596 



IOWA ACADEMY OF SCIENCE Vol. XXV, 191S 



times, so that similar opportunities for solution haA^e been of- 

 fered for a long period of time. 



Gypsum Domes. — About a mile up Two Mile valley there is an 

 exposure of gypsum just above the track level of the Interurban 

 railway. It is the last exposure to be seen in ascending the val- 

 ley and it lies at the end of a point between the main valley and 

 a tributary ravine. Evidently the overburden was cleared away 

 in order to obtain the gypsum. About three feet of red sandy 

 shale lies between the drift and the gypsum. Some of the upper 



P"'ig. 'IQ'Z. — I>omes ;inil iIi'.mhi's in the ,L;>iisiim up Twci .Mil.' rieci^ \;ill,->'. 



layers of gypsum on the floor of the exposure have been arched 

 up until they have formed a number of hollow domes of circular 

 or elliptical outlines. The walls of these domes are six inches to 

 a foot in thickness and are cracked and checkered in a very ir- 

 regnlar manner. The writer has not seen anywhere else the gyp- 

 sum so weathered as it is at this place. At several places the 

 rocks give forth a hollow sound as one walks over them. Solu- 

 tion channels similar to those seen in the Vincent clay pit are 

 present here also and show the same intricate arrangement as 

 well as revealing the weathering which the gj^psum has under- 

 gone. 



