276 THE GEOLOGY OF MINNESOTA. 



I Topography. 



period is shown by their existence beneath the glacial drift. These sink- 

 holes sometimes occur in lines, and with increasing frequency and size 

 toward a large valley, and at last coalesce so as to form a continuous 

 valley, though frequently without running water, that becomes tribu- 

 tary to the larger gorge. These gorges under the drift can sometimes 

 be traced for some distance by a series of successive sink-holes. Some- 

 times streams are lost in them, and re-appear at lower levels. There are 

 several well-known subterranean passages in the county. Lost creek, in 

 Jordan township, and the Brook Kedron, in Sumner, both have under- 

 ground passages for several miles. Canfield creek, south of Forestville, 

 runs underground about twelve miles, and, finally, the south branch of 

 Root river sinks in the N. E. J sec. 19, Forestville, and runs underground, 

 except in high water, to about the center of section 21, where it re-appears. 

 These underground passages are in the area of the Galena. They indicate 

 the corrugated appearance the country presented prior to the overspread- 

 ing of the drift and the loess-loam. The Galena cannot be supposed to 

 have been any more subject to such causes as produced this channeling in 

 the rock than the formations of the Cambrian. .There is some reason, how- 

 ever, why these gorges are found almost entirely confined to that limestone. 

 As has been said, the Cambrian consists of alternating sandstones and lime- 

 stones, which conduces to their breaking down laterally, the sandstones 

 easily crumbling out. The Galena limestone, on the other hand, in con- 

 junction with the Trenton, while they have a thickness of 160 feet, more 

 or less, have, near the bottom, a bed of impervious shale which prevents 

 the downward infiltration of the surface water, and protects the underlying 

 sandstone. Hence the erosions that operate lateral!}', in tearing down the 

 Cambrian formation, are occupied in the Galena and Trenton limestone 

 area, in cutting narrow perpendicular gorges. For this reason the Galena 

 area is everywhere the highest in the county. From the eastern boundary 

 of the Trenton, looking east, one beholds a broad landscape, lying several 

 hundred feet, in some places, below him, the effect of the more rapid denu- 

 dation of the rocks in that portion of the county. Into such narrow gorges 

 neither the drift nor the loess-loam, however deposited, would enter with 

 such compactness as to close up the pre-existing water courses; and when 

 partially closed up, as they were wherever sink-holes have since appeared, 



