Part 9 EROSION BY WATER AND WIND
Aerial oblique view of the South Dakota Badlands, developed on near-horizontally bedded Pierre Shale. Note complete absence of stabilizing vegetation.
Retreating escarpment of the South Dakota Badlands. Note isolated “sod tables” which are more resistant to erosion. • The the White River drainage is eroding soft sediments of the Pierre Shale lying above the more resistant Chadron Formation.
The escarpment is comprised of the Pierre Shale, which contains montmorillonite, a weathering product derived from volcanic ash (tuff). Outcrops containing smectite clay are usually barren of vegetation and often exhibit a crinkled coral-like appearance, due to severe expansion when wetted.
Lacustrine sediments can be particularly susceptible to erosion. This shows spires in Cathedral Gorge State Park near Pioche, Nevada. The material is the Pliocene age Panaca formation, which contains montmorillonite. Smectite clays can swell between 300% and 800% of their dry weight, becoming extremely sticky and slippery when wetted.
Block diagram illustrating the qualitative relationship between runoff velocity and stages of the erosion process
Limiting values of runoff velocity versus erodability for various geologic materials
Raveling cut in Pliocene age siltstone in Orinda, CA. Rapid erosion, acidic nature of the strata, and southwest exposure are factors that combine to prevent effective vegetation of the slope, resulting in rapid erosion.
Stepped cut in preCambrian metamorphic rocks along Interstate 210 near Sunland, CA. The cut is over 200 ft high, covered by volunteer vegetation.
Rapid disintegration and break up of the preCambrian gneiss has allowed creosote brush to gain a toehold on the seemingly impervious crystalline rock. Plants will take hold wherever there are fractures, which transmit water.
Mesas and buttes are erosional remnants of a wetter climate, when erosion equaled or exceeded sediment & detrital transport. When the weather becomes arid, there is insufficient runoff to remove the imposed debris, which accumulates as talus fans and colluvial-filled bedrock depressions.
Residual uplands, erosional remnants or monoliths, like those shown here in Arches NP, are known as inselbergs. In this case the inselberg developed along pre-existing systematic regional joints and secondary joints, within the Slickrock Mbr of the Entrada fm. Joints and shale partings exert enormous influence on slope morphology.
Differential weathering • This rock-defended pillar is known as a hoodoo, earth pillar, or pyramid (this one being in Wyoming) • The capstone is more resistant than the pedestal • Can also form in channels
Rocks of varying hardness weather at different rates. This shows Pedestal Rock near Lee’s ferry, Arizona , where a talus block of Shinarump Conglomerate lies upon the much softer Moenkopi Shale
Ventifacts on desert pavement. Most alluvial fans in arid areas are from matrix-supporteddebris flows. Pavement surfaces develop from ablation of fines by wind, allowing the heavier particles to settle downward, leaving a well-graded/poorly sorted mixture of granule gravel, gravel, and cobbles.
Stripes exposed across mesas adjacent to Lake Powell in southern Utah. These are caused by wind-induced erosion, with the prevailing winds emanating from the southwest, blowing northeasterly.