Anyone who has hiked to Potato Chip Rock knows what north Poway is made of. Big rounded boulders, bare rock domes, and chaparral filling in the gaps. If you own a hillside lot near Mt. Woodson, Garden Road, High Valley or Old Coach, some of that rock is probably in your backyard or on the slope above it.
That rock is mostly an asset. It is strong, it drains, and good footings love it. But a slope covered in loose boulders is not the same thing as solid bedrock, and the City lists rockfall among its principal geologic hazards for a reason. Here is how the boulders got there, what can get them moving, and what you should and should not do about the ones near your house.
The rock has a name
The high ground around Mt. Woodson is Woodson Mountain Granodiorite. The U.S. Geological Survey lists it as a Cretaceous unit of the Peninsular Ranges, named for the mountain itself. Poway’s Safety Element describes it as very resistant to erosion, forming most of the high areas in the city, with large rounded boulders and large exfoliation domes.
Not all of Poway’s granite behaves that way. The same document describes Green Valley Tonalite as highly susceptible to weathering, forming low areas with gentle topography. So two lots a mile apart can both say “granite” on a soils report and still behave very differently. One sits on hard rock with boulders scattered over it. The other sits on deeply rotted rock that digs like sand.
How a block becomes a boulder
The round boulders did not roll in from somewhere else. They grew in place, underground.
Granite is cut by fractures, called joints, that break it into rough blocks. MiraCosta College’s field guide to Woodson Mountain explains the rest: water, gases and organic acids seep into those fractures and slowly rot the rock along them. Corners and edges get attacked from more than one side, so they break down faster, and each block rounds off toward a solid core. Long dry summers and short wet spells keep the process going. Wildfire strips the brush, heavy rain washes away the loose grit, and the rounded cores end up sitting on the surface. Geologists call it spheroidal weathering.
A Forest Service paper on granitic landslides describes this as a sequence of stages: fresh rock, then corestones (solid blocks sitting in rotted rock), then decomposed granite, then fully weathered material. Each stage has its own typical failures. Fresh, jointed rock tends toward rockfalls and block slides, controlled by the joints. The corestone stage, the boulder-studded slope so common around Poway, comes with rockfall and rolling rocks.
That is the important part for homeowners. A boulder on a slope is usually a hard core resting in, or on, softer decomposed granite. The boulder is strong. What holds it in place is not.
What gets a boulder moving
The Safety Element says rockfall in Poway’s granitic terrain comes from oversteepened slopes, caused by seismic shaking or erosion. In practical terms, on a residential lot that usually means one of a few things:
- The ground under it washes away. Decomposed granite erodes easily. A downspout, a broken sprinkler line, a driveway that dumps runoff onto the slope, or a winter of heavy rain can carve out the soil holding a boulder’s downhill side.
- Someone cuts below it. Grading a pad, a path, a pool or a wall footing at the base of a rocky slope can leave boulders above perched on a steeper face than before.
- Roots and time. Roots work into fractures, and joints keep weathering. A crack through a boulder can widen until a slab splits off. Potato Chip Rock is a famous example of a slab that split from a larger outcrop.
- Shaking. Poway’s own faults are classified as inactive, but regional earthquakes can still shake loose rock that was already close to the edge.
- After a fire. Burned slopes lose the brush and roots that held loose material, and the first heavy rains after a fire can move a lot of soil.
What to watch for from your yard
You do not need climbing gear to keep an eye on the slope above you. Twice a year, ideally before the rains and after the biggest storm, look for:
- Soil freshly washed out from under one side of a boulder, leaving a gap or overhang.
- New fans of sand and grit at the bottom of the slope.
- A boulder that has tilted, shifted, or has a fresh, lighter-colored scar where it used to touch the ground.
- New cracks running through a large rock, especially near an edge.
- Leaning trees or bent fence posts around rock outcrops.
- Rock fragments on the ground that were not there before.
Take dated photos from the same spots. Over a few years those photos tell you whether the slope is stable or changing.
What not to do
- Do not dig under or around a boulder to see what is holding it. That is how small problems become big ones.
- Do not pry, jack or try to roll a large rock off a slope yourself.
- Do not point downspouts, pool drains or overflow lines at a rocky slope. Send water to an approved outlet.
- Do not cut into the base of a slope for a project without a plan. Grading and walls in Poway often need permits, and work below rock outcrops may need a geologist’s input.
Where rock meets the foundation
Rock also matters below your house. A 2022 geotechnical report for a Poway project describes decomposed granitic rock producing low-expansion sandy soils, but also notes compressible alluvium, perched groundwater, and possible blasting where the rock is hard. On a single lot, that kind of variability shows up as a pad that is part rock and part fill, or footings that hit boulders at different depths. Those transitions are a common reason one corner of a hillside house moves while the rest stays put.
Excavating through boulders, getting equipment up a steep driveway, and coordinating with a geologist all affect the scope of hillside work. They are cost factors to plan around, not reasons to wait.
When to bring someone in
If a boulder above your house has visibly moved, if soil is washing out from under rock, or if cracks are opening in a retaining wall or the downhill side of your home, request a Free Quote. We look at the house, the walls and the slope together. Rockfall mitigation and slope stabilization are design questions, so we coordinate with licensed engineers as needed, along with an engineering geologist where the slope calls for one.
For more, see our pages on hillside foundation repair, retaining walls, large-lot and estate foundations and drainage. If your house was rebuilt after a fire, our companion post on finding your rebuild’s foundation records is a good next step.