We have guests coming in next week, and the group includes a cyclist. While I know about Healdsburg's Road Cycling (How Sonoma County became hotbed of the cycling world!) I know less about the off-road action. And our guest says he's bringing the cyclocross bike. He's from Minnesota, not Williamsburg in Manhattan or San Francisco's Mission District , so he's probably not one of these guys...
My Mountain Biking friends rave about Annandale State Park near Santa Rosa. I don't know how technical the terrain is so I don't know how appropriate it is for cyclocross. I also know that Sonoma seems to be a hotbed of cross racing, I went and watch my friends do a cross race, but these are usually on courses that are prepared and then taken down.
I asked Greg Durbin - a.k.a. "GregKnottLeMond" whom I follow on Twitter and he pointed me at Lake Sonoma. I know Lake Sonoma has some very difficult road terrain - Skaggs Springs Road and Rockpile Road, but didn't know about the Mountain Biking. He forwarded me a map showing trails near Liberty Glen Campground off of Rockpile Road.
Greg also pointed me at Bike Monkey magazine which appears to be the Mother Lode - cross fiends galore. I noted that there was reference to a race in Liberty Glen, so maybe that's the spot.
Any cross fiends out there have some recommendations? Please add them in the comments!