EXTENT OF THE FAULT 819 



distance, can be traced for hundreds of feet away from the sandstone. 

 This breaking up of the rock is intensified by weathering. The several 

 lava flows to be noted along this line dip strongly to the south. Angles 

 of dip are reported, varying from 40 to 70 degrees in the Black River 

 Falls and Copper Creek districts, while the alternation of amygdaloidal 

 and compact phases of the rock are constant, giving the flows a thickness 

 varying, Doctor Berkey, says, from 25 feet to 50 feet, and both the com- 

 pact and amygdaloidal phases of the diabase are vigorously shattered. 

 The sandstones which lie to the north of this fault line are relatively 

 depressed below the level of the Keweenawan just described. They dip 

 northward and away from the lava flows strongly at first and gradually 

 less until at a mile or two from the contact a nearly horizontal attitude 

 is assumed. These sandstones no doubt continue without interruption 

 northward to the Saint Louis exposures, where they are found to dip 

 southward at an angle of 10 degrees. The sandstones as well as the 

 diabases are profoundly shattered at the contact zone and fault line and 

 are generally bent upward, thus suggesting that the movement which 

 produced the fault was an upthrust of the diabases along a fault plane 

 sloping to the north — that is, beneath the sandstones.* 



Farther northeastward, at Pratt, a small station on the Chicago, Saint 

 Paul, Minneapolis and Omaha railroad, near the middle of township 45, 

 range 6 west, the diabases are exposed at several places. They show all 

 the salient features of the rocks described for the exposures to the west- 

 ward. They are badly shattered. While ordinarily compact, they have 

 sufl'ered to a notable extent from weathering, and the local areas of 

 amygdaloidal phases disclose the effusive nature of the rocks. This 

 shattered condition is a strong index of the proximity of the fault line, 

 which, if it continue eastward from the falls of Black river, must pass 

 far to the north of these exposures. 



At Ashland and probably in the north part of township 47, range 4 

 west, an artesian well has been sunk to the depth of 3,400 feet and the 

 bottom of the pink medium grained quartzose sandstones was not 

 reached. These rocks were continuous next below the glacial drift and 

 lake deposits. 



MINNESOTA LOCALITIES 



With the foregoing evidence clearly establishing the fault-line contact 

 of the Keweenawan and Cambrian in Wisconsin, we turn to Minnesota. 

 The Cambrian sandstones are known to occur at a sufficient number of 

 localities in northeastern Minnesota to establish their continuity from 

 the Black River gorge, in Wisconsin, and Fond du Lac, in the Saint 



* Ibid., pp. 19, 20. 



