often shaly 

 rock, the thin shale partitions being as thick as one-half or one and a half inch 37 ft. 



2. The descent then is irregular over beds of argillaceous limestone and shale, mostly 

 hid from view. Some of these shale beds are six and eight inches thick, and from them, 

 where enabling under the weather, fragments of fossils fall out, such as Chcrtetes, Pleuro- 

 tomaria and Ortliis. The limestone weathers rough and thin-bedded, and shows lieceptacu- 

 lites. This interval includes about 47J ft. 



3. Then there is a broad shoulder making a talus and including a heavy bed of green 

 shale which overlies, as disclosed further down the river, the limestones of the "Lower 

 Trenton" as seen at Rochester 42 ft. 



The rock-cuts along the railroad north of Chatfield are in sec. 35, 

 Eyota. They are included in the foregoing section of the Trenton rocks, 

 and consist of alternations of limerock and shale each about eighteen 

 inches in thickness, the rock predominating; the whole thickness being 

 about twenty feet. There is considerable drift in this region, but below 

 this point the valley widens out more, so that it soon shows the outlines 

 and width characteristic of valleys cut in the St. Peter sandstone, though 

 there is no sandstone exposure till within about a mile of Chatfield depot. 

 This depot is above the Shakopee but near its upper limit. 



The heavy mass of green shales (No. 3 of the foregoing section) is a 

 familiar feature to the geologist who examines Olmsted county, on account 

 of the effect it has on the topography of the county, in preserving the un- 

 derlying limestone beds from solution and removal by percolating waters. 

 It may generally be seen near the brow of the bench which marks the out- 

 running strike of the Trenton, just before the general surface drops down to 

 the floor of the Shakopee. It contains numerous fossils, mainly brachi- 

 opods and small gasteropods, and an occasional coral. The coral Chii'Mrs 

 is abundant at most of the outcrops. Its thickness seems to reach at least 

 forty feet, but generally not more than six to ten feet can be seen at any 

 one place. In this shaly horizon are embraced thin lenticular sheets of pure 



