Till-; MONUMENT CREEK FORMATION. 253 



The Mm i m in-lit ( 'n-ck formation occurs along tin- southern edge of the 

 Denver field in the steep slopes of a high mesa ami also stretches from its 

 base prairjeward in thin sheets. The floor of tin- lake in which the Monu- 

 ment ('n-ek was deposited was more or less irregular from erosion, anil in 

 inn- part or another consisted of the clays ami sandstones oi tin- Laramie, 

 Arapahoe, or Denver formations. In the foothill region the Monument 

 Creek lies in contact with the Arapahoe; between Platte River and Cherry 

 ('n-ek a tew hundred feel "I Denver beds exist, which further to the east 

 disappear. North and east of Coal ('reek, mi the eastern edge of tin- field, 

 both Denver ami Arapahoe are wanting ami tin- Monument Creels rests 

 directly upon tin- clays of the Laramie. 



The Monument < 'n-ek consists of conglomerates, sandstones, ami bright, 

 vari-colored, arenaceous shales. These alternate with one another, hut 

 the conglomerates are especially prominent in the upper division, while 

 tin- sandstones and shales are about equally distributed throughout the 



whole formation in beds from 20 to 40 feet thick. Only a portion of 

 the lower ilivisiun of the .Monument ("n-ek extends within tin- Denver 

 field. This displays marked regularity in the succession ami composi- 

 tion of it> beds, except at the very base, where, owing to the uneven 

 Hour, the material varies from conglomerate through sandstone to arena- 

 ceous shale. A short distance above the base are two broad hands of 

 green shale, separated by one of pink ami capped by a line grit or sand- 

 stone, which is suit ami friable ami easily disintegrates. Tin-si- arc suc- 

 ceeded, beyond the Held, by other similar beds of shale, sandstone, and 

 conglomerate. 



Tile sandstones ami grits of the lower division are mostly of Archean 

 in sedimentary debris; in the upper division eruptive material of several 

 kinds, including a rhyolitic tuff, occurs. Between the two divisions is a 

 local development of rhyolitic tuff, which probably supplies tin- fragments 

 of this rock in the beds above. 



The Thickness uf the Monument ('n-ek ur of its parts is undetermined. 

 Loth divisions vary, owing to erosion, past or recent. A rough estimate is 

 900 feet for tin- lower division, 4(H> fin- the upper, ami 40 or 50 feet for the 

 thickness of the intervening rhvolitic tuff. 



