Naturalist Segment: Winter in Santa Cruz, When Mountain Bikers Think Like Geologists
January 6, 2020
Up next in our Science Committee's Naturalist Segment is this fascinating insight about wet conditions and erosion, written by SeanPaul Laselle.
Words by SeanPaul Laselle It’s that time of year again when mountain bikers all over the Bay Area stick their heads out the door like helmeted versions of Punxsutawney Phil to make a critical decision: are the trails dry enough to ride? Most of us are aware of the negative effects of riding wet trails. You’ve probably heard the rule of thumb that one should wait to ride for one day per inch of rain following a storm. Likewise, if the trail is wet enough to leave tire tracks everywhere, it is best to hold off and come back when it’s drier. These are the times when going for a road ride, hike, or binging a TV show are appropriate alternatives to mountain biking. But if you’re really dedicated to riding trails throughout the winter and have figured out which areas and trails are not impacted by riding in wet conditions, then congratulations! Not only are you showing that you care about the well-being and long term sustainability of our local trails, but you’ve also got some solid geologic intuition as well!
Here in Santa Cruz, the geology is a bit more complicated, resulting in smaller, more complex pockets of sandy goodness. One example of the bedrock geology influencing our trails is the first switchback climb up from the corral on the Wilder Ridge trail which is located along an uplifted seacliff cut into the Santa Margarita Sandstone. This geologic unit is much older than the sands in Fort Ord and was deposited in a marine environment roughly 10 million years ago before being uplifted to its present elevation. This same unit is responsible for the rare Sandhills of Santa Cruz and the well-drained hiking trails on Bear Mountain.
As you climb farther up Wilder Ridge past the picnic table, you get into the Santa Cruz Mudstone. This is what causes the rocky beginning to Zane Gray and the very fine-grained material that erodes out of these mudstone outcrops and creates soils that do not drain well. A quick descent down the old paved road to the junction of Eucalyptus, Enchanted, and Baldwin ends in a sandy area which is, you guessed it, derived from the Santa Margarita Sandstone. Climbing farther from here, all the way to Twin Gates, the underlying surface is primarily composed of weathered granite that was intruded about 70 to 90 million years ago (it underlies all the younger sedimentary rocks across a large swath of the Santa Cruz Mountains). The soil that developed as the granite weathered has relatively low permeability and drains poorly, as evidenced by the extremely rutted out junction at the northern end of the Old Cabin Trail.
As mentioned before, the geology in Wilder Ranch is relatively complex. Just because a trail is located on the same section of a map as the Santa Margarita Sandstone doesn’t necessarily mean it drains well. Many of the soils that have developed on the flat parts of the marine terraces are composed of sediment eroded out of sandstones, siltstones, mudstones, and granite. This, in combination with low relief, and fire roads that were not constructed with drainage in mind, results in the very wet and rutted out sections of trail that we see.
We’ll continue to explore the relationship between marine terrace soils and trail conditions in Wilder in a future Naturalist Segment -- stay tuned for more!
SeanPaul LaSelle's love for science and cycling goes back to the days of trying to eat rocks on hikes and sitting in the baby seat on his dad's bike in Washington state. Now in Santa Cruz, as a sedimentologist, a member of the MBOSC science committee, and trail crew leader in training, SeanPaul is excited to help MBOSC in its mission to practice and promote sustainable mountain biking.
Words by SeanPaul Laselle It’s that time of year again when mountain bikers all over the Bay Area stick their heads out the door like helmeted versions of Punxsutawney Phil to make a critical decision: are the trails dry enough to ride? Most of us are aware of the negative effects of riding wet trails. You’ve probably heard the rule of thumb that one should wait to ride for one day per inch of rain following a storm. Likewise, if the trail is wet enough to leave tire tracks everywhere, it is best to hold off and come back when it’s drier. These are the times when going for a road ride, hike, or binging a TV show are appropriate alternatives to mountain biking. But if you’re really dedicated to riding trails throughout the winter and have figured out which areas and trails are not impacted by riding in wet conditions, then congratulations! Not only are you showing that you care about the well-being and long term sustainability of our local trails, but you’ve also got some solid geologic intuition as well!
“I don’t like sand. It’s coarse and rough and irritating, and it gets everywhere.” - Anakin Skywalker (Don’t be like Anakin Skywalker, embrace the sand)Perhaps the most well known and somewhat-local destination for wet riding is down in Fort Ord National Monument. It’s nice and sandy down there, which means that the area holds up well even in wet conditions. We can thank the series of ice ages that occurred during the Pleistocene epoch (from about 12 thousand to 2.6 million years ago) for this unique environment and the cooler, drier climates and sea levels up to 130 m lower than the present. This led to the development of sand dunes and sandy river sediments in the Salinas Valley at various times during the Pleistocene, and ultimately delivered us the sandy trails that are grippier and more fun to ride when wet!
Here in Santa Cruz, the geology is a bit more complicated, resulting in smaller, more complex pockets of sandy goodness. One example of the bedrock geology influencing our trails is the first switchback climb up from the corral on the Wilder Ridge trail which is located along an uplifted seacliff cut into the Santa Margarita Sandstone. This geologic unit is much older than the sands in Fort Ord and was deposited in a marine environment roughly 10 million years ago before being uplifted to its present elevation. This same unit is responsible for the rare Sandhills of Santa Cruz and the well-drained hiking trails on Bear Mountain.
As you climb farther up Wilder Ridge past the picnic table, you get into the Santa Cruz Mudstone. This is what causes the rocky beginning to Zane Gray and the very fine-grained material that erodes out of these mudstone outcrops and creates soils that do not drain well. A quick descent down the old paved road to the junction of Eucalyptus, Enchanted, and Baldwin ends in a sandy area which is, you guessed it, derived from the Santa Margarita Sandstone. Climbing farther from here, all the way to Twin Gates, the underlying surface is primarily composed of weathered granite that was intruded about 70 to 90 million years ago (it underlies all the younger sedimentary rocks across a large swath of the Santa Cruz Mountains). The soil that developed as the granite weathered has relatively low permeability and drains poorly, as evidenced by the extremely rutted out junction at the northern end of the Old Cabin Trail.
As mentioned before, the geology in Wilder Ranch is relatively complex. Just because a trail is located on the same section of a map as the Santa Margarita Sandstone doesn’t necessarily mean it drains well. Many of the soils that have developed on the flat parts of the marine terraces are composed of sediment eroded out of sandstones, siltstones, mudstones, and granite. This, in combination with low relief, and fire roads that were not constructed with drainage in mind, results in the very wet and rutted out sections of trail that we see.
We’ll continue to explore the relationship between marine terrace soils and trail conditions in Wilder in a future Naturalist Segment -- stay tuned for more!
SeanPaul LaSelle's love for science and cycling goes back to the days of trying to eat rocks on hikes and sitting in the baby seat on his dad's bike in Washington state. Now in Santa Cruz, as a sedimentologist, a member of the MBOSC science committee, and trail crew leader in training, SeanPaul is excited to help MBOSC in its mission to practice and promote sustainable mountain biking.
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