Bajadas Coalesced alluvial fans Bajadas Death Valley Bajada

Bajadas Coalesced alluvial fans

Bajadas Death Valley

Bajada

N Bajata East of Panguitch

Alluvial Slope parallel drainages, wide interfluvs with eolian activity No sign of fan form Steep or shallow gradient Often fine grained

Alluvial slope

Pediments

Pediment Originally defined by GK Gilbert in Henry Mts Erosion and retreat of mountain front by planation surfaces (lateral planation of streams) Streams swipe back and forth cutting bedrock bench and leave only veneer of sediment gen def: Erosional surface cut by streams on bedrock or older sediment on Piedmont leaving veneer of alluvium Conceptually like piedmont strath terrace

Pediment Form May be gullied Commonly have inselbergs Abrupt change in slope at base of Mtn

Mountain front Pediment

Pediment surface in granite

Inselbergs on Pediment

Characteristics of Pediments 1. Slope away from the Mt front 2. Erosional in origin 3. Usually cut into the same rocks that underlay Mts best developed on hard bedrock 4. May be covered by veneer of alluvium (no more than a channel depth 5. Usually associated with arid environments maybe just better exposed in arid environments

Processes formation of Pediments: 1. Lateral stream planation Does not explain interfluvs, inselbergs 2. Sheet wash / sheet floods from hillslopes Rare, not erosive enough 3. Striped etch plains (Pediplains) Weathering is primary process Weathering front creates regolith to certain depth Core stone and etch front during past climate Subsequent erosion removes regolith Most applicable to granite terrains

Inferences from Pediments 1. Disequilibrium landforms - Formed under different weathering conditions now relict forms that have yet to adjust to present conditions 2. Balance between sediment supply and transport across piedmont sed load transport = basin seds abut Mtn front sed load transport = pediment forms

Inferences from Pediments 3. Base-level stability Get alluvial fills/fans where accommodation space is being formed (tectonic drop down of valley) Get pediments where there is NO accommodation space and mountains laterally retreat, leaving a transport slope graded to a relatively stable base level

Remember hillslope forms