CHANNELS OF THE LATER STAGE 4G9 



froiri the east to join the clmnuel of the old Ponca Creek about 3 miles 

 east of Choteau Creek. 



A view across the Mosquito Valley from the northeast is given in 

 figure 2, plate 27, from point A on the map, plate 25. 



2. Geological evidence is as follows : 



Slaughter Creek shows chalk and shale throughout its southwesterly 

 course as far up as the old Mosquito Valley, wliere the shale gives place 

 to alluvial material. 



Near the southwest corner of section 2, township 93 north, range 63 

 west, or about one and a half miles west of the point where Cold Springs 

 Creek comes out on the bottom lands (point B on the map, plate 25), 

 there is a fine spring in a deep ravine about 10 rods back from the face 

 of the chalk wall. The sides of this ravine show that a channel has been 

 cut 25 to 30 feet into the shale and underlying chalk. This is inferred 

 from the level of the spring, which is about 60 feet above the bottom 

 land opposite. The channel, which merges into the river valley about 

 20 rods farther east, is filled mainly with silt closely resembling loess 

 and showing but little trace of stratification. It contains widely scat- 

 tered pebbles. Probably the lower part is sand, although no clear ex- 

 posure was found. The spring is highly charged with iron. 



Chalk banks appear along Cold Springs Creek north of the line of the 

 north bank of this old channel, which may be a third of a mile in width. 

 Eastward from this point for several miles the channel is marked by 

 numerous knolls which have been carved out of the silt which originally 

 filled it. A view of some of these is given in figure 2, plate 27, from a 

 photograph taken at point C on map, plate 25. 



The crossing of Choteau Creek Valley, by the old Mosquito Creek 

 channel is not clearly marked, but the valley of a small eastern tributary 

 of the Choteau shows that the chalk has been removed to the proj^er 

 depth in a valley north of a high hill just east of the mouth of Choteau 

 Creek. This feature is indicated on the map, plate 25. 



THE ANCIENT CHOTEAU 



The course of the ancient channel of Choteau Creek has not been 

 traced so clearly as the others and the principal evidence is topographic. 

 Inside the principal moraine, which here lies on a high ridge of pre- 

 Glacial or at least pre-Wisconsin age, there is a practically continuous 

 depression extending from the upper valley of Pease Creek across the 

 southern end of Lake Andes and along the middle portion of Choteau 

 Creek, running east of that stream for a few miles above its junction 

 with the Dry Choteau. Below that place it may have run southeast to 



