464 J. E. TODD PRE-AVISCONSIN CHANNELS IN DAKOTA-NEBRASKA 



of the ice and of the country west flowed south through a channel pass- 

 ing near Creighton and Plainview into north fork of Elkhorn River. 

 This old channel is now at an altitude of about 1,700 feet. 



As the ice receded the main drainage outlet shifted eastward to a 

 lower valley which passes near Hartingion and Coleridge and connects 

 with the valley of Logan Creek. This channel now has an altitude near 

 1,600 feet. Still later the drainage flowed along the present course of 

 the Missouri, where the altitude is 1,200 feet. During the following 

 interglacial epoch the channels about Elk Point were deepened about 

 100 feet — that is, nearly to their present level. At that time the valley 

 of the Missouri Eiver above Yankton was not in existence, and the Nio- 

 brara crossed the line of the present Missouri Valley east of Springfield, 

 South Dakota, and entered the James Eiver Valley a few miles north of 

 Yankton. Ponca Creek also crossed the Missouri Valley and after a 

 northward bend joined the old Xiobrara near Springfield. A stream of 

 similar size came past Fort Eandall — whether from Pease Creek or Lake 

 Andes has not been ascertained — passed several miles north of Green- 

 wood, South Dakota, crossed Choteau Creek near its mouth, and joined 

 Ponca Creek a few miles farther east. 



The following facts are the evidence on which the above statements 

 are based. The events are considered in chronologic order: 



Channels of the Earlier Stage 



the creighton-plainview cha-^nel 



The principal evidences indicating tbat an ancient stream flowed past 

 Creighton are the existence of a shallow valley connecting the upper 

 portion of Verdigre and Bazile creeks with the tipper portion of the 

 north branch of the Elkhorn and the relations of tliis valley to the 

 earlier drift. Tt is about a mile wide and ')0 or 10 feet deep near Plain- 

 view, where it is least affected by erosion. It lies iietween the till- 

 covered area to the northeast and the area of Tertiary sands to the 

 southwest, on which there is but little drift. Traces of the channel 

 northward have been obliterated by the erosion of present streams and 

 by the effects of the Wisconsin ice. 



THE HARTIXGTON-COLERIDGE CHAXXEL 



That a stream flowed past Hartington and Coleridge is clearly indi- 

 cated by the extensive gravel deposits shown on the map, plate 25. 

 This gravel is first prominent a few miles east of Niobrara, Nebraska, 

 415 to 450 feet above the Missouri, and extending eastward across Bazile 



