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, years ago and the beginning of arid conditions in the early Holocene about 9, years ago. McDonald et al , 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.
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Outline Introduction. Postulated epigene pedimentation processes. Mantled pediments in sedimentary terrains. Contrasts between pediments and platforms. Full text PDF k Send by e-mail.
Introduction 1 The landforms known as pediments were so called by Gilbert , p. How does a blowout form? What is a ventifact, and how does one form? Describe and explain the general cross section of an unanchored desert What is a playa, and why does one form? What is the difference between a playa and a salina? Why is a desert lake in a basin of interior drainage likely to be a saline lake? Explain differential weathering and erosion.
Describe the formation of an What processes are involved in the formation of an alluvial fan? What is the arrangement, or sorting, of alluvial material on the fan? Describe two landforms associated with basin and range topography. What is the role of chemical weathering Which is not a type of dune?
What are cross beds, and how do they form?
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