36 GEOLOGY OF ILLINOIS. 



nois river range from two to five miles in width. These bottom lands, how- 

 ever, are not so much elevated as those on the west side of the county, and are 

 generally too low and wet for cultivation. A portion of them are heavily tim- 

 bered with cottonwood, sycamore, soft maple, elm, ash, hackberry, honey 

 locust, linden, black walnut, water oak, hickory, etc. 



Loess. The river bluffs on both sides of the county, are capped with this 

 formation, which ranges in thickness from ten to sixty feet or more. It always 

 overlies the Drift, where both are present, and hence is of more recent origin, 

 and it also differs in its character and appearance from the Drift deposits. It 

 generally consists of buff or brown marly clays and sands, usually stratified, 

 and often so coherent as to remain in vertical walls twenty or thirty feet in 

 hight, when an artificial cut is made through it. On analysis, it generally 

 affords from seventy five to eighty per cent, of silica, from ten to fifteen per 

 cent, of alumina and peroxide of iron, from three to four per cent, of lime, and 

 one to two per cent, of magnesia. Its greatest thickness is usually on the top 

 of the river bluffs, and in the lateral valleys immediately adjacent to them, and 

 from thence it thins out generally towards the summit level of the interior. 

 In the vicinity of Chambersburg, in the northeast part of this county, the 

 Loess forms the main portion of the bluff, so far as can be seen, and appears 

 to be at least sixty or seventy feet in thickness. The timbered lands adjacent 

 to the bluffs on both sides of the county, are usually underlaid by this forma- 

 tion, and it furnishes a light porous subsoil, which is admirably adapted to the 

 growth of fruit trees, vines and small fruits. At many localities, it contains a 

 variety of fossil shells, which present the usual bleached and water-worn ap- 

 pearance of the dead shells of our ponds and bayous. It also affords a variety 

 of calcareous concretions, which assume many imitative forms, some of them 

 resembling potatoes, and others taking discoidal forms like the "clay stones" 

 in the drift clays of New England. It gives origin to the bald knobs so fre- 

 quently met with along the river bluffs, and is often rounded into natural 

 mounds, which have been very generally used by the Indians as burial places 

 for the dead. The bones of extinct mammalia are often found in the marly 

 beds of this formation, and are associated with both land and fresh water shells, 

 which would indicate this to be a sedementary accumulation in a fresh water 

 lake, or rather series of lakes, into which the land shells and bones of land ani- 

 mals were carried by rivers, or smaller streams of running water. Bones of 

 the Mammoth, the Mastodon, and the Casteroides, or fossil beaver, have been 

 found in this formation in this State, and also the flint arrows and other imple- 

 ments of primeval man. 



Drift. The lowest division of the Quaternary system comprises a series of 

 variously colored clays, containing gravel and boulders, to which the term 

 " Drift " is usually applied, because the materials of which it is composed have 



