THE DATE* OF THE GLACIAL PERIOD. 553 



mile in width, and the rock has a freshly broken appearance, 

 the large fragments thrown down by the action of the water 

 on the easily crumbled sand-rock, as the falls have receded, 

 still existing in the talus along the bluffs. Throughout this 

 distance (about eight miles) the strata are horizontal, the thick- 

 ness of the drift-sheet overlying them nearly uniform, and all 

 other conditions, so far as they can be seen, that would affect 

 the rate of recession, seem* to have exerted an unvarying in- 

 fluence. The inference is inevitable that the rate of recession 

 has been practically uniform between the two points named. 

 There is an aspect of age, and long weathering, presented by 

 the rock in the bluffs of the Mississippi below Fort Snelling. 

 It has a deeply changed color, a light- yellow, oxidized exterior, 

 which marks all old bluffs. The blue color is found at greater 

 depths from the surface than it is in the rock of the bluffs 

 above Fort Snelling. This stained condition also pervades the 

 lime-rock at the mouth of Bassett's Creek and at the quarries 

 in the ancient river-bluffs near the mouth of Shingle Creek, 

 on both sides of the river. Another notable difference between 

 the bluffs above Fort Snelling and those below consists in the 

 absence of caves, and subterranean streams entering the river, 

 above Fort Snelling. Although the Trenton limestone exists 

 in full force about St. Paul, in the bluffs east and north of the 

 city, yet it had been cut through by some means prior to the 

 drift so as to allow the entrance and exit of streams of water 

 at levels below its horizon through the sandstone. None such 

 are found above Fort Snelling. The surface drainage is shed 

 by the limestone, and is precipitated over the brink of the 

 gorge, forming several beautiful cascades. When such streams 

 enter the river below Fort Snelling, they either enter some 

 subterranean passage and appear at the mouths of caverns in 

 the sandstone, or as springs in the drift along the talus, or 

 they find an ancient ravine down which they plunge, by a se- 

 ries of rapids over bowlders, to the river-level, rarely striking 

 either the lime-rock or the underlying sand-rock. Again, the 

 rock-bluffs at St. Paul, and everywhere below Fort Snelling, 

 are buried under the drift-sheet. Their angles are sometimes 

 seen jutting out from some wind-beaten corner, but nearly 

 everywhere thev are smoothed over by a mantle of drift and 



