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 it.

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.