392 C. W. TOM UN SON 



advances from Nevada northeastward, so that the thickness of its 

 deposits diminishes toward the northeast because of both a pro- 

 gressively shorter total duration of sedimentation in that direction 

 and, correspondingly, a progressively longer duration of the inter- 

 vening emergences, with accompanying erosion. A small amount of 

 sand, diminishing toward the northeast, was deposited in the shore- 

 ward portion of the sea during each advance. Aside from this, little 

 or no clastic material was laid down in the Devonian sea in the 

 central Rocky Mountain region until the later part of the period. 

 The Upper Devonian muds. — In the Upper Devonian the cal- 

 careous sediments accumulating on the sea bottom in Utah, Wyo- 

 ming, and Montana (see Fig. 12) were polluted by an influx of mud, 

 which in Utah attained a thickness of several hundred feet. The 

 Three Forks formation comprises the resulting shales and lime- 

 stones, together with a small thickness of arenaceous sediments, 

 only locally present, which may date from the beginning of the 

 Mississippian submergence rather than from the close of the 

 Devonian. The sea which occupied parts of Colorado and central 

 Utah in Upper Devonian time seems to have been connected 

 directly with the Three Forks sea for a short time only, near the 

 middle of the epoch. 



THE PRE-MISSISSIPPIAN INTERVAL OF EMERGENCE 



Depth of erosion. — It is certain that all of Wyoming, most of 

 Montana, and much of Colorado was above sea-level and under- 

 going erosion at the close of the Devonian period. The Lower 

 Mississippian sediments (Madison limestone) overlap all older 

 Paleozoic formations (see Fig. 13). The gradual truncation of 

 the Bighorn dolomite and the upper part of the Deadwood forma- 

 tion by the Madison limestone in the southern part of the Bighorn 

 Range is clearly due to pre-Mississippian erosion, and the varying 

 thickness of the Three Forks formation in northwestern Wyoming 

 and part of southwestern Montana is due to the same cause. The 

 relief of the final surface on which the Madison limestone rests, 

 however, is nowhere very pronounced. 



Where the Mississippian strata rest upon Ordovician rocks a 

 part of the hiatus may be attributed to erosion during Silurian and 



