270 G. B. RICHARDSOX THE MONUMENT CREEK GROUP 



base of the Dawson arkose is marked by a conglomerate lying uncon- 

 formably on the Laramie formation. 



The Castle Eock Conglomerate 



The uppermost part of Hayden's Monument Greek group, that part 

 lying above the unconformity exposed on Castle Rock which has been 

 traced throughout the area mapped, is named the Castle Rock® con- 

 glomerate, from Castle Rock, a prominent hill immediately north of the 

 town of that name. The Castle Rock conglomerate has a maximum 

 thickness of about 300 feet. It outcrops in detached areas on the divides 

 between the tributaries of South Platte River from the vicinity of Elbert 

 to the vicinity of Sedalia, a distance of about 40 miles. 



The Castle Rock conglomerate is a massive indurated deposit, which 

 is the remnant of a once larger extent of the formation that has been 

 removed by erosion. It is situated in the south central part of the Denver 

 Basin, where the strata lie almost flat, but, conforming with the pitch of 

 the syncline, have a distinct though slight northward dip. 



Everywhere the Castle Rock conglomerate rests on an undulating, 

 eroded surface of the underlying Dawson arkose, and there is an abrupt 

 change in texture of the material from the medium or fine-grained arkose 

 of the upper part of the Dawson to the coarse Castle Rock conglomerate. 

 The bottom of the conglomerate is composed of rounded pebbles, up to 

 several inches in diameter, of quartzite, quartz, red sandstone, gneiss, 

 and granite, and of larger, usually irregular, blocks of rhyolitic rocks. 

 These are imbedded in a finer-textured matrix of quartz and feldspar. 

 The conglomerate is remarkably persistent in its general features, yet it 

 is of variable texture, coarser and finer-grained phases being intimately 

 associated, and in places the rock is an arkosic sandstone, which, however, 

 is streaked with variable conglomeratic lenses. 



A number of Titanotherium bones have been found in the Castle Rock 

 conglomerate at various localities. Especially noteworthy is a collection 

 from the plateau east of Elbert, including a well-preserved lower jaw, 

 with teeth identified by Mr. J. W. Gidley, of the U. S. National Museum, 

 as Titanotheiiuiti trigonoceros (?). These bones correlate the Castle 

 Rock conglomerate with the Cliadron formation, the name given by Dar- 

 ton to the "Titanotherium beds" which constitute tlie lower formation 

 of the White River group of Oligocene age. 



» Castle conglomerate was suggested by Willis T. Lee for this formation, but the name 

 is not admissible because of prior usage. 



Willis T, Lee : Geology of the Castle Rock area. American Geologist, February, 1902, 

 p. 103. 



