552 The America)! Geoloyist. DeceiniK-r, 1»'J5 
During the long period of erosion which ensued as the 
result of^the elevation, the streams, whose activity was thus 
greatly nenewed, excavated their new valleys to substantiall}' 
their present dimensions before the occurrence of the next 
episode recorded in the Ozark plateau. This excavation 
meant the removal of. first, the Lafayette deposits in the 
stream bed ; next, the cutting of the underlying solid rock 
to near baselevel ; and then a widening of tiie valley, by the 
undermining of its walls, to several times the width of the 
present streams in the case of the larger valleys, and to many 
times their width in the minor valleys. This period was un- 
doubtedly long, and meanwhile the first glacial stage or epoch 
had come and gone, leaving the Kansan drift sheet over the 
region north and east of the Ozarks. The first interglacial 
epoch, indicated in the region northward by a long period of 
subaerial erosion on the previouslj' ice-covered areas, I have 
been unable to separate from the preceding epochs in those 
portions of the Ozarks that I have studied. The Kansan 
glaciation was probably attended by increased precipitation 
in this region and great erosion, but its etfects have been 
obliterated during subsequent epochs. 
The Columbia Formation. 
The beds of loess which are found in great thickness along 
the Missouri valley within the state of Missouri and far to the 
northwest up that stream, and which also spread out in thin 
sheets over the lower upland country near by, do not continue 
into the Ozark region with any large development. Even in 
the border portions of the uplift the higher upland ridges are 
free from loess, although no higher than man}'^ loess-covered 
ridges to the north. But along such streams as the Osage and 
Gasconade rivers, the loess, or rather a loamy deposit resem- 
bling loess, extends to a great distance within the Ozark re- 
gion. It is at first a pretty definitely marked deposit, and 
forms low terraces, occasionally narrow second bottoms, and 
even spreads out over the lower ridges of the valleys. As the 
headwaters of the streams are approached, it becomes less dis- 
tinct, although still occasionally forming low and imperfect 
terraces; and finally, at an elevation exceeding l.OOO feet 
above the sea, this dejiosit is generally not present in any 
identifiable form, ('rossing the watershed and descending on 
