Welcome to a window into Southern California’s geologic past! This outcrop along the eastbound I‑10 at the Wildwood Rest Area exposes sediments that record hundreds of thousands of years of landscape evolution in the Yucaipa Valley.
Map showing the tectonic and geological setting of the Wildwood Rest Area, and surrounding topography.
The layered deposits here belong to the Yucaipa Formation, a sequence of alluvial-fan and valley-fill sediments that accumulated during the Pleistocene Epoch, roughly 2 million to 500,000 years ago. These sediments were derived from the nearby San Bernardino Mountains, which were actively uplifted by movements along the San Andreas Fault system and associated faults, such as the Crafton Hills Fault. As the mountains rose, erosion carried enormous amounts of rock and soil downslope. Flash floods, debris flows, and seasonal streams deposited this material in fan-shaped spreads at the mountain fronts, gradually filling the valley floor.
Cross section along the A to A' line showing the thickenss of aluvial sediments from the San Bernadino Mountains. Note Interstate intersects the cross section at the Wildwood Rest Area.
The outcrop reveals alternating layers of sand, silt, and gravel, each representing a distinct depositional event. Thicker, coarse-grained beds formed during high-energy debris flows, when water and rock surged down mountain channels carrying boulders and cobbles. Finer, thinner layers accumulated during calmer periods, when slower-moving water deposited silt and sand across the fan surface. This repeated alternation of coarse and fine layers records a dynamic environment dominated by fluctuating water flow, erosion, and sediment supply.
Over time, the fan deposits coalesced, forming a broad sedimentary wedge that now makes up much of the valley. Some layers were stable long enough to develop soil horizons and support vegetation, evidenced by root traces preserved in the sediment. The gently dipping to near-horizontal orientation of the layers indicates that these deposits accumulated in a relatively stable basin, although the surrounding region remains tectonically active.
The Yucaipa Valley itself is a tectonic basin formed by complex interactions between faults, uplifted blocks, and subsiding troughs. The sediments here record not only the history of water and debris flows but also the effects of active tectonics. While the outcrop itself shows little deformation, nearby faulting controlled sediment supply and shaped the valley’s structure over thousands of years.
Today, these sediments continue to play a crucial role in the landscape. Their porosity allows them to store groundwater, supplying local communities with a vital resource. Their erodible nature, however, also makes slopes like this one prone to small landslides and ongoing surface erosion, demonstrating that geologic processes here are ongoing.
As you study this outcrop, imagine the ancient environment: rivers and debris flows cascading down newly uplifted mountains, depositing layers of sand, silt, and gravel; valleys gradually filling with sediment; and tectonic forces subtly tilting and shaping the landscape. This site offers a tangible connection to Southern California’s dynamic geologic history, from mountain-building and faulting to flood and fan deposition, all captured in the layers of rock before you.
Works Cited
https://pubs.usgs.gov/of/1992/0446/report.pdf
https://cawaterlibrary.net/document/geology-and-hydrogeology-of-the-yucaipa-groundwater-subbasin-san-bernardino-and-riverside-counties-california/
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Yucaipa Channels " on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with the outcrop (see photo on bottom of cache page) directly behind you. This photo MUST be attached to your "found it" log.
3. Describe the (a) range of colors and (b) range of clasts size. For example, are the clasts all sand grain sized, or boulder sized?
4. Describe the tectonic forces that have deformed the channel. This outcrop is about equidistant from the San Jacinto and San Andreas Fault, among other smaller fault of the region. Are the channel layers flat, angled, distorted, folded? Explain what you see.
5. How many ancient river channels are preserved here? How many can you see in the outcrop? Hint: look for thin lines or bands of small pebbles/rocks.
6. Are these anciet river channles more prevalent near the top of the outcrop (young) or are they near the bottom (old)? What may this suggest about the tectonic forces in this area? Explain your answer.