PUBLIC PARKS OP IOWA 59 



section 23, Scott township, Fremont county. The clay which fills the 

 caverns in the limestone south of Malvern is probably also of the same 

 age, and the same may be said of a highly disintegrated, ferruginous and 

 soft clayey rock resting on the eroded surface of the Coal Measure lime- 

 stone southeast of the center of section 13, Tp. 67 N. f R. XLJI W. 



While the age of the small outcrops enumerated above must be re- 

 garded as uncertain, the Dakota sandstone can be positively identified 

 in two exposures in Mills county. One of these is in the low slope of the 

 east bluffs of the Nishnabotna a little south of the cente^r of the north- 

 east quarter of section 22, two miles south of Henderson, and the other 

 is half a mile distant, northeast of the southeast corner of section 14. 

 Sandstone was quarried for many years at the former place, but the 

 quarry is now partly filled. The face of the quarry appears to have been 

 about ten feet high. The rock is a gray sandstone in heavy ledges, in 

 places yellow or even brown. It breaks with equal readiness in all di- 

 rections. Where the rock is hardest, the sand grains are held together 

 by an opaque, white, thin layer of silecious cement which apparently 

 is a product of interstitial leaching and redeposition. The solvent effect 

 of underground waters are seen also in the absence of the ferruginous 

 cementing material which is common in the Dakota sandstone else- 

 where, and which makes the sandstone in section 14 almost black. In 

 the quarry in section 22, the yellow or red oxides of iron color the rock 

 in places where perculating water has not had free passage, as along 

 shaly seams and in concretions. At the base of the quarries the sand- 

 stone rests on gray clay, or is interbedded with this, and on the faces of 

 some ledges there are marks which show how the two kinds of sediments, 

 while yet in a plastic slate, have been worked into each other and broken 

 into lumps which have slid into new positions, evidently under pressure 

 of superincumbent sediments. 



The sediments are of the littoral kind: mud, sand and gravel alterna- 

 ting. It is the first deposit of the advancing sea. The gravel is well 

 worn, and consists largely of the most resistant material of the under- 

 lying Coal Measure rocks. No limestone fragments were seen, but in 

 one block were some angular cavities which might have contained chips 

 of such rock, afterward removed by solution. A study of several lots of 

 pebbles of various sizes show that the larger ones are mostly made up 

 of chert from the Coal Measures and this often contains silicified frag- 

 ments of fossils. Most of the finer material is common quartz, and may 

 be seen in the following table, which is based upon observations of sev- 

 eral hundred pebbles and grains of the conglomerates and sandstones. 

 Evidently the larger fragments are nearly all derived from the local 

 rocks, the more resistant material 'of the Missourian. 



The geographical condition under which the Missourian deposits were 

 laid down are to be made out from the physical character of the beds 

 themselves as well as from the plant and animal remains which they 

 contain. These indicate off-shore conditions, such as prevail on a con- 

 tinental shelf, alternating with more shallow and less open waters. A 

 considerable part of the shales contain fine, arenaceous material such as 

 is common in the deposits out on a continental shelf. The limestones in- 



