572 GEOLOGY OF CENTRAL WISCONSIN. 



times very coarse, giving- to the 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 brown ferruginous layers (1442); all very coarse and rice- .... 



like; some of the thin layers very regular and persistent 54 



Total height of river cliff 85 



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, botli 

 in low mural exposures on the river bank, and also in high isolated peaks. Of these, 

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

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

 The total height of the peak above the river is 230 feet, the upper 50 to 75 feet being 

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

 wide 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 Cris, on the N. E. qr. of the S. E. qr. f of Sec. 30, T. 18, R. 6 E., Adams 

 county, has already been mentioned as one of the most striking of the great sandstene 

 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 lies in a nearly due north and south line. 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 within 80 feet of the top. On the east there is also quite 

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

 the rock is somewhat broken down, making an easy ascent. The summit is without 

 the pinnacles that characterize Petenwell and others of the more western outliers, 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. 



SHAPE OP THE SUMMIT OF ROCHE A CRIB. 

 Scale 300 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 Kilbourn City, and represents quite accurately the cliff at the southern 

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

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

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

 as follows: 



