POST LARAMIE MOVEMENT. 31 



That the Mesozoic strata were uplifted along the foothills above the 

 general level of the beds, and their upturned edges exposed to erosion, 

 is proved by the fact that rolled fragments of the rocks of the differenl 

 formations are found in the succeeding Arapahoe and Denver formations. 

 The period of erosion that succeeded the movement of elevation must have 

 been of long duration, since as much as half the total thickness of the 

 Laramie was removed from portions of the field before the succeeding 

 Arapahoe beds were laid down. 



ARAPAHOE FORMATION. 



After an erosion of the Laramie beds which removed from portions of 

 the Denver Basin 600 feet or more of the previously deposited sediments, 

 a considerable fresh-water lake was formed and sedimentation again set in. 

 What the exact area of this lake was it is not possible now to determine; 

 its extent was undoubtedly considerably larger than that covered by its 

 beds at the present day, especially to the northward. To the southward 

 vertebrate fossils characteristic of the post-Laramie formations have been 

 observed by Professor Marsh in Monument Park, and remnants of beds 

 resembling the Arapahoe and Denver series have been observed near 

 Canyon City which may have been contemporaneously deposited, but 

 whether the lake was continuous along the mountain front or there were 

 several small isolated basins it is as yet impossible to determine. For the 

 present discussion it will be assumed that the Arapahoe Lake was confined 

 to the Denver Basin. 



In it were deposited more than 600 to 800 feet of sediments, the excess 

 above these figures being the unknown amount that was eroded off before 

 the Denver beds were deposited. Of these sediments the lower 50 to 200 

 feet were conglomerates, the upper 400 to 600 feet arenaceous clays. In 

 the persistent band 40 feet in thickness at the base of the formation have 



been found among the pebbles coal, silicified w 1, and white sandstone 



from the Laramie; limestone from the Niobrara; the characteristic cherty 

 conglomerate from the Dakota; limestone and red sandstone from the dura 

 and Trias; and silicified limestone with casts of Beaumontia. The last 

 named, which are most abundant in the southern portion of the field, must 

 have come from Carboniferous limestones in Perry Park or beyond, and 



