544 -E. RUSSELL LLOYD AND C. J. HARES 



withdrew still farther to the east and with the beginning of Lance 

 time continental sedimentation was inaugurated over a large part 

 of the eastern Rocky Mountain and Great Plains region. The 

 last advance of the sea into the region of the Dakotas was approxi- 

 mately contemporaneous with the final dying-out of the Cretaceous 

 dinosaur fauna, but there was no marked break in the continuity 

 of sedimentation until the close of the Fort Union. Then the 

 area suffered deformation and the erosion of thousands of feet of 

 strata before the deposition of the later formations which rest 

 unconformably on the Fort Union and all the underlying formations. 



IS THE LANCE FORMATION TERTIARY OR CRETACEOUS? 



In a general consideration of the age of the late Cretaceous or 

 early Tertiary formations in the Rocky Mountain and Great 

 Plains regions there are five lines of evidence which must be taken 

 into consideration, and the decision at which any geologist will 

 arrive will depend largely upon the relative importance which he 

 gives to one or another of these lines of evidence. In a paper 

 dealing primarily with the stratigraphy of a limited area in the 

 Great Plains the larger and more general problem can be outlined 

 only very briefly. 



In the opinion of the writers the greatest weight should be 

 given to the evidence of diastrophism. This, however, is of 

 undetermined significance. The greatest break in the sedimentary 

 record comes at the close of the Fort Union where there was every- 

 where, so far as known, in the Rocky Mountain and Great Plains 

 regions an interval of long erosion and in many places of extensive 

 folding. An earlier but apparently much less extensive break in 

 the continuity of sedimentation is represented by erosional uncon- 

 formities of greater or less extent at the base of the Raton, Dawson, 

 Arapahoe, and "Upper Laramie" formations in New Mexico, 

 Colorado, and Wyoming. An attempt to show that this uncon- 

 formity is general throughout the northern plains region has, as 

 previously pointed out, met with little success. In the Gulf and 

 Atlantic Coastal Plains provinces the interval between the Creta- 

 ceous and Tertiary systems is marked by a great change in marine 

 faunas and presumably by a profound break in the sedimentary 



