668 GEOLOGY AND TOPOGRAPHY OF THE LEAD REGION. 



CHAPTEE III. 

 GEOLOGICAL FORMATIONS. 



POTSDAM SANDSTONE. 



Geographical Boundaries. South of the Wisconsin river it is 

 found as the bed rock of Otter, Mill and Blue Mounds creeks, and 

 their various tributaries, as far south as town 7, on the various ranges. 



7 7 5 



Here it disappears, owing to the southerly dip of the formation, and 

 the sudden rise of the country to the south. The valley of the Wis- 

 consin river also lies in this formation, forming a very level plain 

 from two to three miles wide, and extending from Sauk City to a 

 point about four miles above the mouth of the river. Good natural 

 exposures are seen in the bluffs on each side of the river, in which the 

 various strata may be traced uninterruptedly for miles. 



The greatest exposed thickness is seen in the valley of the Wiscon- 

 sin river, where it is about 300 feet from the water to the bottom of 

 the Lower Magnesian limestone. Examinations north of the Wiscon- 

 sin river make the entire thickness of the formation about 1,000 feet. 



Lithological Characteristics. The following section, taken from a 

 bluff about three miles northwest of Lone Rock, in the IS". E. qr. of 

 Sec. 34, T. 9, R. 2 E., will serve as a general guide to the formation : 



i A. Lower Magnesian Limestone. 



1. Irregularly-bedded, white and yellowish Lower Magnesian limestone, con- ft- /. 



taining no flints 60 



2. Transition beds of arenaceous limestone, in which rounded grains of sand 



are cemented together by limestone 5 



B. Potsdam Sandstone. 



3. Sandstone, white, very heavy-bedded, containing at the bottom occasional 



horizontal seams of lime. Friable 45 



4. Thin-bedded, earthy, straw -yellow shales, containing Dicellocephalus and 



Lingula 45 



5. Very finely laminated blue shale 3 



6. Thin-bedded, yellow, argillaceous limestone 2 



7. Earthy material 3 



8. Very hard and compact brown limestone, building stone 4 



9. Thin-bedded gray limestone, building stone 9 



10. Yellow and white friable sandstone, to the valley, mostly unexposed, cov- 

 ered by slope of hill 190 . . 



Total thickness 360 6 



