Tart II. GEOLOGIC RELATIONS AND FLORA OF THE LARAMIE OF THE 



DENVER BASIN. 



LITHOLOGIC .4ND STR.\TIGR.\PHIC REL.4TIONS. 



It is not my intention in the present con- 

 nectioh to attempt a complete exposition of 

 the hthokigy and stratigraphic rehitions of the 

 Laramie in this area but simply to give enough 

 of this history to serve as a setting for the 

 paleobotanic data. One w-ishing to go thor- 

 oughly into this phase of the subject should 

 consult the monograph on the geology of the 

 Denver Basin."' from which much of the fol- 

 lowing brief account is condensed. 



With the exception of a narrow belt along 

 the foothills which is occupied by older for- 

 mations, practically the entire Denver Basin 

 is presumably underlain by the Laramie. As 

 a surface formation it is confined chiefly to the 

 northern portion of the basin and to a very 

 narrow strip parallel with the foothills and at 

 a distance from them of 1 or 2 miles. In this 

 narrow belt the formation crops out in detached 

 areas separated by overlapping deposits and 

 extends from a point a few miles north of 

 Denver to the vicinity of Colorado Springs. 

 The beds are steeply tilted eastward along 

 the foothills, but the dip becomes less toward 

 the east, and in the central part of the basin, 

 where the beds lie almost flat, the Laramie is 

 covered by the Arapahoe, Denver, and Dawson 

 formations. 



The Laramie is the youngest Cretaceous for- 

 mation in the Colorado Front Range region. 

 It rests conformably on the Fox Hills sand- 

 stone, from which it is distinguished where 

 in continuous outcrop by its lighter color and 

 the presence in the topmost layer of the Fox 

 Hills of numerous marine invertebrates."^ In 

 this area the Laramie was reported by the 

 authors of the Denver Basin monograph to 

 range in thickness between 600 and 1,200 feet, 

 and Richardson, in the Castle Rock folio, 

 reports a thickness of 1,200 feet in the foothill 



M Emmons, S. F., Cross, Whitman, and Eldridge, G. H., U. S. Geol. 

 Survey Men. 27, 1S96. 



« It was stated by Eldridge that none of these invertebrates were 

 known to pass into the Laramie, but according to Henderson (p. ST) at 

 least two brackish-water species (Oslrea glabra and Melanm tcyomimj- 

 etuU) are now found to be common to the contiguous portions of both 

 formations. 



region to the south of Denver. Near Colorado 

 Springs, the southernmost point at which the 

 Laramie is recognized, the thickness is reduced 

 to less than 400 feet. 



According to Eldridge the Laramie is '"divisi- 

 ble into two parts, a lower of sandstone and 

 an upper composed of clay. The former has a 

 uniform thickness of about 200 feet; the latter 

 varies.'"" 



In the lower division there are three per- 

 sistent beds of sandstone, two of which occur 

 at the base and have a thickness of approxi- 

 mately 60 feet, while the upper one has a 

 thickness of 8 or 10 feet. 



The intervening one.s not only disappear but van- in the 

 horizon at which they occur. The coal beds also van,-, 

 one of several seams being workable at one locality and an- 

 other in another. 



The sandstones are white and are composed almost ex- 

 clusively of quartz, clear and opaque white. The material 

 is somewhat loosely held together by cement, usually white 

 but occasionally tinged brown by iron oxide. 



In the upper division the thickness, 



owing to uneven denudation from the top, varies between 

 400 and 1,000 feet. The strata are chiefly clays, through 

 which are distributed small lenticular bodies of sandstone, 

 innumerable concretionarj^ ironstones from 2 to 4 feet in 

 diameter, and narrow local seams of impure lignitic 

 material. One or two beds of lignite are also present in its 

 upper portion east of Denver. 



The interpretation embodied in this quota- 

 tion was generally accepted prior to 191.5, and 

 the coal formerly mined at Scranton, Colo., was 

 believed to be in the upper division of the 

 Laramie. In recent years certain fossil plants 

 were collected near beds of coal which are sup- 

 posed to be the same as the Scranton coals. 

 These plants indicate Tertiary rather than 

 Cretaceous age. 



83 This division into a lower and upper part has been interpreted by 

 some as comparable to the use of "Lower Laramie" and "Upper Lara- 

 mie" by Veatch in describing the section in Carbon County Wyo., 

 but a ca'"eful reading of Eldridge 's context shows clearly that no such 

 separation was impUed or intended. I am informed by Mr Cross, the 

 only survl\'ing author of the Denver monograph, that these divisions 

 were made simply to show that the lower part is made up prevailingly 

 of sandstones and the upper part of clays. There is not the slightest 

 known evidence of an unconformity between them, and no stratigraphic 

 or formational importance is to be attached to this use of "lower" and 

 "upper." 



83 



