STRUCTURE OF THE DRIFT BORDER. 51 



passed below the level of the outer border plain before entering the black 

 muck which is thought to separate the Illinoian drift from the Kansan. In 

 such cases the well is supposed to have struck into a valley which had been 

 excavated in the earlier sheet of drift, though there is a bare possibility 

 that an older soil horizon is struck. The following section of a well on 

 the farm of F. Smith, about a mile south of Yarmouth, will illustrate the 

 condition just mentioned: 



Section in well on farm of F. Smith, a mile south of Yarmouth, Iowa. 



Feet. 



Yellow till, becoming gray below (Illinoian) 36 



Sand, with thin beds of blue clay and also of cemented gravel, probably in part Illinoian and in 



part alluvial 73 



Black muck, containing wood (Yarmouth) fi 



Sand and gravel, probably alluvia] 8 



Gray silt, apparently pebbleless, probably alluvial 15 



Blue till (Kansan) 42 



Depth 180 



This well is on the crest of the ridge at a level 60 or 70 feet above the 

 outer border plain. The black muck is therefore at a level about 40 feet 

 below the plain. A well in the neighboring section on the south, at an ele- 

 vation 25 feet lower, enters rock at a depth of 182 feet. 



One of the thickest drift sections found along this drift border is in a 

 well made by Anton Totemeir near New London, Iowa, in sec. 19, T. 71, 

 R 4 W., which struck rock at a depth of 276 feet. The section of the well 

 indicates that only the upper 40 feet should be referred to the Illinoian drift 

 sheet, The section as reported by Mr. Totemeir is as follows: 



Section in well of Anton Totemeir, near Neiv London, Iowa. 



Feet. 



Pebbly yellow clay (Illinoian) 30 



Pebbly blue clay (Illinoian) 10 



Deeply stained, reddish -brown pebbly clay (Kansan) 12 



Blue pebbl y clay, with thin beds of sand, possibly including pre-Kansan as -well as Kansan 224 



Total 276 



The well mouth being about 750 feet above tide, or 240 feet above the 

 Mississippi River in Burlington, the rock floor at this well is but a little 

 lower than the bed of the present Mississippi at Burlington. Of the several 

 wells along this ridge in Des Moines County none have been found to enter 

 rock at less than 120 feet, and probably at least half this drift is older than 

 the Illinoian. 



