212 GEOLOGY OF THE DENVER BASIN. 



The strata of the Arapahoe and Denver have a total thickness much 

 greater than that of the Laramie, and by their texture speak for epochs of 

 sedimentation presumably equal in duration to that of the Laramie. 



EVIDENCE OF STRATIGRAPHIC RELATIONS. 

 Stratigraphic break between Laramie and Arapahoe. Erosion of tile llppel' liml) of the 



monoclinal foothill fold has destroyed the zone where the Arapahoe beds 

 must have originally overlapped the Laramie and some older deposits with 

 angular unconformity, unless it be that the Arapahoe basin was entirelv 

 excavated out of Laramie strata. Minor unconformity of this latter kind is 

 shown in the plains area, as described by Mr. Eldridge. 



The evidence of the lithologic character of the Arapahoe beds has 

 been given, ami the following deductions from that evidence seem 

 unavoidable. Somewhere tributary to the Arapahoe sea many thousand 

 feet of the Mesozoic strata were upturned and had already been greatly 

 eroded prior to the new epoch of sedimentation, so that widely separated 

 horizons contributed simultaneously to the basal conglomerate of the 

 Arapahoe. It is most natural to assume that the present foothill fold, 

 which is known to mark the line of progressive or repeated movement, 

 elevated the western shore-line deposits of the older formations, and that 

 the erosion of this western area produced the various pebbles of the 

 Arapahoe. But the position -of the land mass of these eroded Mesozoic 

 sediments is of secondary importance compared with the fact- of a great 

 orographic movement which terminated the long succession of conformable 

 Cretaceous sediments at the close of the Laramie. As the Arapahoe 

 deposit seems to have been local in character, it was at first possible to 

 assert that this orographic movement was not of importance in the broad 

 area of the Rockv Mountains. But the facts of the Middle Park and 

 Livingston beds, and the inferences present knowledge justifies in other 

 described localities, all tend to show that this stratigraphic break was 

 extensive, ami recent discoveries have uniformly increased its importance. 



Relation between Denver and Arapahoe beds. As far IIS lvllOWll till' 1 >ellVer lake 



basin was eroded out of the Arapahoe strata, and to the northeast of 

 Denver it would seem that the preceding formation was entirely destroyed. 



