388 C. W. TOMLINSON 



4. Relation of Ordovician to Silurian in Utah. — In the vicinity 

 of Blacksmith Fork, northern Utah, the Laketown dolomite 

 (Silurian) has been found to be separated from the underlying 

 Upper Fish Haven dolomite (Richmond) by a well-marked dis- 

 conformity. 



HISTORICAL SKETCH 

 UPPER CAMBRIAN AND LOWER ORDOVICIAN 



Extent of submergence. — The present extent and variations in 

 thickness of the sediments deposited in the central Rocky Mountain 

 region during the Upper Cambrian -Lower Ordovician submergence 

 are shown in Fig. 7. It will be noted that this is not a paleogeo- 

 graphic map in the usual sense, as no attempt has been made to 

 indicate the original extent of these sediments, other than to show 

 them as continuous across relatively small areas, from which they 

 have certainly been removed by post-Cretaceous erosion. 



The much greater thickness of this group of sediments in western 

 and northern Utah and in Nevada, as compared with Wyoming 

 and Montana, means either that (1) sedimentation continued 

 longer or (2) took place more rapidly in the first-named region, or 

 that (3) during the succeeding interval of emergence erosion removed 

 a larger part of the series in question from Wyoming and Montana 

 than from the adjoining region to the southwest. Certainly the 

 first and last, and perhaps all three, of these factors combined to 

 produce the net result. A logical supposition is that the sea 

 retreated from northeast to southwest, so that deposition continued 

 in the Great Basin after erosion had begun on recently emerged 

 areas in Wyoming and Montana. The wide-spread uniformity 

 in type of the sediments in question indicates a similarly extensive 

 uniformity of conditions of deposition, and suggests that the second 

 factor was not of great importance. 



Paucity of clastic sediments. — The beginning of the Upper 

 Cambrian transgression in Utah was marked by a deposit of sand 

 of varying thickness. From that time until the close of the Beek- 

 mantown epoch no part of the region in which the St. Charles, 

 Garden City, Gallatin, and Upper Deadwood formations are found 



