THE GEOLOGY or MINNESOTA. 



[Natural drainage. 



The area of Olmsted county embraces G58.42 square miles of land, or 

 421,391.08 acres; its water area is 3.94 square miles, or 2,520.20 acres; mak- 

 ing a total of 662.36 square miles, or 423,911.28 acres. 



SURFACE FEATURES. 



Natural drainage. Streams are plentiful and their fall is moderate. 

 The central, northern and western parts of the county are drained by the 

 Zumbro river. This stream runs north into Wabasha county, where it 

 turns east and makes its way to the Mississippi. It comes into Rochester 

 from the southwest, and within the city limits Bear creek from the south- 



ill 9f 



east, Silver creek from the east and Cascade creek from the west, empty 

 -ai8f- 



into it. Near the north line of the county it receives quite a stream, re- 

 to 

 suiting from the union of the middle and north forks of the Zumbro. The 



southern tier of townships are drained by Root river, which, very sinuous, 

 takes a generally east course to the Mississippi. This has no affluents of 

 much size in the county, except at Chatfield where a small stream known as 

 Mill creek joins it from the north. On the eastern border of the county 

 some small branches of the Whitewater river reach within the county. 



There are no lakes in this county, but it contains {a few small ponds 

 which in no sense deserve the name of lakes. Streams which sink into the 

 ground and disappear are occasionally met with. They occur in Farming- 

 ton, Elmira, Haverhill and Viola townships, and are especially frequent 

 where the sandstones of the Cambrian have combined with the magnesian 

 limestones to produce a gorged and broken condition of the strata in pre- 

 glacial times, followed by a thin spreading of till or of loam. The same 

 conditions produce sink-holes and subterranean streams in the area of the 

 Galena limestone wherever the drift is light so as not to have filled com- 

 pactly and completely the pre-existing gorges.* These phenomena are 

 more particularly noted on a subsequent page where these formations are 

 discussed. 



Living springs of cool, pure water, of the best quality, are not rare. 

 They are by far the most common on the south or west sides of the bluffs 

 where the green clay derived from the lower rocks of the Trenton period 

 comes to the surface. This clay is impervious to water. The formations 



Compare the report on Fillmore county. 



