Geology Cafe

Field Trip to a landslide in Coyote-Harvey Bear County Park

Geology Field Trips & Guidebooks
 

Field-Trip Overview: This field trip is to examine a massive slump/landslide area on the east side of Coyote Ridge. The slump has developed in poorly consolidated sediments (Piocene-Pleistocene Packwood Gravels) that form the bedrock along the mountain front. Perennial springs are associated with the slumping. Blocks of basalt are also exposed along the trail above the landslide.

Directions:
Take exit 362 on Highway 101 for San Martin Avenue.

Take San Martin Avenue east for 1.5 miles.

The entrance to Harvey Bear Ranch (County Park) is on the left. Park in the auto parking area. the Willow Creek Trail starts at the parking area.

Geologic Setting: The Packwood Gravels (Pliocene to early Pleistocene age) are poorly consolidated alluvial deposits (sand, mud, and gravel) deposited in an ancestral valley that existed in the area before the modern Santa Clara Valley. The Packwood Gravels have been uplifted and exposed in the shutter ridge (Coyote Ridge) along west side the Calaveras Fault zone. The Packwood Gravels have porous zones that yield springs year round along the hillslopes at the base of the ridge near Harvey Bear Ranch (historic ranch homesite). The springs contribute to the formation of slumps and landslides in the unconsolidated sediment. The perennial source of water is probably a combination of annual winter rainfall and water seeping through the shutter ridge from the Coyote Reservoir side of the ridge.


  Map showing the location of the Harvey Bear Ranch entrance
Field-trip walk: From the Harvey Bear Ranch parking area, follow the Willow Springs Trail (the main trail that starts near the parking area that leads up into the hillsides of Coyote Ridge).

Less than a quarter mile up the trail is the toe of a large landslide on the right (south) side of the trail. The landslide is a large slump displaying classic features: a head scarp, lateral crevasses, a rumpled and disrupted surface on the massive slide, and massive piles of earth at the bottow (toe) of the slide. A small spring-fed stream has carved a ravine at the base of the slope and is a likely cause of the landslide. The poorly consolidated character of the soil and underlying bedrock (the Packwood Gravel) is prone to landsliding in the steep slopes along the western side of Coyote Ridge.

It is possible to explore around the landslide, but be cautious in that the slope is moving and there may be open crevasses and loose blocks of material ready to fall or roll.

Continue up the Willow Springs Trail to the vicinity of a watering trough (for livestock and horses). Look around the watering trough for wildlife tracks and examine the mosquito fish that the County personnel put in such tanks to control mosquito infestation. Near the trough is a large oak tree and scattered around the tree in the soil are an abundance of blocks of basalt. The basalt shows that volcanism occurred in the vicinity at the the the Packwood Gravel was accumulating in the ancestral valley. Lava flows problably flowed down the valley from volcanic vents along the ancestral valley associated with the Calaveras Fault (San Andreas Fault system).
Harvey Bear Ranch landslide features
Harvey Bear Ranch entrance Landslide features
landslide at Harvey Bear Ranch landslide at Harvey Bear Ranch
Toe of a slump along Willow Springs Trail Head scarp and lumpy terrain of the slump
basalt blocks oak trees
The hike to the top of Coyote Ridge is a splendid day hike in the winter months to early spring with the weather is cool and wild flowers are blooming in abundance. Blocks of basalt on the slopes of Coyote Ridge. Santa Clara Valley near Morgan Hill as seen from Coyote Ridge

Santa Clara County Parks information
http://www.parkhere.org/portal/site/parks

Park Trails Hotline: (408) 355-2200, press 7, then press 2 for trail closure information.
For general open/close information, please call (408) 225-2147.
 
 
 
http://geologycafe.com/fieldtrips/coyote_harvey_bear.html
9/7/2012