We also advise that observation of these roadcuts be done as a passenger and not as driver...and assume no liability for accidents on this cyberspace geologic tour Hwy 101 through Marin County. This short excursion forms only about 1 page and 1/3 of the 82 mile, San Francisco - Cloverdale section covered in Alt and Hyndman's, 1975, Roadside Geology of Northern California (Mountain Press Publishing, Missoula, Montana) but it is a very lively portion! (We still recommend this guide if you are traveling California's roads, even if they didn't put the College of Marin's Geology Department on the map!)
As you approach "mellow Marin" on the Golden Gate Bridge, the Marin Headlands in the Golden Gate National Recreation Area forms the shore to the west of the bridge (ocean side). It would be worth a brief side trip here, if you have the time since it is here that you can see good examples of the old ocean floor rocks that have been bulldozed on to the edge of North America. Exit at Alexander Avenue, turn left and go under the bridge. Now be careful, just before getting back on the bridge..it will cost you $3.00 if you miss this turn...turn right and go up hill. It is hard to miss, since the number of tour buses going here is phenomenal. Beyond the fabulous rocks, the views are exceptional..at least on a day with no fog... and one seen in numerous car commercials on the tube. The ribbon cherts are especially prominent but there are exposures of greywackes and pillow lavas as well, you just have to look a little harder. As you come back down the hill, you will see this view of the 101 and Alexander Ave. roadcuts. These extraordinary roadcuts expose actually a coherent slab of old ocean floor that somehow stayed intact as it was raised from the deep ocean floor. In the Alexander Ave. roadcut you find pillow lavas, on the east side of the 101 cut are the resistant chert units that stick out of the hillside, and on the western side are the greywacke/shales that buried the cherts before the entire section was ripped off the ocean floor.
Turn left and go back under the highway, bearing right on to 101 North...careful now..it is a short, uphill, entrance and there is a super exposure of chert with trees growing over it just to the right. In the first roadcut you can see the thick bedded greywacke on the western side and you might even be able to see the chert interbedded with thin layers of greywacke on the eastern side.
The second roadcut on the west side of the freeway has a core of pillow lava. Look up the hill and you can see the overlying resistant chert layers producing a prominent capping of the hill to the north (you can see this in the photo above on the skyline at the back). If you are real quick you might have noticed that the chert layers have been displaced between the first two outcrops...(they are on the east side of the freeway in the first outcrop and on the west side of the freeway at the second outcrop). There is a particularly picturesque, white chert exposure with trees growing out of it just before the second roadcut. (Normal, reverse, or lateral faulting? Look, we can't give you ALL the answers)
Entering the "Rainbow Tunnel" you might notice the thin bedded cherts into which the tunnel is cut. (While the comedian Robin Williams received his early drama instruction at the College of Marin, his social commentary on the county's dominately white population, generated a referral to the tunnel as a giant "ethnic detector"). Exiting the tunnel...(this is very hard to see going north)... the roadcut to the left going west is composed of ribbon cherts on the south end but the majority of the road cut is composed of cross-sections of pillow lavas.
Topping the hill, you pass into a section of melange and just north of the Rodeo Exit and on the west of the freeway you can see a very large tectonically resistant block that has been slowly weathering out of the sheared matrix and starting to loom over the highway. Someone has shown some interest since the block has painted numbers on it, presumably done to monitor it's position...again, presumably to see if it is moving downhill. There is probably no problem going north but coming back south...just to play safe better drive in the fast lane through this section.
To be continued, with your help.