314 TRANS. ST. LOUIS ACAD. SCIKXCE. 



practically destitute of alluvial deposits ; each considerable tribu- 

 tary being tiivisible into three portions, in the first of which the 

 declivity is high and the gathering waters do not deposit their 

 detritus, in the second of which the declivity is less and deposi- 

 tion results from seasonal and non-periodic variation in stage, 

 while in the third the stream approaches base-level and flood- 

 plain deposition prevails. 



No tei racing was observed in the region, if the occasional rock- 

 shelves and spurs due to the presence of obdurate ledges in the 

 sub-terrane be excepted. 



Glacial Deposits. — The entire area, with the exception of the 

 valleys of the rivers and their principal tributaries, is overspread 

 by a mantle of glacial drift which conceals the inequalities of the 

 subjacent surface, and upon which the characteristic topography 

 already described is impressed. The observed thickness of the drift 

 deposits varies from a foot or two to the 30 or 40 feet sometimes 

 exposed in shafts and borings ; and the average thickness doubt- 

 less approaches and perhaps exceeds the latter figure. Drift ma- 

 terials are occasionally (though rarely) exposed beneath the allu- 

 vium in the bottoms of the river channels ; but in such situations 

 they are perhaps rearranged. 



So far as seen, the drift consists of massive or obscurely stra- 

 tified, tenacious, and nearly impervious yellow or bufl' clay^ 

 containing rather rare bowlders and pebbles disseminated through- 

 out, but most abundantly below ; the pebbles and bowlders con- 

 stituting about one or two pr. ct. of the mass. Perhaps half of the 

 pebbles and bowlders are erratic, i.e. crystalline rocks from north- 

 ern Minnesota or still farther northward ; while the remainder are 

 local or sub-local — i.e. limestone, sandstone, shale, etc., mainly 

 from the Palaeozoic rocks of northern Missouri and Iowa. Many 

 of the bowlders and pebbles, both local and err.itic, are beautifully 

 striated and polished. In one case (in an abandoned railway 

 cutting a mile and a half northwest of Macon), a block from the 

 first limestone ledge above the Bevier coal was found to be dis- 

 tinctly striated and to have its angles truncated, though moved 

 but a few feet from its original position. Calcareous nodules, 

 identical widi loess-ki/idchen, abound in the drift, especially in 

 the upper portions. They sometimes occur in vertical or hori- 

 zontal lines, in which they occasionally merge and form nearly 



