Sweeney Ridge is located east of the inactive Pilarcitos Fault
and immediately west of the San Andreas which is the major active
earthquake fault in North America. Over millions of years each
fault has slipped repeatedly, crushing and grinding the rocks
which now form the valleys of San Pedro Creek and Crystal Springs
Reservoirs. In addition, there are several smaller branch fractures
between these major breaks which created the intervening valleys.
Movement along these faults has broken off the rocks of Sweeney
Ridge from the rest of North America. The rocks of Sweeney Ridge
slide northwesterly, at a rate of between one and two inches per
year, toward Alaska.
On the lower part of the ridge, the rocks are highly shattered
because of their proximity to the fault. Often these rocks are
covered by an assortment of broken fragments, including mud, sand,
and angular pebbles. This "slope wash" has been carried
down from higher elevations by rain and gravity. There are some
scattered outcroppings of relatively solid rock along the ridge.
The rocks are part of the Franciscan Formation and are about
100 million years old. This area was once covered by the ocean
and evidence of submarine volcanism can be found to the north
and south of this area.
The primary rock seen in most locations is sandstone. Often called
graywacke, it ranges from light tan to dark gray. It is easy to
identify from a freshly broken surface because it looks and feels
like coarse sandpaper.
Shale is present in some places. It tends to break easily into
many small, flaky pieces which do not have the gritty feel of
sandstone. The shales are often interlayered with chert, a harder
rock, usually reddishbrown in color in this area, which breaks
into large blocky chunks.
The most difficult of all the rocks to identify is basalt. Basalt
is the type of lava extruded during eruptions of Hawaiian volcanoes.
Here it is very old and decomposed. It usually looks more like
a hardened mudstone. Some basalt is evident near the radar tracking