TOPOGRAPHY AND COMPOSITION 111 



hence farms having the general level of all the surrounding country are 

 cultivated in places to the water's edge. 



CLAY CONSTITUENT OF THE TILL 



Color and cowposiiion. — The lowan till, as a rule, is a pale yellow clay, 

 rich in minuteW divided lime carbonate and carrying many pebbles, 

 cobbles, and small boulders that, with scarcely an exception, are all of 

 foreign origin. Limestone fragments are comparatively rare. The upjjer 

 part of the till has been modified by a number of agents, the most pro- 

 nounced change being due to the development of a deep, black, surface 

 loam varying from a few inches to two or three feet in thickness. 



Condition as to iveathering and leaching. — The absence of signs of weath- 

 ering in the upper zone of the lowan till is a very prominent character- 

 istic, especially to one familiar with the strongly pronounced ferretto 

 zone resulting from weathering in the older sheets of drift. The lowan 

 is practicall}'' unweathered. Oxidation is no more perfect at the surface 

 than at the bottom, and the reaction for lime carbonate is as energetic 

 at the grass roots as it is ten feet beneath the surface. Absence of oxi- 

 dation and leaching is one of the distinguishing characteristics of the 

 lowan when compared with the older drift-sheets of the Mississippi 

 basin. 



10 WAN BOULDERS 



Composition, size, and weathering. — Among the more conspicuous feat- 

 ures of the lowan drift are huge boulders of crystalline rocks scattered in 

 great numbers over the surface of the region in which the formation is 

 t3'picall3^ developed. In no other drift-sheet are boulders so prominent a 

 characteristic of the prairie landscapes. In composition the majority of 

 the boulders are light colored, coarse textured granites, in which large 

 crystals of feldspar are the most abundant constituent. A few quartzite 

 boulders occur, and there are some of the dark fine grained traps, basalts, 

 and greenstones, but these are inconspicuous in size as well as in num- 

 bers. The prevailing types are granites, and the great size of some of 

 the individual masses is such as to excite the interest and attention of 

 even the most casual observer. Granite boulders 30 feet in diameter are 

 not uncommon. In certain localities, for the distribution is not uni- 

 form, blocks having diameters ranging from 12 to 20 feet may be counted 

 by the score on every square mile, while boulders varying from 2 to 6 

 feet across can be reckoned by the hundreds within the limits of an 

 ordinary farm. Furthermore, the granites, as a rule, show little or no 

 signs of deca}^ from weathering during the period of their exposure since 

 the retreat of the lowan ice. The majority are perfectly sound even at 



