54 IOWA ACADEMY OF SCIENCES. 



THE PINE CREEK CONGLOMERATE 



BY J. A. UDDEN. 



In the right bank of the west branch of Pine creek, a short 

 distance north from where it leaves section thirty-four in range 

 one west, township seventy-seven north, there is a pebbly 

 sandstone, unlike the coal measure conglomerate in the sur- 

 rounding country. This sandstone is mostly brown in color, 

 changing to yellow. It has a rather coarse texture, compared 

 with the coal measure rocks, and is somewhat more variable in 

 this respect. The best exposure appears in a small gully, 

 which comes down the hill from the west, some twenty rods 

 north of the south line of the section. In all a thickness of 

 about sixteen feet is seen. Springs issue from the base of this 

 rock, along the slope to the creek, indicating finer impervious 

 underlying beds. The lower part of the section has one ledge 

 which is two feet in thickness. But the bedding is irregular 

 and the layers vary much in thickness in short distances. 

 Some of the ledges are strong enough to be used for building 

 stone, while one or two are loose sand. Even the hardest lay- 

 ers break easily under the hammer. In these the sand and 

 gravel is cemented by a black matrix of peroxide of iron. The 

 uppermost ledges are somewhat finer than the lower. Two 

 sets of quite regular joints here cut the rock. One set bears 

 west of north and the other north of east. Along these joints 

 the ferruginous material is most profusely deposited. Some 

 of the ledges are cut up into rhomboidal blocks about a foot in 

 length and from eight to ten inches in width. These have a 

 shining black hard crust, half an inch or more in thickness, 

 which on some of the blocks has separated from the lighter 

 and softer rock within, forming thin, straight and smooth 

 plates. Above this brown sandstone there is a yellow loose 

 sand containing small boulders of greenstone and granite. On 

 top of this sand there is boulder clay and loess. Small expos- 

 ures of the conglomerate occur for a distance of a quarter of a 

 mile along the west side of the creek to the south of this 

 place. 



