118 THE GEOLOGY OF MINNESOTA. 



[Gravel and sand. 



but spreads eastward, covering much of the country to Red lake, the Big 

 Fork river and Rainy river. Toward the south it tapers to very narrow 

 limits and ceases at Brown's Valley. In the southwestern corner of the 

 state is found still another modification of the till. While in the northwest 

 it passes vertically from till to loam, though by a marked line of separation, 

 in Rock and Pipestone counties it changes horizontally into loam, by a 

 gradual and imperceptible transition from the characters of one deposit to 

 those of the other. This change begins in central Pipestone county, and is 

 completed before reaching the southern boundary of Rock county. At first 

 the loam is confined to the surface, but it increases in depth toward the 

 south, and the till gradually becomes converted to a pebbly clay and finally 

 to a loam that shows the action of water in its deposition. Thus the four 

 corners of the state, drained each in its own direction from the central por- 

 tion of the state, exhibit four remarkable variations from the typical sheet 

 of till that covers the rest of Minnesota in common with much of adjoining 

 states, and each presents an interesting problem of glacial geology. The 

 greater part of the till is blue or gray, but throughout the northeastern 

 and much of the east-central portions of the state it is red, or has the color 

 of non-hydrated iron-peroxide. 



Gravel and Sand. Along the valley of the Mississippi river, and also of 

 most of the larger valleys of the state which drain southerly, are found 

 deposits of stratified gravel and sand. These are not everywhere present 

 along these valleys, but instead of them the surface consists of clay or of 

 till; and in many large tracts they are not found at all. Below St. Paul 

 this gravel-and-sand is confined to the river gorge, and constitutes a high- 

 terrace flat. Such a terrace also skirts the St. Croix valley as far north as 

 Taylor's Falls, the Root river and Zumbro valleys, and that of the lower Min- 

 nesota. Above St. Paul such stratified gravel-and-sand deposits are found 

 more generally, and sometimes are spread over extensive plains, though 

 still occupying restricted areas. Such plains are found along the St. Louis 

 river and its upper tributaries; along the Mississippi and its tributaries in 

 Cass, Wadena, Benton, Sherburne and Stearns counties; also along the 

 Pomme de Terre and Chippewa, and the Otter Tail and Crow rivers. These 

 are not always immediately tributary to any present drainage valley, but 

 frequently exist as isolated plains, particularly in Cass, Wadena, Meeker, 



