200 G. L. COLLIE — WISCONSIN SHORE OF LAKE SUPERIOR 



Ripple-marks. — Ripple-marks are another common feature of tlie sand- 

 stone. Three types are recognizable, each of which is abundant. The 

 first type is one in which the two slopes of the ripple-marks are unequal, 

 both in length and pitch, and the crest is sharp normally. This type oc- 

 curs in those layers which contain a large [)ercentage of clay, and are for 

 that reason more impressionable. In the second t3''pe the two slopes are 

 nearly equal and the crest is rounded. The third type is one in which 

 two sets of ripple-marks intersect at var3dng angles, two sets of ridges 

 being thus formed, between which lie rudely quadrilateral depressions. 



Mad-cracks. — Associated with the ripple-marks, often on the same 

 slab, are abundant mud-cracks. Frequently iron has segregated in the 

 original cracks, and on exposed surfaces the more resistant iron stands 

 out ill welt-like ridges. 



Pebbly sandstone. — Thin bands of pebbles occur in the sandstone, oc- 

 cupying small areas. The pebbles are usually well water-worn and 

 carefully assorted. These features of the sandstone, together with those 

 above mentioned, indicate the shallow-water conditions under which 

 the sandstone was laid down. 



Thickness. — A well-boring at Ashland shows that the sandstone at that 

 point is over 2,500 feet thick. The sandstone thins out toward the con- 

 tact with the Keweenawan southward. It is reasonable to suppose that 

 it also thins out to the north as the distance from tlie original source of 

 supply increases. It is })robable that the thickness at Ashland repre- 

 sents, a})proximately, a maximum. 



General ToinxiRAiMiY of the Region 



The most prominent topographic feature of the area under considera- 

 tion is the Penokee and Douglas ranges, the former of which rises to a 

 heiglit of 1,000 feet above the level of the lake. At tlie Montreal river 

 the foot of the range forms the lake shore, a low cliff, whose face is the 

 dip ])lane of tlie rocks. Immediately across the river the range trends 

 to the southwest, gradually diverging from the w^estward trend of the 

 shore. The highest portions of the Penokee range in Wisconsin are 15 

 miles from the lake. 



The north face of the range, at the contact with tlie sandstone, is a 

 steep escarpment of varying height. This northward- facing cliff is with- 

 out question a fault scarp.* This is shown by the extreme brecciation 

 of the rocks at the contact; also by the upturned condition of the sand- 

 stone, which is on the down-throw side. No exact figures can be given 

 as to the amount of the displacement, but it })rol)ably amounts to several 

 hundred feet. The width of the zone of brecciation and the upward 



♦Grant : Copper-bccariug Rocks of Douglas Co., Bull. Wis, Geol. Sur. No. 6, pp. 17, 18. 



