9510

• UCREFRP Catalog Number: 9510
• Author: Jones, A. T.
• Title: A Cross Section of Grand Canyon Archeology
• USFW Year: 1986.
• USFW - Doc Type: Excavations at Five Sites Along the Colorado River.
• Copyright Material: NO

and dolor:li tes such as the Cambri.an Tapeats and the Permi an Coconino
sands tones and the Mi ssi ss i ppi an Redwall 1 imestone fonn prominent
cliffs. Igneous basalts of various ages and the Older Precambrian
metamorphic rocks are the most resistarttrocks in the canyon.
Whi 1 e the rocks exposed in the Grand Canyon are 01 d, the canyon
itself appears to be you ng. Whil e the exact age of the Colorado Ri ver
is still the subject of a lively controversy (t~cKee, ~Iilson, Breed and
Breed 1964; Hunt 1969), recent potassiur.l-argon (K-Ar) dating of lava
flows in the \'/estern portion of the canyon suggest that the river had
not established its present course until after about 5.5 million years
ago (Lucchitta 1979), but had cut to within 15 m (50 feet) of its
present position by about 1.2 million years ago (Hunt 1969; Young 1982).
No doubt th~ formation of the Grand Canyon has been closely associated
with rapid epeirogenic uplift of the region during the Miocene and
Pliocene, the opening of the Gulf of California between about 10 and
3 million years ago, and headward erosion and stream capture by the
Hualapai drainage system (Huntoon 1976; Lucchitta 1979).

!

t.,.::.l.
~-,' .i
Lol

Cl imate
In addi ti on to severe contrasts in temperature and preci pi tati on
typical of continental climates, the Grand Canyon region includes a wide
range of climate zones based on elevation and local variations in slope,
aspect, and microcl ima t ic factors. Mean annual precipitation values
range from about 12 to 25 cm in the Inner Gorge to greater than 75 cm
(30 inches) on the Kaibab Plateau (Whitney 1982). Generally,
precipi tation occurs during the winter months due to frontal systems
from the Pacific Ocean, and as convective thunderstorms during the late
summer. The Kaibab Plateau, \'/ith an average of greater than 318 cm (125
inches) of snowfall per year, receives most of its precipitation in the
winter. On the South Rim, however, summer precipitation.roughly equalS
winter precipitation.
Temperature in the canyon is characterized by strong yearly,
seasonal and diurnal fluctuations, and by a steep gradient due to
elevation differences and to the normal lapse rate. During the summer,
daytime high temperatures average about 240 C (mid-70s F) on the Kaibab
Plateau, in the upper 20s C (mid-80s F) on the Coconino Plateau, and

~'

J',

!'-.

I'.
i".
K:
~... ;

d

f...'j,
r !
L

1 :
1 1
L-J

[........1
U

~- .

15