This ride begins in a plush virgin redwood stand in the san geronimo valley. You may want to explore the soft trails beneath the giant redwoods before you begin the hellish ascent. With a giant fallen redwood, this area could be a trials riders dream. The ride begins with a hill that can be likened to trying to ride your bike up Half Dome, passing from bay trees and Oak to a wide open grassy ridge with few trees. Although the trail will leave Open Space part way up the descent, I've never had any problems passing through this private ranch land- and all the times I've been up this trail, I've never once encountered another person on any part of this ride. At all junctions, keep going up hill along the ridge. There are beautiful views in all directions from the ridgeline, especially of West Marin/Pine Mountain Area. The ridgeline fire road is wide, with many opportunties to exceed 40 mph (50 for you big ringers). The trail will eventually make its way up to the 1592' summit of Loma Alta, Marin's best kept downhill secret. When you come to the junction with three other trails, take the middle and to the top of Loma Alta and a wicked downhill will be waiting. This is by far and away my favorite downhill in Marin County. The only draw back is that about 1/3 the way down there is a cattle gate that must be opened, unless you think you can jump a 4' barbed wire fence! The downhill is around 1.75 miles and drops 1500', those who like the rocky ridge trail in MMWD, will love this highspeed descent. The top portion of the downhill flanks down a ridgeline with minimal turns and many roller coaster like hills with opportunities to catch big air at VERY high speeds. Half way down the grassy hills disappear into a short ascent of about 75yards (stay to the right). I often find myself sprinting up this short hill only to begin the more technical part of the downhill. There are many tight turns, and a few hairpins that will test the most skilled riders at high speeds. The ride ends up at San Domenico School. You can either ride the trail back, or take Butterfield Rd to Sir Francis Drake in Fairfax, with about a 25 minute ride via pavement back to Roys Redwoods.
The best part about this ride is bay far and away the Downhill from Loma Alta to San Domenico, once you've made it through the cattlegate, if you've got the balls and the equipment you can easily top out at 50+ mph without have to worry about anyone on the trail, except for maybe deer! :)
Take Sir Francis Drake west through Fairfax and into San Geronimo Valley. You will see the San Geronimo Golf Course on both sides of the road. You will hang a right onto Nicasio Valley Road shortly after passing the entrance to the club house. About 1/4 up the road there will be parking on the side of the road with Roys Redwoods Open Space on the right. Once you park head about 150 yards up Nicasio Valley road and there will be a gated fire road, the beginning of the ride :)
Summary: I've ridden this trail and around the area a few times. The ascent and ridgeline are fun, but only receive a 3/5. The downhill by far and away makes up for it, catapulting this ride to a 5/5 easily. There are many different ways to the peak of Loma Alta from Loma Alta Open Space in Fairfax, Lucas Valley, and even Terra Linda. The descent isn't the most difficult at moderate speeds, but this should be a full fledged adrenaline rush which ups the ante. Downhill riders will love this trail, and cross country riders can explore the hills around Loma Alta to discover some hidden single track.
Other recommended trails in the same area: There are many fun fireroads and a few single tracks to be discovered on Loma Alta Open Space Land, and the surrounding areas.