070 THE GEOLOGY OF MINNESOTA. 



[Topography. 



125 feet will furnish a power of 20 feet. About twenty rods from Weisbach's, a tunnel of 45 > 

 feet will afford 64 feet head of water. The rock is limestone, in horizontal bedding. 



Topography. That portion of the county which is covered with a thick 

 deposit of foreign drift presents the usual monotony of surface character- 

 istic of the drift latitudes. This includes the most of the range of town- 

 ships across the western end of the county, and some portions of the next 

 range east. There are, however, even within this drift area, a number 

 of narrow, deeply cut valleys, with precipitous rocky bluffs, having very 

 much the nature of canons, like those of the driftless territories of the west. 

 Toward the east these deeply cut valleys are more numerous. All the 

 little streams, and a great many narrow valleys that have no running watef 

 in them, have high rocky bluffs along their whole course. These valleys 

 and streams, constituting the drainage system of the county, converge 

 toward the valley of Root river. The valley of this stream with its prin- 

 cipal tributaries presents some of the most remarkable and instructive phe- 

 nomena of erosion to be found in the state. It passes nearly at right angles 

 across the strike of the formations. These are alternating limestones and 

 sandstones, with an occasional bed of soft shale. The Trenton limestone, 

 underlain by the easily eroded St. Peter sandstone, the same as at the falls 

 of St. Anthony, although about a hundred and sixty feet in thickness, is 

 eaten into by the retroaction of the water as it plunges over the falls at the 

 point where the streams cross the line of its superposition over the St. Peter, 

 until they have each excavated in the Trenton a deep channel from fifteen 

 to thirty miles in extent. Through the line of the strike of the St. Peter 

 these valleys are widened out, the surface of the low ground within the 

 bluffs being usually one of rich meadow with undulating surface, from one 

 to two hundred feet below the general level. The Cambrian formations are 

 entered upon by the streams while they are yet a good many miles within 

 the general area of the Trenton. As these formations consist of two lime- 

 stones, separated and succeeded by sandstones, they repeat the succession 

 of phenomena witnessed in the erosion of the Trenton and St. Peter. As 

 the water leaves the Shakopee limestone and enters upon the Jordan sand- 

 stone, it passes over a series of rapids, or a fall of several feet perpendicular, 

 which falls or rapids undergo a process of recession under the same causes 

 as produce the recession of the Trenton-St. Peter falls. Again when the 



