Itirer ]'(i//ciis of I In- Ozark J'hdcaii. — II crslirn . ':\\~ 
wide as the present streams, the greater portion of the period 
of erosion is known to have been occupied by the stream in 
widening its valley by meandering and undermining its walls, 
which talve place not more than a few times 7iiore rapidly in 
a short than in a long gorge. My observations show that the 
cross-section of a gorge does not de|)end so much on its length 
as it does on the depth and on the nature of the rock. A 
comparison of short valleys and ravines, having nearly equal 
drainage areas, of Pleistocene age in northwestern Illinois, and 
of the age of the lower troughs in southern Missouri, shows 
that the latter are not many times larger thaii the former. In 
short, from an examination of all the phenomena in both re- 
gions, I feel safe in asserting that the lower troughs or present 
immediate valleys of the streams in the Ozark region have 
not required more than two or possibly three times as long for 
their excavation as the Pleistocene gorges and ravines of 
northwestern Illinois. 
The earliest known glaciation of Illinois, although many 
times older than the last or that wliich formed the great mo- 
raine systems, was still perhaps nearer the middle than the 
beginning of the Quaternary era ; and, as the erosion of the 
rock-gorges of Illinois has occupied j^erhai^s three-fourths of 
the time since that first glaciation, its length is probably not 
over half that of the (Quaternary era. Therefore the i)erio(l 
of erosion of the lower troughs of the Ozark region, if only 
two or three times as long, would correspond approximately 
to the whole of this era. This reasoningis far from conclusive, 
but, as I hope to show, there is other and even stronger evi- 
dence that the channeling of these valleys began near the be- 
ginning of Quaternary time. 
Mi:AXDKKIN(i CoUKSKS OK THE StKKAMS. 
The cause of tlie exceeding crookedness of theO/ark valleys 
has been much discussed during the past few years, two chiel" 
Jiypotheses being advaiu-ed. One attributes the interminal)le 
windings of the streams to the effect on them of systems of 
Joint i)lanes develojx'd in the Ozarks. ^^'!lile some of the 
minor crooks of the vjiUeys may be due to this cause, I <-an 
find nothing l)ut contradictory evidence in the application of 
this theory to the larger valleys. Slight aiitielinals are locally 
developed in the Ozarks, but the streams maintain their 
