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Marin High Country
San Geronimo Ridge lies between the exurban communities of Forest Knolls and San Geronimo to the north and deep canyon of Kent Lake to the south in Marin County. The description is that of a shuttle hike from the White Hill Preserve trailhead just west of Fairfax, CA to the Leo T. Cronin Fish Viewing Area. There's a big catch, however: parking at the Cronin Fish Viewing Area is limited to 60 minutes during salmon and steelhead spawning season (January-March), and typically closed during the rest of the year so most hikes are out-and-back from either White Hill or one of the access points to the north by San Geronimo.
The White Hill Preserve parking lot is located on the west side of Sir Francis Drake Blvd. near the divide between the two watersheds that respectively drain into San Francisco Bay to the east and Tomales Bay to the west. The parking lot is actually about 100 yards south of the true trailhead, and there is an alternate route to the top of the first ridge right off the parking lot. The true trailhead is part of the Bay Area Ridge Trail, and it is reached by heading north/uphill from the parking lot between road barriers. The next portion goes a 0.9 mile through open areas exposed to the roadway noise below and deep forests of Douglas fir and California bay before somewhat reaching the top of the ridge.
The goal is to stay on the Bay Area Ridge Trail, and that's fairly easy to do because it's well-signed. The signage is very weathered here, but the correct route is indicated by arrows and a circular emblem showing a rough outline of the Bay Area in blue and white. I neglected to take a picture of one. The next stretch goes about 1.5 miles near the top of the ridge through more open areas. Most of the north side is private property, and a lot of it is grazed. The stretches of forest here are typical of the Bay Area's common mix of Douglas fir, California bay and coast live oak forests being islands in seas of grass and coastal chaparral. Views of the San Geronimo Valley to the north and the deep canyons of Kent Lake enchant on both sides
Soon thereafter the trail goes through an area of serpentine. The plants are stunted and the soil is greener and grayer here, and the transition from the typical Marin vegetation to the uniqueness of the serpentine environment is startling. There is literally a precise point where the trail goes from being red/brown/blonde to green/gray. You are now on the San Geronimo Fire Road 3.5 miles from the trailhead, and the trail goes up and down. The Sargent cypress is the most notable plant here. It typically forms dense stands in serpentine soil, and looks like a juniper shaped like a mushroom.
Continue west on the San Geronimo Fire Road for another 2 miles as it leaves the serpentine and heads into second growth forest. Isolated groups of redwoods have made their appearance before now, but they are now common on the north side of the route which is still on top of the ridge. Dougies are prevalent, but there is a noticeable difference between the forest on the north side and chaparral to the south down to Kent Lake 800' below.
Continue following the Bay Area Ridge Trail west as it goes up and down, not as dramatically as a few miles back, before plunging down the Lagunitas Creek drainage below Kent Lake's spillway. The forest has grown quite tall here and the Leo T. Cronin Fish Viewing Area has a small parking lot to serve the people who want to see spawning salmon and steelhead on the relatively large Lagunitas Creek. This is either a great place to be picked up or merely turn around, and walk the other way.
Check out the Official Route and Triplog.