PRE-GLACIAL RIVER VALLEYS OF MINNEAPOLIS 



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These may be capped by a few feet of Decorah shale, which repre- 

 sent remnants of erosion. There are also 

 several depressions or basins, probably 

 gouged out by ice scouring, which, in 

 several instances, are deep enough to 

 extend through the limestone into the 

 underlying sandstone. Another expla- 

 nation for these basins is that they repre- 

 sent sink holes caused by the caving of 

 the limestone over solution cavities pro- 

 duced in the underlying strata by ground 

 waters. There is one cave about half a 

 block long at Second Avenue South, 

 between Fourth and Fifth streets, in the 

 downtown district. This has not re- 

 sulted in any noticeable surface de- 

 pression, but in order to avoid such a 

 possibility in the future, which might 

 prove disastrous to buildings in the 

 vicinity, it has been thoroughly walled 

 up with concrete supports. This cave 

 was due to mechanical action rather 

 than to solution. The cavity occurs at 

 the contact between the limestone and 

 sandstone, but lies entirely within the 

 sandstone. It is clearly due to the 

 action of the ground water which has 

 washed away the somewhat loosely con- 

 solidated sand at the contact, where the 

 ground water tends to accumulate be- 

 neath an impervious bed of shale which 

 separates the sandstone from the over- 

 lying limestone. This locality is only 

 a few blocks from the river gorge along 

 the sides of which the ground water 

 comes to the surface. The production 

 of open cavities or channels in the sandstone by circulating ground 



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