The effectiveness of the above processes in degrading the trails is governed in large part by the trail conditions. Trail grade, alinement, drainage, tread material characteristics (specifically infiltration properties and erodibility), trailside vegetation, and local geomorphology all make the trail more or less susceptible to erosion. For example,
trails that have gentle gradients and hard trail tread materials (i.e. bedrock) are relatively resistant to erosion. In contrast, fluvial or recreational erosion can cause large volumes of erosion on trails that have steep gradients and soft trail tread materials (i.e. forest soil). Thick soil cover and vegetation in close proximity to a trail can absorb surface water and limit fluvial erosion. Properly located drainage structures can also limit fluvial erosion by diverting surface water off the trail. Decayed or plugged culverts can increase fluvial erosion by causing streams to overtop their banks and flow on the trail surface.
LOCATION AND BEDROCK GEOLOGY
Joaquin Miller Park is located in Alameda County on the southwestern slope of the East Bay Hills between Highway 13 and Skyline Boulevard. The City of Oakland owns and operates the park. Numerous small creeks drain the upper reaches of the park into Palo Seco Creek, a major tributary to Sausal Creek (Figure 1). The park consists of a rugged upper section located east of Sunset Trail and a gently sloping lower section located west of Sunset Trail (Figure 1).
The rugged upper section of Joaquin Miller Park is underlain by Upper Cretaceous Oakland Conglomerate and Joaquin Miller Formation bedrock (Radbruch, 1969). The Oakland Conglomerate underlies the flat ridgetops located on the northern edge of the park and is composed of pebble and cobble (up to 8 inches in diameter) conglomerate in a yellowish-brown, weathered sandstone matrix. The Joaquin Miller Formation underlies the steep sided ridges and canyons in the middle of the park and is composed of thinly bedded to massive (up to 10 feet thick) beds of yellowish-brown sandstone, shale, and minor conglomerate. The rocks in the upper section of the park have a northwesterly strike, a moderately steep northeasterly dip, and comprise one limb of a large anticlinal fold.
The lower section of the park is characterized by gentle topography
and is separated from the steep upper section of the park by the
northwest-trending Chabot fault located in the vicinity of the
park visitor center and Palo Seco Creek. The lower section of
the park is underlain by Upper Jurassic massive shale and interbedded
sandstone of the Knoxville Formation, Upper Jurassic to Cretaceous
greenstone and serpentine of the Franciscan Formation, and Pliocene
Leona Rhyolite bedrock (Radbruch, 1969). The southwestern border
of the park is less than a quarter mile from the active, northwest-trending
Hayward fault. The rock formations in the lower section of the
park also strike to the northwest.
