Introduction to the
Mojave National Preserve
& Ancient Lakes
Weathering & Erosion
Faults & Active Tectonics
Pediments & Alluvial Fans
& Older Surfaces
Sand Dunes & Dust
3D Geology Tour
Stream Terraces and Older Surfaces
Stream terraces form when streams carve downward into their floodplains,
leaving discontinuous remnants of older floodplain surfaces as step-like
benches along the sides of the valley. Stream terraces are common throughout
the Western United States. In the context of this discussion on the Mojave
region, older surfaces represent flattened areas (plateaus, mesa,
uplands areas, hillside benches) that are stable or isolated, neither
experiencing significant rates of sediment buildup (aggradation) or down
cutting by erosion. These older surfaces may have no clear or obvious
connection to a more modern drainage system in a particular area. Terraces
and older surfaces preserve or display unique characteristic soil profiles
or weathering characteristics because of their long-standing isolation
from stream erosion.
Many factors influence why streams episodically carve into their floodplains,
forming stream terraces. Because stream terraces are typically widely
distributed along steams throughout a region, changing climatic conditions
are likely a most important contributing factor to their formation. Streams
broadened their floodplains when sediment supplies are high and down cutting
by stream erosion is abated. In cool, wet periods, plants typically cover
the landscape, and hence sediment supply is low; enhanced moisture increases
stream flows, and streams draining mountainous regions will cut downward.
During dry periods, plants don't provide enough cover to prevent intense
erosion during infrequent storms. As a result, high sediment yields may
result in the backfilling of stream channels. This natural feedback system
is much more complex than this because many other processes occur simultaneously.
Under cooler, wetter conditions during an ice age, soil development and
weathering processes proceed faster due to more frequent wetting and drying,
more freeze-thaw cycles, and increased biological activity (particularly
root penetration). Soils formed during extended wet periods can be released
as sediments once the groundcover is removed during drought conditions,
especially by wildfire followed by a rainstorm.
Climate is also a factor in the development of caliche (calcium-carbonate-rich
crusts or soils that form in desert conditions). In North America, caliche
is found in arid or semiarid regions of the western states. In many places
in the Mojave region these calium-carbonate-rich crusts form a resistant
caprock along stream terraces.
|Caliche-cemented gravels (pale zone topped by a ledge) form the
resistant cap rock of older Pleistocene terrace surfaces along the
sides of the modern wash. In the distance, the surface of an older
quaternary alluvial fan is preserved intact (partly due to a resistant
caliche bed preserved at the surface). The high core of the Providence
Mountains in the distance consists mostly of Paleozoic limestone and
dolomite rock formations; these rocks provide calcium carbonate to
the alluvium and enhance caliche development.
|A boulder of the caliche-cemented gravel has been eroded and re-deposited..
It displays rock fragments similar to the modern stream gravels surrounding
|Morning sunlight highlights the incised remnants of an older (Pleistocene
or Pliocene) alluvial fan along the mountain front of the Granite
Mountains. The smoother modern (Holocene) alluvial fan surface stands
out in the foreground (in mountain shadows). The incised and eroded
condition of this fan suggests different possibilities.
|A desert pavement (a surface gravel deposit of tightly packed pebbles,
layered just one pebble thick and generally devoid of vegetation)
is abundant on Pleistocene-age surfaces, particularly in the mid-fan
regions. Pavements such as this occur in areas where the stream flow
is restricted to relatively stable channels nearby. Note how little
relief exists on this alluvial fan surface on the eastern flank of
the Providence Mountains.
|A close-up view of a desert pavement shows that gaps between rock
fragments are small or rarely visible (hiding the accumulated dust
underneath). Wind and episodic rains keep the surface free of dust,
and plants have a difficult time becoming established due to lack
of soil. The surface temperature difference between night and day
during the summer may range over 100 degrees Fahrenheit. This daily
temperature difference may play a role in the formation of these pediment
surfaces. Most of the rock fragments shown here are dolomite and limestone.