434 '^HE GEOLOGY OF MINNESOTA. 



[Cretaceous beds. 



road cut shows a mixture of Cretaceous clay with the Cambrian, the top of the whole being thinly 

 and irregularly covered over and chinked up with coarse drift. The Cambrian is more or less 

 broken and tilted, at least the bedding seems to have. been cut out into huge blocks by divisional 

 planes, which, either by weathering or water- wearing, were widened, the blocks themselves being 

 subsequently thrown to some extent from their horizontally, tipping in all directions. The 

 opened cracks and seams were then filled with the Cretaceous clay, which is deposited between 

 these loosened masses, and sometimes even to the depth of twenty feet below the general surface 

 of the top of the rock. The clay sometimes occupies nooks and rounded angles, sometimes shel- 

 tered below heavy masses of the Cambrian beds. The clay is uniformly bedded, about horizon- 

 tally, with some slope in accordance with the surface on which the sedimentation took place. 

 But the most interesting and important feature is t/ie condition of these old Cambrian surfaces. 

 They are rounded by the action of water, evidently waves. The cavities and porous spots are 

 more deeply eroded, making little pits on the face of the rock; or along the lines of section of the 

 sedimentation planes with the eroded surface, there are furrows due to the greater effect of water. 

 The rounded surface of these huge masses of limestone is coated with a thickness of about a half 

 inch, or an inch and a half, of iron ore, which scales off easily, and is easily broken by the ham- 

 mer. While this scale of iron ore is thicker near the top and on the upper surface of the blocks, 

 yet it runs down between the Cretaceous clay and the body of the-rock." 



Another deposit of greenish clay (Fig. 25) similar to the two last described, enclosed in a 

 cavity of the Shakopee limestone and in part appearing to be a stratum overlain by it, was noted 

 beside the carriage road from South Bend to Mankato close east of its bridge over the Blue Earth 

 river. 



Surface. 



iH 



FIO. 25. CRETACEOUS CLAY BENEATH THE SHAKOPEE LIMESTONE, MANKATO. 



a. Shakopee limestone. 6. Bedded greenish clay, weathering white, but little sandy, c. Sandy, 

 bedded greenish clay. d. Drift, mostly coarse fragments of Shakopee limestone. 



In the S. \V. } of section 20, Lime, the quarry of J. R. Beatty & Co. exhibits a thickness of 

 twenty to twenty-five feet of the Shakopee limestone. The top of this ledge is waterworn and 

 hollowed in shallow pot-holes. Near the middle of the quarry face, as it was at the time of exam- 

 ination, these waterworn cavities reach to a depth of fifteen feet, their sides being in part en- 

 crusted with an iron-rusty scale, an eighth to a half of an inch thick. They are filled with very 

 coarse ferruginous gravel, much waterworn, so that sometimes its pebbles up to three or four 

 inches in diameter are almost perfectly spherical. In some of these crevices scanty traces of white 

 clay occur witli the gravel, the former being probably Cretaceous, while the latter seems to be 

 older than the glacial drift, and may be Cretaceous or of earlier date, possibly representing the 

 period in which these hollows were eroded. Close west of this quarry is found a thick bed of 

 whitish, very fine earth (analysis 2, page 438), containing too little clay for brick-making. 



Professor Winchell writes as follows respecting these probably Cretaceous deposits at locali- 

 ties recently examined by him near Mankato. "At the quarry of the Standard Cement company, 

 lately opened in the east bank of the Blue Earth river about a third of a mile south of the rail- 

 road bridge, the Shakopee limestone is separated from the Jordan sandstone by a course of light 

 green or often nearly white shale or clay, highly siliceous and aluminous, having a thickness of 

 about three feet. The hydraulic qualities of the Shakopee limestone seem to be associated with 

 the occurrence of this bed of shale, and to be altogether an accidental and local character. The 

 formation has before been known to be somewhat hydraulic, but here this quality is so far 

 extended as to make a valuable source of hydraulic lime. In the Shakopee limestone here are 

 also numerous pita and gorges, rounded off with age and crusted over with a ferruginous scale 



