312 W. T. LEE MORRISON A CRETACEOUS FORMATION 



that until i-ccciitlv (17) thev were reganled us parts ol: the Dykuta. The 

 Purgatoire ami its i)()ssil)le time equivalents, Lakota and Fuson, and the 

 h)\ver part <ir the so-called Dakota, where a Lower Cretac-eoiis porticiii has 

 not 3'et been ifc(),i:iii/,c(h overhip the Tilorrisoii in only a few places (12). 

 Tlie Dakota appeals to liave entirely covci'ed the areas now occupied hy 

 the l.'ockv Moiiiitains. Althon,i:h it is now ai)scnt in nriny places because 

 of .subscipU'Ut erosion, ivinnants (d it occur on all sides of the luountains 

 ;)nd in iiiteniionlane ai'eas at elevation^ ran.iiinu' to a nraxiniuni of i;'.Ji)i» 

 feet ahtive sealevel (v!\M. With very lew ext-eptions, the Morrison is 

 found below the Dakota (or Purgatoire. where that formation has been 

 identified). 'I'he uniform distribution of the^e sandstones, taken in con- 

 nection with their regularity in character and thickness, indicates that 

 the sands were spread out over a nearly level phiin. Although the Da- 

 kota in the KNicky Mountain ]egion is |)laiit-liearing and consequently a 

 so-called fresh-water fornuition, it is ])robal)ly a deposit of sand cleansed 

 bv wind and wave at the ailvancing fi'onl of the Cretaceous sea. It is 

 believed that the shai]» chan.ge from the .Moi'i-isou to the overlying sand- 

 stone is due to thi'^ change in the I'onditiou- n\' deposition rather than to 

 anv lapse of time between them. 



The essential points in this pai't of the geologic histoiy of tlie Pocky 

 ilountain i-egion may he sumnuiii/.ed a> follow^: That i^art of the region 

 above seale\cl alieady degraded to a peneplain near the close of the 

 Jurassic period was disturbeil hy a movement that increased its relative 

 altitude and expelled the .Jurassic sea. The culmination of this move- 

 ment is regaided as the close of the Jurassic period. This uplift was 

 accompanied by continued degradation and followed by slow su])sidence, 

 which doubtless was inteimittent and oscillatory, but whicli finally re- 

 sulted in the formation of the liasin occu])ied by the interior sea of Upper 

 Cretaceous time. Th.e Moirison was deposited on this graded plain by 

 the streams made sluggish by reduced gradients. The region was partly 

 submerged in late Lower Cretaceous time by the shallow Avaters of the 

 sea, which reached at least as far inland as the present mountain front. 

 This partial submergence was followed, either immediately or after a 

 slight interval, l)y the greater sul)mergence of Tapper Cretaceous time, the 

 first sedimentary expression of which is the Dakota sandstone, which in 

 turn overlaps the marine Lower Cretaceous formations of the Pocky 

 Mountain region and extends over areas that were above sealevel in 

 Lower Cretaceous time. 



