THE GEOLOGY OF MINNESOTA. 



[Topography. 



The highest portions of adjoining undulations vary from a few rods to a half mile or 

 more apart ; and their elevation is sometimes -5 to 15 feet, and again 20 to 30 feet, or rarely 

 more, above the depressions, to which the descent is usually by very gentle slopes. These 

 hollows have a form that is like that of the swells inverted, being mostly wide, and either in long 

 and often crooked courses of unequal length, variously branched and connected one with another, 

 or in basins from one to one hundred acres or more in extent, which have no outlet but are sur- 

 rounded by land 5 feet or perhaps 10, 20 or 30 feet higher upon all sides. The small swamps 

 which often fill the depressions are called sloughs or marshes, the former name being the most 

 common in this prairie region, while the latter is applied to them in wooded parts of the state. 



Many others of these depressions contain bodies of water, which vary from a few rods or a 

 hundred feet to five or ten miles in length. All these are called lakes, and the term pond, which 

 would be applied to them in the northeastern United States, is here restricted to reservoirs made 

 by dams. The lakes of these counties usually lie in shallow basins, bounded by gently ascending 

 shores, which, however, are here and there steep to the hight of 10 or 15, and rarely 20 to 25 feet. 

 These higher banks are mostly at projecting points of the shore, and they have been formed by 

 the undermining action of the waves. The foot of such banks is plentifully strown with boulders 

 that had been contained in the till, all the fine parts of which have been thus washed away. Other 

 parts of the lake shore, adjoining tracts of lowland or marsh, are frequently bordered by a flat- 

 tened ridge of gravel and sand, often with intermixed boulders, heaped up by the action of ice in 

 winters, in its ordinary freezing, thawing, and drifting, when broken up, before the wind. 

 These ice-formed lake-ridges rise only from three to six feet above the line of high water of the 

 lake, and are from two or three to five or six rods wide. They occur most frequently in situations 

 where they separate the lake from a bordering marsh, whose area evidently was at first a part of 

 the lake. 



The most notable features of the topography of this region are the valleys or channels that 

 have been eroded in its broadly smoothed and approximately flat expanse by creeks and rivers. 

 The smaller streams generally flow 15 to 30 feet below the general level, with valleys from a few 

 rods to a quarter of a mile wide. The valley of the Redwood river is of small depth, 25 to 50 

 feet, along all its course above Redwood Falls. At and below this town, within a distance of one 

 mile this river descends a hundred feet in a succession of picturesque cascades and rapids, over 

 granite and gneiss, decomposing portions of which form towering cliffs, 100 to 150 feet high, on 

 each side, from an eighth to a quarter of a mile apart. This gorge, extending one and a half miles 

 before it opens into the broader bottomland of the Minnesota river, is quite unique in its grand 

 and beautiful scenery, with dense woods along its bottom through which the river flows, but 

 crowned above by the verge of prairies whose vast expanse, slightly undulating but almost level 

 in this extensive view, stretches away farther than the eye can reach. 



In Redwood county the Cottonwood river lies in a depression from a third to a half of a mile 

 wide, composed of level alluvial bottomland. 40 feet below the average surface. Through North 

 Star and Burnstown, in western Brown county, this river flows about 50 feet below the average 

 hight of the region, with a bottomland usually from a fourth to a third of a mile wide, of sand or 

 gravel, or in part of fine silt, elevated 10 to 15 feet above the river at low water but overflowed by 

 its highest floods. At Iberia, near the center of Brown county, four miles south of Sleepy Eye, 

 the Cottonwood valley is 75 to 100 feet deep, and from a half to two-thirds of a mile wide, con- 

 taining, on the northwest side of the river, terraces of gravel and sand, covered by a fertile soil, 

 similar to that of the upland prairies. These terraces occupy a width of two-thirds of a mile, and 

 form three or four successive levels or steps, 15 to 50 feet above the river. The bluffs that enclose 

 this valley here and below are usually very steep, varying in slope from 30 to 45. They have 

 been formed, like the higher bluffs of the Minnesota valley, by the undermining action of the river, 

 flowing along their base and wearing them away in its process of excavation. Mostly these slopes 

 are wooded and lie at considerable distance from the river; but the stream may, in its gradual 

 change of channel again impinge upon them, as it is now doing on its southeast side one and a half 

 miles northeast from Iberia, exposing there a freshly undermined section of drift, 75 to 100 feet 

 in hight, composed of yellow till for its upper 20 or 25 feet, and of dark bluish till below. East- 

 ward the valley of the Cottonwood river, before uniting with that of the Minnesota, gradually 

 increases in depth to 175 feet, with a width varying from a third of a mile to one mile. 



