Pediments and Alluvial Fans
The term, mountain front, is an imaginary borderline between a mountainous area and a low, gently dipping plain (either a pediment or alluvial fan). A pediment is a gently sloping erosion surface or plain of low relief formed by running water in arid or semiarid region at the base of a receding mountain front. A pediment is underlain by bedrock that is typically covered by a thin, discontinuous veneer of soil and alluvium derived from upland areas. Much of this alluvial material is in transit across the surface, moving during episodic storm events or blown by wind.
Pediment-forming processes are much-debated, but it is clear that rocks such as granite and coarse sandstone (and Tertiary conglomerate made up of boulders of these rocks) form virtually all pediments in the Mojave Desert. These rocks disintegrate grain-by-grain, rather than fracturing and then being reduced in grain size by alluvial transport processes.
Alluvial fans are aggrading deposits of alluvium deposited by a stream issuing from a canyon onto a surface or valley floor. Once in the valley, the stream is unconfined and can migrate back and forth, depositing alluvial sediments across a broad area. View from above, an individual deposit looks like an open fan with the apex being at the valley mouth. Typically the fans formed by multiple canyons along a mountain front join to form a continuous fan apron, termed a piedmont or bajada.
Large areas within the Mojave Desert are pediment surfaces. These pediments reflect both the antiquity of some mountain structures in the region and the persistent arid climatic conditions in the region. Perhaps the most notable pediment in the region is Cima Dome, a very broad, shield-shaped upland area within the Mojave National Preserve (below). This great, gently-sloped upland area represent a region where desert-style weathering and erosion has stripped away most of the relief to the point that the erosion keeps pace with surface weathering and that surface gradient is gentle enough to prevent gully-style downcutting. Isolated rocky hills or knobs that rise abruptly from an erosional surface in desert regions are called inselbergs.
The development of pediments and alluvial fans is progressive with the uplift of mountains and subsidence of adjacent basins. Pediments reflect a relative "static equilibrium" between erosion of materials from upland areas and deposition within an adjacent basin. The slope of the landscape is gentle enough that weathering and transport of sediments from upland areas and the pediment that no significant stream incision occurs. In many areas throughout the Mojave region it is nearly impossible to see where a pediment ends and alluvial fans begin, however, geophysical data and water-well drilling shows that in many places sediment filled basins do occur adjacent to pediment areas.
The impact of climate change on alluvial fans has been the focus of much research. Studies show that a period of elevated alluvial fan deposition occurred between the time of the Last Glacial Maximum (about 15,000 years ago) and the beginning of arid conditions in the early Holocene (about 9,400 years ago). McDonald et al, (2003) suggest that the climatic transition from seasonable wet conditions to arid conditions, punctuated by extreme storm event (possibly associated with tropical cyclones) may be responsible for this change. Today, heavy rainfalls rarely provide enough precipitation to allow enough surface runoff to occur on highly porous soils and colluvium. Only during major stream event will water discharge in volume and intensity to move material from mountain source areas to lower fan areas. In addition to extreme storm events,the buildup of alluvial fan deposits at this Pleistocene/Holocene time transition may be linked with the transition from widespread plant cover to the more barren character of the modern Mojave landscape. Die-back of plants would decrease rooting, making more mountain-side material available for erosion and transport to alluvial fans.
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USGS Western Region Geology and Geophysics Science Center