614 W. T. LEE STRATIGRAPHY OF COAL FIELDS OF KEW MEXICO 



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POS T-CRE TACEOUS 

 ' UNCONFORMITY 



- — "Laramie" formation 



*< — Lewis shale 



Dulce 



-Monero 

 "Mesaverde formation 



']■• — Mancos sKale 



A-^ Dakota tandstorte 



acter on either side of the mountains 

 and sediments above the Dakota sand- 

 stone^ varying in thickness from 1,000 

 to more than 2,000 feet, give little 

 indicaton that they were derived 

 from a land-mass in the Eocky Moun- 

 tain area rather than from land- 

 masses at greater distances. Inas- 

 much as the remnants of them are 

 now separated by a distance of less 

 than 90 miles, as shown in figure 3, it 

 seems reasonable to assume that they 

 once extended continuously over the 

 area now occupied by the mountains. 

 Although minor warpings of the sur- 

 face may have occurred, causing dif- 

 ferences in the thickness of these sed- 

 iments, or even causing slight emer- 

 gencies in some places, there seems 

 to have been no orogenic disturbance 

 in the mountain region before the 

 close of the Cretaceous which left any 

 unmistakable imprint on the strati- 

 graphic record. At the close of the 

 Cretaceous the first great upheaval of 

 the mountains occurred and erosion 

 naturally began on all sides of them. 

 It is recorded on the east, south, and 

 west sides by the post-Cretaceous un- 

 conformity described in this paper. 

 The period of erosion was sufficiently 

 long to remove thousands of feet of 

 consolidated rock, and doubtless the 

 sediments thus derived accumulated 

 in formations that are transitional in 

 time between Cretaceous and Ter- 

 tiary. Such formations should be 

 found at a distance from the moun- 

 tains, while those above the uncon- 

 formity near the mountains should represent time somewhat later than 

 the beginning of the Tertiary. 



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^-'Dakota sandstone 



/- — Benton-Piarre 



.POST-CRETACEOUS 

 UNCONFORMITY. 



