28 J. E. TODD — HYDROGRAPHIC HISTORY OF SOUTH DAKOTA 



quite large pebbles in the Potsdam sandstone. Probably the Archean 

 core of the hills was land exposed to the attack of the sea and contrib- 

 uting largely to the material of that formation. 



As Paleozoic time advanced, the western area gradually diminished, 

 both by erosion and subsidence, while the eastern area, on the whole, 

 increased several fold by gradual elevation. 



An episode marked by the Silurian beds near Deadwood was a differ- 

 ential subsidence of the Black Hills region to the north, with a probable 

 elevation on the south. The beds mentioned are found only around the 

 north end of the hills. 



Of the Devonian little is known. Thirty feet or more of green shales 

 are doubtfully referred to that age, but no distinct break is found. Dur- 

 ing the Carboniferous a shallow sea surrounded the island, over which 

 several feet of limestone were formed. Harney peak at that time, as we 

 believe, was not wholly submerged. 



With the close of the Paleozoic the sea began to retire from the Black 

 hills, and probably continued the same movement around the eastern 

 area. The extent in either case is not definitely known. As no Paleo- 

 zoic beds have been recognized in the numerous wells which have been 

 put down to crystalline bed rock east of the Missouri, it is doubtful 

 whether the sea occupied that region during the Paleozoic and early 

 Mesozoic. If it did at any time, all of the deposits there formed seem 

 to have been completely removed. 



The sea attained a minimum of extent in our state during the Tri- 

 assic, if we read rightly the significance of the gypsiferous red clays barren 

 of fossils ; but with the beginning of the Jurassic began a steady gain. 

 The gain seems to have been very rapid during the early part of the 

 Dakota epoch, but the sea was quite shallow. This advance continued, 

 probably, until the later part of the Niobrara stage of the Colorado. 

 During the Fort Pierre epoch we may suppose it again retired, until at 

 the opening of the Laramie it merely verged on our northern border. 

 The evidence is that during that age the northern part of our State alter- 

 nated between shallow seas and marshes with lakes. We have no dis- 

 tinct trace of the drainage of that epoch, unless it be in the Little Mis- 

 souri and the upper part of the Belle Fourche ; also in the upper courses 

 of the streams draining the Black hills. We can easily believe that the 

 drainage in the state elsewhere was generally toward the north, as we 

 have indicated on the map. 



The same general relation continued through the Eocene. The streams 

 gradually deepened their valleys. This fact is attested by the eroded sur- 

 face of the Montana below the Tertiary beds generally, and by deposits 

 of gravel which are found under the Tertiary beds along French creek 



