573 GEOLOGY OF CENTRAL WISCONSIN. 



times, very coarse, giving to tlie rock an appearance of being made Ft. In. Ft. 



up of grains of rice 6 . . 



21 . Alternating thin and heavy layers, light- colored, friable (1441), with 



some dark brovm ferruginous layers (1442); all very coarse and rice- 



like; some of the thin layers very regular and persistent 64 . . 



Total height of rivercliff >"■> 



Top of Elephant's Back above river 310 



None of the sandstone of this section has any trace of calcareous or dolomitic ingredi- 

 ents. 



On the Wisconsin river, above the Dalles, the sandstone is very frequently seen, botJi 

 in low mural exposures on the livor bank, and also in liigh isolated peaks. Of ihese, 

 the most remarkable, as to height, is that known as Petenwell PeaJi, which rises 

 abruptly from the west margin of the river, on Sec. 9, T. 18, R. 4 K., Juneau county. 

 The total height of the peak above the river is 230 feet, the upper 50 to 7o feet being 

 a, narrow vertical crest, worn into partly separated crags, not more than 20 to 30 feet 

 ■ndde on top and about 300 feet in length. The rock of this crest is a light-colored, fri- 

 able sandstone, with a hard, vitrified crust. Below there is a long talus of sand, with 

 exposures of thin crumbly rock at base. The country around is a level sand plain, 40 fett 

 above the river. 



The Roche a Oris, on the N. E. qr. of the S. E. qr., of Sec. 30, T. 18, R. 6 E., Adams 

 county, has already been mentioned as one of the most strUring of the great sandstone 

 outliers of the central plain. It rises abruptly from the surrounding level stretch of 

 sand, a wedge-shaped mass of bare rock, 225 feet high, 1,300 feet long at base, and 

 about 1,100 at the summit, which is a nearly level area 10 to 200 feet in width. The 

 greatest length of the rock hes in a nearly due north and south Hne. The southern end 

 is a sheer precipice, over 200 feet in height. On the west side there is a steep talus of 

 sand creeping up in places to vrithin 80 feet of the top. On the east there is also quite 

 a long talus, but the chffs are generally as much as 150 feet high. At the north end 

 the rook is somev/hat broken down, making an easy asoent. The summit is without 

 the pinnacles that characterize Petenwell and others of the more western outUers, and 

 is grassed and wooded with a few small pines and scrub oaks. It has the shape and 

 dimensions indicated in Fig. 36, the measurements being made to the edge of the ver- 



Fig. 36. 



SUAFE OF TUB SUHMIT OV KoCUB A ClU8. 



Scale 800 feet to the inch. 



tical cliff on all sides. The view given in Plate XIV is taken from a photograph by Mr. 

 H. H. Bennett of Kilboum City, and represents quite accurately the diff at the soutliern 

 end of the bluff. Prom top to bottom of this cliff, the rock is a friable aggregation 

 of rolled quartz grains, showing only slight and somewhat indefinite variations in tlie 

 different layers. A detailed section along tlie face of the cUff, beginning above, is 

 as follows: 



