FILLMORE COUNTY. 275 



Topography.] 



Explanation. 



At a the Galena and Trenton have their full thickness, about 160 feet. Such a point may be 

 found at Fountain ; b represents an outcrop of the Galena, or Upper Trenton, as seen along the 

 gorges that are frequent in the Galena area. Such an outcrop is visible at the "Big Spring" a 

 few miles northwest of Fountain, where the water rashes out in a great volume near the base of 

 the bluff, and probably on a level with the top of the green shale. At c is a marshy tract, or one 

 that is gently sloping, having a springy margin, near the brow of the lower bluff. Such spots 

 are visible particularly at Chatfleld, in the northern part of the village, near Jacob's limekiln, and 

 west of there. A fine illustration of the effect of this shale on the surface drainage may be seen in 

 section 35, Holt, where a copious spring issues from near the top of the mound of Trenton, the water 

 being shed by the shale overlying, and gathered by troughs into a tank for watering stock, d rep- 

 resents the outcropping edge of the Trenton. It is this which is seen in the summits of the isola- 

 ted mounds, and which forms the conspicuous shoulder that exists wherever the strike of the 

 Trenton crosses the county. The slope e is occupied by the St. Peter sandstone. Sometimes this 

 is quite precipitous, and its upper forty or fifty feet are very apt to be, but its lower portion is very 

 gently descending, so that it is impossible to determine where it is replaced by the Shakopee which 

 underlies it. The horizontal distance between & and d is sometimes several miles. This is apt to 

 be the case along the eastern margin of the Trenton area. Indeed the detached Trenton areas in 

 Holt and Amherst,~and notably that in Pilot Mound, townships comprise only this lower portion 

 of the Trenton. The Shakopee limestone, /, underlying the St. Peter, is that which occurs along 

 the tops of the immediate bluffs of the river, as at Preston, Lanesboro, Clear Grit, and Whalen. 

 At Chatfield it is seen at the mill, and rises about thirty feet above the river. 



The strike of the Galena and Hudson Elver formations is often driven 

 back several miles from that of the Lower Trenton. The limerock which 

 lies below this shale is about twenty feet thick. The singular Trenton 

 mounds, which have already been mentioned, are composed of the Lower 

 Trenton, protected by a greater or less thickness of this green shale, and a 

 portion of the St. Peter sandstone. The preceding diagram illustrates the 

 manner of weathering down of the Trenton and St. Peter. Instances of this 

 may be seen in almost any square mile, in the loam-covered area, along the 

 out-running strike of the Trenton. 



Throughout the Galena and Upper Trenton areas are found a great 

 many depressions that are well known as "sink-holes". These consist of bro- 

 ken down spots in the drift, or loam, where it had been spread over a pre- 

 existing canon in the rock. In some places they are very numerous, but 

 they are confined, so far as known, with but very few exceptions, to these 

 limestone areas. They throw some light on the condition of the rocky sur- 

 face prior to the period which witnessed the spreading of the drift. The 

 rock was wrought, at least in Fillmore county, in very much the same man- 

 ner as we now see it along the river gorges. The immense valleys of erosion 

 which we see, not only in Fillmore county but also throughout the tract 

 that has been denominated the "driftless area", were excavated before the 

 glacial period. Where the streams of the present time run in such gorges 

 they have been so located by the exigencies of surface drainage and ero- 

 sion since the last glacial epoch. That these gorges antedate the last ice 



