232 r. AV. SARDESON THE REDSTONE QUARTZITE 



In fact, the leaf-bearing sandstone is not under the green shales, but is 

 quite the reverse. No such wide distinction as Dakota formation and 

 Niobrara (that is, Colorado formation) is at all applicable, and not even 

 a division into lower and upper formation is well defined. All strata are 

 fresh-water deposits and are referal^le to the Dakota formation, although 

 they possibly are contemporaneous with the marine Colorado formation 

 or Niobrara. The name Big Cottonwood formation or beds will he useful 

 in distinguishing this part of the Cretaceous in the region here described. 



This formation extends continuously for 7 miles in the valley of the 

 Big Cottonwood river, and is evident as far west as Springfield. From 

 the Big Cottonwood valley it extends up the Minnesota to New TJlm and 

 down the valley toward Cambria. It lies on three sides of the Eedstone 

 quartzite. Tlie formation probably runs continuously westward toward 

 Marshall, j\Iinnesota, while it is cut off to the northeast and south, form- 

 ing thus an area 20 miles or more long and 10 miles or less wide. It rep- 

 resents a river delta or filled valley of a stream which originally descended 

 from east to west, as is shown in the cross-bedding in the strata. The 

 Big Cottonwood formation now dij^s from west to east at about the same 

 rate as the gradient of the Big Cottonwood river, the formation having 

 been tilted. 



The Big Cottonwood formation comprises coarse, clean sandstone and 

 fine conglomerate, shales, jootters' clay, limestone, and lignite. Alternate 

 strata of almost any two kinds of materials may be seen locally. Owing 

 to the prevailing and often very strong cross-bedding, no two exposures 

 afford the same succession of strata. The color is red, green, white, yel- 

 low, blue, or brown. The deposit is clearly that of a river. Coarse, clean 

 white or 3'ellow sand occurs, in which there are seams of fine, tough, blue 

 clay cross-bedding the sands at an angle of 10 degrees or less, and the sand 

 frequently contains pieces of lignite, besides occasionally imjjrints of 

 leaves. 



There is no uniform division into upj^er and lower parts, but in general 

 there are some differences between the lower and uj^per deposits. Lime- 

 stone, Avhich occiu's at one end of the field, is in general median in posi- 

 tion. It is found more particularly in the vicinity of the Eedstone. The 

 top of the formation is generally clean sand with clay seams or beds, 

 which occur irregularly, but yet so that 40 feet of strata comprise 10 feet 

 or less of cla}', either in seams from a quarter of an inch to 1 foot thick 

 or in several instances in a single thick bed. Below that there is red and 

 green shale, sandy shale, clay, and some clean white sand, in strata which 

 are 1 to 10 feet thick, but run unevenly. The lower part of the forma- 

 tion is to 200 feet thick, as far as known, varying with the depth to sub- 

 jacent rock, and comprises on the whole the greater part of the formation. 



