Part 9Part 9

EROSION BY EROSION BY WATER AND WATER AND

WINDWIND

Aerial oblique view of the Aerial oblique view of the South Dakota BadlandsSouth Dakota Badlands, , developed on near-horizontally bedded developed on near-horizontally bedded Pierre ShalePierre Shale. . Note complete absence of stabilizing vegetation.Note complete absence of stabilizing vegetation.

Retreating escarpment of the South Dakota Badlands. Note Retreating escarpment of the South Dakota Badlands. Note isolated “isolated “sod tablessod tables” which are more resistant to erosion. ” which are more resistant to erosion.

The the White River drainage is eroding soft sediments of the The the White River drainage is eroding soft sediments of the Pierre ShalePierre Shale lying above the more resistant lying above the more resistant Chadron FormationChadron Formation. .

The The escarpmentescarpment is comprised of the Pierre Shale, which contains is comprised of the Pierre Shale, which contains montmorillonitemontmorillonite, a weathering product derived from volcanic ash , a weathering product derived from volcanic ash (tuff). Outcrops containing (tuff). Outcrops containing smectite claysmectite clay are usually barren of are usually barren of vegetation and often exhibit a crinkled coral-like appearance, due vegetation and often exhibit a crinkled coral-like appearance, due to severe expansion when wetted.to severe expansion when wetted.

Lacustrine sedimentsLacustrine sediments can be particularly susceptible to can be particularly susceptible to erosionerosion. . This shows spires in Cathedral Gorge State Park near Pioche, This shows spires in Cathedral Gorge State Park near Pioche, Nevada. The material is the Pliocene age Panaca formation, Nevada. The material is the Pliocene age Panaca formation, which contains which contains montmorillonite. montmorillonite. Smectite claysSmectite clays can swell can swell between 300% and 800% of their dry weight, becoming extremely between 300% and 800% of their dry weight, becoming extremely sticky and slippery when wetted. sticky and slippery when wetted.

Block diagram illustrating the qualitative Block diagram illustrating the qualitative relationship between relationship between runoff velocityrunoff velocity and and stages of the erosion processstages of the erosion process

Limiting values of runoff velocity versus Limiting values of runoff velocity versus erodability for various geologic materialserodability for various geologic materials

Raveling cut in Pliocene age siltstone in Raveling cut in Pliocene age siltstone in Orinda, CA. Rapid erosion, acidic nature of the Orinda, CA. Rapid erosion, acidic nature of the strata, and southwest exposure are factors that strata, and southwest exposure are factors that combine to prevent effective vegetation of the combine to prevent effective vegetation of the slope, resulting in rapid erosion.slope, resulting in rapid erosion.

Stepped cut in preCambrian metamorphic rocks along Stepped cut in preCambrian metamorphic rocks along Interstate 210 near Sunland, CA. The cut is over 200 ft Interstate 210 near Sunland, CA. The cut is over 200 ft high, covered by volunteer vegetation.high, covered by volunteer vegetation.

Rapid disintegration and break up of the preCambrian Rapid disintegration and break up of the preCambrian gneiss has allowed creosote brush to gain a toehold gneiss has allowed creosote brush to gain a toehold on the seemingly impervious crystalline rock. Plants on the seemingly impervious crystalline rock. Plants will take hold wherever there are fractures, which will take hold wherever there are fractures, which transmit water. transmit water.

MesasMesas and and buttesbuttes are erosional remnants of a wetter are erosional remnants of a wetter climate, when erosion equaled or exceeded sediment & climate, when erosion equaled or exceeded sediment & detrital transport. When the weather becomes arid, detrital transport. When the weather becomes arid, there is insufficient runoff to remove the imposed there is insufficient runoff to remove the imposed debris, which accumulates as debris, which accumulates as talus fanstalus fans and and colluvial-colluvial-filled bedrock depressionsfilled bedrock depressions..

Residual uplands, erosional remnants or monoliths, like those Residual uplands, erosional remnants or monoliths, like those shown here in Arches NP, are known as shown here in Arches NP, are known as inselbergsinselbergs. In this case the . In this case the inselberg developed along pre-existing inselberg developed along pre-existing systematic regional jointssystematic regional joints and and secondary jointssecondary joints, within the Slickrock Mbr of the Entrada fm. , within the Slickrock Mbr of the Entrada fm. Joints and shale partings exert enormous influence on slope Joints and shale partings exert enormous influence on slope morphology. morphology.

Differential weatheringDifferential weathering This rock-defended pillar is This rock-defended pillar is

known as a known as a hoodoohoodoo, , earth earth pillarpillar, or , or pyramidpyramid (this one (this one being in Wyoming)being in Wyoming)

The capstone is more The capstone is more resistant than the pedestalresistant than the pedestal

Can also form in Can also form in channelschannels

Rocks of varying hardness weather at different rates. This shows Rocks of varying hardness weather at different rates. This shows Pedestal Rock near Lee’s ferry, Arizona , where a talus block of Pedestal Rock near Lee’s ferry, Arizona , where a talus block of Shinarump Conglomerate lies upon the much softer Moenkopi Shinarump Conglomerate lies upon the much softer Moenkopi ShaleShale

VentifactsVentifacts on on desert pavementdesert pavement. Most alluvial fans in arid areas . Most alluvial fans in arid areas are from are from matrix-supportedmatrix-supported debris flowsdebris flows. . Pavement surfacesPavement surfaces develop from ablation of fines by wind, allowing the heavier develop from ablation of fines by wind, allowing the heavier particles to settle downward, leaving a well-graded/poorly sorted particles to settle downward, leaving a well-graded/poorly sorted mixture of mixture of granule gravelgranule gravel, , gravelgravel, and , and cobblescobbles. .

Stripes exposed across mesas adjacent to Lake Powell Stripes exposed across mesas adjacent to Lake Powell in southern Utah. These are caused by wind-induced in southern Utah. These are caused by wind-induced erosion, with the prevailing winds emanating from the erosion, with the prevailing winds emanating from the southwest, blowing northeasterly.southwest, blowing northeasterly.