58 PUBLIC PARKS OF IOWA 



GEOLOGY OF MILLS COUNTY. 

 By John A. Udden, Geologist. 



The uplands consist of an old drift plain, modified by erosion and by 

 the deposition on its surface of a blanket of loess. But little is left of 

 the old surface of the flat drift plain. The only remnants left are some 

 flat strips of land on the highest divides farthest away from the largest 

 streams. These strips* are usually less than one-fourth of a mile in 

 width, often much less. The widest flats seen were between the head- 

 waters of Mill creek and Rock creek in Locust Grove township in Fre- 

 mont county; in the vicinity of the town of Tabor; on the divide be- 

 tween Mud creek and Silver creek southeast of Silver City; on the 

 divides north of Glenwood, north of Emerson and north and south of 

 Hillsdale. The total area of these upland strips do not cover more than 

 at most a few square miles of land in the two counties. 



Excepting these flat areas the divides everywhere consist of ridges, 

 more or less convex in cross sections. These are broadest farthest away 

 from the principal drainage basins and as we approach the margins of 

 the uplands they become more and more contracted and narrow. In 

 the bluffs of the Missouri they are frequently only three or four feet 

 across, with a steep slope on either side. The average elevation of these 

 summits of the uplands for the two counties is about 1,170 feet above sea 

 level, and it varies a hundred feet above and below this figure. The 

 eastern two-thirds of the uplands in this area fall about thirty or fifty 

 feet below the average, while the highest divides approaching the Mis- 

 souri river bluffs rise above it in places as much as ninety feet. From 

 north to south they have a general descent of about a foot and one- 

 third to the mile. 



By far the greater area of the uplands is formed of slopes which ex- 

 tend on either side from the creeks and ravines up to the crest of the 

 ridges and flats on the divides. Farthest away from the larger drainage 

 lines these slopes have a gentle grade and even near some of the larger 

 creeks they may be a half mile in length and 100 or 125 feet in height. 

 But near the Missouri bottoms they become more steep and frequently 

 rise at a high angle to 150 or even 200 feet above the bottoms. Along 

 these bluffs they are sometimes too steep to be tilled. Elsewhere they 

 constitute the main farm land in the region. The distance from the 

 foot of the lowest to the top of the highest slopes embraces a vertical 

 range of about 360 feet. 



There is no doubt that small patches of Cretaceous deposits lie under 

 the drift in several places on the uplands, where they cannot now be 

 seen. Sand and soft "sand-rock" have been found under the boulder 

 clay a mile east of Emerson. Another well in the west bluffs of the 

 Nishnabotna west of Henderson penetrated some gravel which may have 

 been of the same age. On the eroded surface of the limestone in the 

 quarry at Henton there are seen some disintegrated lumps of a brown 

 sandstone which resembles the Cretaceous in appearance. It contains 

 almost exclusively well rounded pebbles of quartz and chert. Blocks of 

 the same conglomerate, always highly ferruginous, occur associated with 

 small exposures of Coal Measure rocks two miles farther south and have 

 been again noted on top of these older rocks east of Wabonsie lake in 



