PUBLIC PARKS OF IOWA 137 



in that the surface features of the prairie give no indication of the pres- 

 ence of the gorge until one is in close proximity to its edge. The erosion 

 forms devoloped in the quartzite area are well shown at Jasper pool in 

 Lyon county, Iowa. 



The special area in question is located about twelve miles northeast 

 of Sioux Falls. Corson station, on the Great Northern Railway, is just 

 within the southern limit of the area and is a central point in the great 

 quartzite region. Split Rock creek, a tributary of the Big Sioux, meanders 

 through the area from north to south and is flanked on either side by a 

 chain of hills, the summits of which rise to a height of nearly 100 feet 

 above the channel of the stream. The valley of this stream measured 

 from crest to crest, is about one mile in width. Near the northeast cor- 

 ner of the northeast quarter of section 15, an isolated hill, Keyes knob, 

 rises within the valley and is the most prominent topographic feature in 

 the vicinity. Iowa Geological Survey, Vol. VI, pp 71-4. 



