274: GEOLOGY OF EASTERN WISCONSIN. 



Toward the top, there are to be found some very sandy layers that appear to rest, in a 

 measure, unconformably upon a worn surface beneath, and probably represent the St. 

 Peters sandstone, which would naturally be calcareous in such a situation. 



A portion of the rock of this dome is a coarsely brecciated, very hard crystalline dolo- 

 mite, but the greater part belongs to the more common class of coarse, impure magne- 

 sian limestone that characterizes this formation. 



Passing by a number of minor exhibitions of the same phenomena, we find, in the 

 vicinity of Ripon, this peculiarity developed to a wonderful extent. Just west of the 

 Ceresco Mills, in the lower part of the city, there rises a rock hill, having an elongated 

 oval contour, somewhat enlarged and curved at the eastern extremity. Its base is 

 skirted on the east and north by streams above which its crest rises to an average height 

 of about one hundred feet. Its axis lies east and west, and does not much exceed one- 

 fourth of a mile in length. It is terminated at the west by a deep ravine, beyond which 

 rises a second ridge of similar form and height, and somewhat greater length, but which 

 is less conspicuous because of its connection with the plateau on the south. The ridge 

 first mentioned is nearly isolated by a deep ravine on the south, which reveals its form, 

 though the rock exposures are chiefly confined to the eastern end. On the northeast 

 slope, thick, heavy, rough beds of Lower Magnesian limestone show a dip of 15 to 18 

 to north of east. On the easternmost point, the dip is 18 to the eastward, while on the 

 southeast curve a slant of 25 to 30 to the southeast is observed, and farther along the 

 south side of the hill, a more southward and southwestward dip seems to be indicated, 

 though the exposures are very unsatisfactory. A small quarry above the limekiln, and 

 near the top of the hill, shows within itself dips of 3 E., 12% N. E., and 7 N. W., 

 with ah 1 intermediate inclinations. This irregularity is not caused by the folding or con- 

 tortion of even bedded layers, so much as by the thickening and thinning of irregular 

 'ones. Great lumps, as it were, occur at certain points, swelling two or three fold the 

 thickness of the layer, or causing it to disappear entirely in a structureless mass. The 

 rock is greenish blue, somewhat argillaceous, aud contains a few fossils. 



Down the slope from this quarry, and within six rods of it, a bed of St. Peters sand- 

 stone lies immediately against the flank of the Lower Magnesian arch, the top of the 

 sandstone being 30 feet below the crest of the ridge. The actual junction of the sand- 

 stone with the limestone was not visible at the time of either of my visits, but I was 

 credibly informed that in a filled portion of the sand pit it had been previously exposed. 



Wells in the vicinity show that the surface of the limestone descends to at least 100 

 feet below the top of the hill. There is here then within a horizontal distance of less 

 than one quarter of a mile, a rise and fall of the upper limits of the Lower Magnesian 

 limestone of not less than 100 feet. 



FIG. 29. 



382ft. 



AND SOUTH SECTION, NEAB RIPON. 

 1. Lower Magnesian limestone. 2. St. Peters sandstone. 3. Trenton limestone. 



Within less than a half mile south of this arch, the Lower Magnesian strata again 

 mount into the Trenton horizon, more than 100 feet above the intervening depression, 

 and again to the south, less than half a mile, another arch appears, but passes 

 under the bluff to the west before attaining so great an altitude as the two preceding 

 ones. The rock forming this one is unusually irregular in hardness, and weathers out 

 into the most fantastic shapes, so that the loose worn masses are much admired as lawn 

 ornaments. 



The accompanying figure will assist in illustrating the foregoing facts. The irregu- 



