620 THE GEOLOGY OF MINNESOTA. 



[Glacial drift. 



sculptured on the northwest side of projecting knobs. "The whole rock," 

 as Prof. Winchell writes, " including the upper surface and the sides of the 

 mounds, is planed off. The best exhibition of these markings is seen on 

 the northwestern slopes, in which direction there is a system of jointing 

 planes, dividing the granite into blocks that have at first sight a strong 

 semblance of dip, the masses breaking off more nearly at right angles on 

 the southeast side." Again, at Mr. Frankhaus' in Yellow Bank, glaciation 

 from northwest to southeast has rounded the projections of the rock, and 

 marked it with large furrows; but the fine striae both there and near Big 

 Stone lake have been effaced by weathering. 



The sheet of drift which overspreads these counties probably averages 

 a hundred feet or more in thickness. It is principally till, or unmodified gla- 

 cial drift. Its material was gathered by the ice from a large region on the 

 north and northwest, being quite certainly derived in large part from beds 

 of Cretaceous clay and shale. Most of its boulders are granite, gneiss and 

 schists, similar to the bed-rocks of this district and of northern Minnesota. 

 About half of the gravel contained in the till, and a small proportion, per- 

 haps averaging one in twenty, of its boulders larger than one foot in diam- 

 eter, are fossiliferous magnesian limestone, whose nearest exposures, in 

 the direction from which the ice-sheet moved, are in the vicinity of Winni- 

 peg, in Manitoba. This rock, pulverized and in masses as pebbles and 

 boulders, is thus a considerable ingredient of the drift, whence it is dis- 

 solved by infiltrating waters. 



Soft rain-water, soaking through the drift, is changed to hard water before it finds its way 

 into wells or issues in springs. The carbonates of lime and magnesia which it has taken up form 

 a scale on the inside of tea-kettles and the boilers of engines; and are occasionally deposited by 

 springs as an incrustation of moss, leaves, or other objects, or as a porous bed upon the surface 

 of springy ground. Interesting springs of this kind occur at the foot of the bluffs on the south- 

 west side of Big Stone lake, two and a half miles from its mouth. Their calcareous deposit is 

 commonly called "petrified moss," from the fact that it becomes covered with growing moss, the 

 lower part of which is being slowly encrusted and its form preserved by this accumulation. It 

 is a light gray, very porous mass, one to two feet thick. Other deposits of similar character oc- 

 cur near by, where no springs now exist, on the dry bluff-side, some 75 feet above the lake. 



From the Cretaceous strata the drift obtains a small admixture of the sulphates of lime, 

 magnesia and soda, which are also held in solution by the waters of wells, springs, lakes and 

 streams; but their amount is seldom sufficient to impart a perceptibly alkaline taste. Salt lake, 

 crossed by the west boundary of Lac qui Parle county, is an exceptional case, being rendered so 

 bitter that horses and cattle refuse to drink of it. Where shallow pools have dried up, they 

 sometimes leave a whitish alkaline efflorescence, resembling frost, gathered by the inflowing and 

 evaporating waters of many years. The till also contains rarely small fragments of Cretaceous 

 lignite, similar to that which is mined thirty-five miles west of Bismarck, Dakota. 



