44 
Psyche 
[March 
has the unusually steep average gradient of 43 feet per mile for the 
50 miles length of the canyon. Within the monument boundaries the 
rate of fall is 95 feet per mile — all without a major falls. The 
downcutting power of the river is so great that the minor tributaries 
flowing from the rims generally have not been able to keep pace and 
are left as hanging tributaries or narrow precipitous gulches. Only a 
few actually reach the river. The canyon is probably not more than 
two million years old, representing an average cutting speed of one 
foot per thousand years. 
Several historical accidents contributed to form the Black Canyon 
(Hansen, 1962) : 1) the Gunnison River was already cutting through 
soft overlying lava flows when the Gunnison Uplift gradually rose 
for a second time. The river was committed to the channel and was 
just able to keep cutting through the hard basement material in pace 
with the rising land. 2) The rock down-stream from the upthrust 
was much softer, thus maintaining the river’s cutting gradient through 
the canyon. 3) The water catchment capability of the mesas which 
were formed by the upthrust is meager and most of the streams are 
dry except for a month during the spring snow melt. Thus side 
canyon erosion is negligible or directed away from the canyon rims. 
4) Fractures and joints in the rock are vertical or, in many places, 
virtually non-existent. Lateral erosion then results in perpendicular 
sheering and nearly vertical walls. All of these factors contribute 
to the formation of a canyon with an average depth equal to or 
greater than its average width, with cliffs falling sheer 1000-2000 feet 
to the river below. At the Narrows the depth is 1750 feet, the 
distance between the rims is 1100 feet, and the width of the canyon 
at the river is 40 feet, a depth-to-width ratio unmatched by any other 
major North American canyon. 
The mesa formed by this block upthrust and split by the Gunnison 
River averages 8000 feet in elevation ± 300 feet, whereas the sur- 
rounding valleys to the south, west and north average less than 6000 
feet. The vegetation on the mesa top is characterized by a chaparral 
composed of Gambel’s oak, service berry, mountain mahogany and 
isolated stands of Utah juniper and pinyon pine. Grassy openings 
are scattered through the thick brush and the populations of Arphia 
conspersa reach their greatest density in these open areas. Average 
rainfall is estimated from records kept at the Monument Head- 
quarters on the South Rim to be about 10 inches per year, at least 
half of it falling as snow during the winter. For the past three 
years at Monument Headquarters the prevailing winds at 1 :00 P.M. 
have been south to northwest, with the mode in the west and south- 
