C. P. Reyes — Pedrock Sandstone of Marion Co.< Iowa. 275 



eastward. On the right a line-grained ferruginous conglome- 

 rate — an extension of the Redrock sandstone — rises in vertical 

 cliffs to a height of one hundred and fifty feet. The inclina- 

 tion is 5° to the westward ; but the dip is perhaps even greater 

 to the southwest. The strata are visible down to the water's 

 edge. The direct line of contact between the arenaceous and 

 calcareous beds is not shown, as the detritus brought down by 

 the streamlet and the alluvial material deposited at its mouth 

 by the DesMoines during high water completely conceals the 

 stratified rocks for several yards on each side of the entrance. 

 In his ascent of the DesMoines river in 1852, Owen observed 

 the same exposure and thought that it indicated a fault of one 

 hundred and fifty feet or more. It is more probable, however, 

 that the case is one similar to .that exhibited at the Redrock 

 quarry ; and that the limestone area at the time of deposition 

 of the sandy material was a slowly sinking island or low promon- 

 tory, which was eventually completely covered by the arena- 

 ceous deposit. 



At all appearances here was an extensive sandstone formation, 

 with a maximum thickness of more than one hundred and 

 fifty feet, lying unconformably upon the St. Louis limestone 

 and with coal-bearing strata imposed unconformably upon it. 

 At first it was thought that the sandy member represented 

 shore or estug^y deposits of the Kaskaskia sea. Such, however, 

 was found no. f .o be the case. A few miles below, exposures 

 were observed howing fully seventy-five feet of dark sandy, 

 clayey and bituminous shales between the sandstone and the 

 concretionary limestone. The shales carry at least two work- 

 able seams of good coal, one of which attains a thickness of 

 five to seven feet and has a very considerable geographical 

 extent. 



As exposed along the DesMoines and Skunk rivers the 

 upper portion of the St. Louis strata is made up of blue and 

 gray fragmentary limestone, overlain usually by several feet of 

 gray, highly fossiliferous, marly clay. The most characteristic 

 and widely distributed fossils are : Spirifera Keokuk Hall, 

 Pentremites koninckiana Hall, Athyris subquadrata Hall, 

 Zaphrentis spinidifera Hall and Productus margimcintus 

 Prout. There are also a number of monticuloporids, tere- 

 bratulse, lamellibranchs, and a few gasteropod and trilobite 

 remains. In many places the St. Louis formation exhibits 

 considerable surface erosion due to subaerial agencies that 

 acted before the deposition of the lower Coal-measures ; and 

 the soft marly upper member lias been largely removed. 

 The superimposing strata thus rest sometimes on limestone, 

 sometimes on marl. 



Am. Jouk. Sol— Third Series, Yol. XLT, No. 244. — April, 1891. 

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