Geology and Soils in Deserts of the Southwestern United States - Design with the Desert: Conservation and Sustainable Development - page 41

Colorado River established its current course and began to build its delta in the Gulf of
California with sediments derived from the Colorado Plateau. The Grand Canyon we see
today is the result of downcutting and sediment removal by the Colorado River over the
past several million years. The current arid climate became dominant in most areas of
the southwestern United States between 10,000 and 8,000 years ago (see Chapter 6), after
which time surficial geological processes typical of desert regions became important to
further evolution of the landscape.
2.5 Surficial Weathering Processes
Mechanical and chemical weathering processes reduce masses of exposed rock to smaller
and smaller particles, making sediments available for transport by wind and/or water
(Figure 2.3). While the products of mechanical weathering are most evident in deserts,
in the form of fragments that retain the mineralogy and chemistry of the source rocks,
chemical weathering processes typically facilitate the physical reduction of rock masses
by creating or exacerbating existing zones of weakness.* Common mechanical weathering
processes active in deserts include heating and cooling over each day/night cycle, which
leads to continual expansion and contraction of rock masses. This can weaken the rock
mass over time by formation of microcracks and fractures. Fractured rock has greater per-
meability, providing infiltration pathways for water and salts. Salt weathering is caused by
the formation and expansion of salt minerals in cracks, which can cause rock to flake apart
or disintegrate over time. In deserts where freezing and thawing occurs, water in cracks
FIGURE 2.3
Mechanical weathering and mass wasting are the dominant processes that are gradually eroding the spires
and mesas of Monument Valley in Utah and Arizona. Both the resistant cliff face and the less-resistant slope-
forming rocks are undergoing mechanical erosion. The smaller rock fragments on the lower slope are derived
both from the thinly layered siltstones forming the gently sloping base of the butte and the more massive, cliff-
forming sandstone above. Mass wasting processes are moving the smaller rock fragments downslope onto the
adjacent valley floor. (Courtesy of W.L. Stefanov.)
* For a more thorough presentation, see Cooke et al. 2