GEOLOGIC HISTORY. 
67 
river meanders as subject to scour and fill, and to entertain the sug- 
gestion that the deeper portions down to 100 feet or more may be 
similarly affected. " 
Some instructive data were secured during the construction of 
bridges across Mississippi and Missouri rivers. N. M. Fenneman a 
has made a special study of this subject and states that the depth of 
scour, as shown by the occurrence of buried wreckage and also by 
direct soundings, is 100 feet or more, and that the occurrence of 
gravels resting on freshly polished rock at a maximum depth of about 
335 feet is interpreted as showing that the river transports gravels of 
considerable size and actively erodes its bed to this great depth. It 
seems possible, however, that some part of this material may be due 
to permanent filling in a glacial or preglacial valley, such as are known 
to exist in the Mississippi Valley. 
It is probable that the depth to bed rock shown by the soundings 
at the various dam sites is largely due to the normal action of the 
river, cutting deep in times of flood and filling the channel at times of 
low water. It is equally probable, however, that the channel is to 
some extent permanently filled. 
RESUME OF HISTORY. 
For convenience of reference, the succession of epochs and their 
possible time relations are tabulated below. 
Table of events in geologic history in western Arizona. 
Period. 
Epoch. 
Deposition epoch. 
Erosion epoch. 
Recent 
Pleistocene ... 
\ 
11. Formation of flood plains; accu- 
mulation still in progress, 
no 
Rejuvenation of streams; Colorado 
reexcavates old valley in part and 
cuts several short rock gorges. 
Rejuvenation of streams; Colorado 
River, flowing west of the Black 
Mountains, lowers its bed 2,01 feel 
or more and cuts Aubrey. Mohave, 
Black, and Boulder canyons. 
Uplift of the Colorado Plateau; ero- 
sion of Grand Canyon; How of Colo- 
rado River in the Detrital-Sacra- 
mento Valley. 
9. Deposition of the Chemehuevis 
gravel. 
8 
3 
u 
3 
7. Widespread aggradation and vol- 
canic eruption. About 2,(H)0 
feet of the Temple Bar con- 
glomerate deposited. 
6 
rtiary. 
. 
Pliocene 
Miocene 
Eocene 
5 
[4. Local deposition; eruption of 
rhyolite and andesite. 
Erosion of the Detrital-Sacramento 
and other valleys. 
Peneplanation and recession of the 
Yampai Cliffs. 
( feneral degradation. 
I 
[2. Eruption of andesite. 
a Personal communication. 
