HISTORY OF THE LOWER MISSOURI bO 



In Atchison County the ancient Platte received a tributary that crossed the 

 Missouri between Napier and Craig, where erosion of the soft drift has helped 

 to produce a very wide floodplain on the Missouri. The drift in this tributary 

 is about 180 feet thick near Tarkio, as shown by the artesian wells along 

 Tarkio Creek. On the western side of Grundy County and also in the eastern 

 part of this county large tributaries were received from the north. The east- 

 ernmost headed in Putnam County near the ancient Platte-Mississippi divide, 

 in the Mendota coal field. Some of the branches cut down through the coal 

 bed and the drift-filled gorges encountered in certain of the coal mines are the 

 so-called "clay faults"' of the miners. 



The width of the ancient Platte Valley as determined by detailed mapping 

 in Grundy County is six miles or more. The minimum occurs where it crossed 

 the now largely buried escarpment of the Bethany Falls and other limestones 

 at the base of the Kansas City formation. In comparison, the width of the 

 present Missouri Valley where it crosses this escarpment is about five miles. 



In Livingston County another large tributary seems to have occupied the 

 valley of Shoal Creek and that part of Grand River Valley to the east. Other 

 small tributaries may have been received from the east in Linn and Chariton 

 counties. Throughout this system thick beds of sand and gravel overlain by 

 boulder clay are common. 



The gradient of the old Platte was about one foot per mile between Nebraska 

 City and the mouth of the Grand. The lowest elevation of the surface of bed- 

 rock known at different points along the course is shown in the following table : 



Altitude of Bedrock in the Valleys of the Ancient Platte and its larger 



Tributaries 

 Location Altitude 



Feet 



Nebraska City 758 



Tarkio (on tributary) 726 



Clyde 660± 



Saline (on tributary) 610 



Near Gait, less than 600 



Triplett 554 



The gradient of the present Missouri between Nebraska City and the mouth 

 of the Grand is about 0.8 feet per mile. 



Pbeglacial Kansas Valley 



The preglacial Kansas Valley followed closely the present Missouri Valley 

 between Kansas City and the mouth of the Grand. Its largest northern trib- 

 utary in Missouri entered the State near Saint Joseph, flowed southeast via 

 Trimble and through the valley now partly reexcavated by Fishing River. 

 Where Smiths Fork of Platte River (not to be confused with the "Platte" of 

 the previous pages) crosses the Saint Joseph Valley its floodplain becomes a 

 mile wide, though less than half that width to the north and south. 



A tributary of the Saint Joseph Valley in Platte and Clay counties possesses 

 some peculiar features. Its bed is covered with a stratum of rounded lime- 

 atone boulders overlain by gravel, sand, and drift, the latter resting uncon- 



