By Nancy Soares and John Lull
“The best way to study geology is by kayak” – Tsunami Ranger Capt. Tortuga
The Golden Gate Bridge as seen from just above Horseshoe Cove
You probably don’t know this, but Capt. Tortuga and Tsunami Ranger John Lull are geologists by trade. On Monday, April 9, John and I drove from his place in El Granada on the San Francisco Peninsula to Horseshoe Cove at the base of the north tower of the Golden Gate Bridge and put in for Angel Island. Our mission: study the geology of the island.
Angel Island
It was the first time I had ever paddled San Francisco Bay. We were blessed with some of the most halcyon weather possible: blue sky; bright, hot sun (it felt like it was about 85 degrees) flat water, and practically no current to speak of. We put in at the little marina there in the cove, paddled out past the jetty, and turned left toward Yellow Bluff. Hugging the shore for a short distance, we then set our course for a beach on the island where John wanted to examine sand samples.
Horseshoe Cove launch
It was a peaceful, easy crossing. John pointed out landmarks and watermarks and told me about eddies and currents a paddler can take advantage of to facilitate passage around the Bay. We saw birds and sea lions. The wind was so light there were few sailboats about and a couple of the ones we did see were underway using their motors. We saw some ferries and encountered small wakes but otherwise paddling both going and coming across Raccoon Straight was uneventful. Paradise!
Tiburon in the distance, and behind it Mt. Tamalpais
The views were spectacular in all directions: The Gate, the Bay Bridge, Alcatraz and Angel Island, the little towns of Sausalito and Tiburon nestled on their hillsides looking more like towns on the Mediterranean than on the colder, grayer San Francisco Bay, and of course the stunning San Francisco skyline.
The San Francisco skyline
The first beach we landed on was at Camp Reynolds, an abandoned army base on the island. During the Civil War, with concern mounting over threats to the Bay Area from Confederate sympathizers and naval forces, the federal government established Camp Reynolds on Angel Island in 1863. Fears that the Confederates might slip into the Bay and attack naval installations at Mare Island and the Benicia Arsenal were never realized. After the Civil War, Camp Reynolds became an infantry camp, serving as a depot for recruits, and as a staging area for troops serving in campaigns against the Apache, Sioux, Modoc, and other Indian tribes. By 1876, this was a busy camp with over 2,000 soldiers and a chapel, bakery, blacksmith, shoemaker, laundry, barber, trading store, and photographer. Now Camp Reynolds is a quiet picnic spot where hikers and kayakers can relax and contemplate.
Camp Reynolds
John examined rocks and sand while I checked out the abandoned camp. Then I came back to the beach and John explained the geology of the cliffs. There were some nice examples of rock pillows formed deep below the sea millions of years ago that have now been raised up and are part of the island.
Pillow basalt. Looking closely, you can see a couple of round pillows in the center of the photo.
We launched again and paddled around to the next beach, a no name beach south of Alaya Cove. I’m calling it Blueschist Beach. While John was checking out the sand at this site, I wandered off in the opposite direction beachcombing.
Blueschist Beach
The beach was quite clean but I picked up a few bits of plastic here and there. Then I saw something that looked like a rolled up ball of tin foil. Reaching down to pick it up I discovered it was a rock. A beautiful, shiny silver rock. “Hey John,” I called, “Look what I found!” I showed him the rock and it was instant recognition. Muscovite mica pelitic schist were some of the words that came out of his mouth, but I’ll let him explain it.
Treasure trove – blueschist, green actinolite schist, and blue sea glass
John Lull: The rocks on Angel Island are part of the extensive Franciscan subduction complex that underlies much of the California coast range. They include far-traveled basalt and pillow basalt, erupted approximately 130 million years ago at a mid-ocean spreading center several thousand kilometers away in the Pacific Ocean, deep ocean chert beds deposited on the basalt during its long journey on the Farallon Plate away from the spreading center, and thick sequences of continent-derived sandstone and shale that were deposited on the oceanic crust in the subduction trench formed just off the continent. This oceanic crustal slab and overlying sediment were carried down beneath the North American continent where most of it was consumed in the underlying mantle.
Angel Island blueschist
The subduction process continued for over 150 million years and periodically some of the oceanic crust and sediment was offscraped, underplated, and accreted to the continental margin. As the accreted material built up over time, it was subjected to intense pressure and localized shearing, resulting in zones of extreme deformation and numerous faults.
John and the blueschist
The rocks on Angel Island are a special case in that they were subjected to high pressure-relatively low temperature ‘blueschist facies’ metamorphism. This means they must have been carried deep into the subduction zone (20-30 km beneath the surface) and underplated beneath the over-riding continent. They were later exhumed due to continued tectonic movement and returned to the surface where they are today. Most of the basalt, chert, and sandstone on Angel Island retains its original texture and can be readily identified. Pillow structure can be seen in the basalt and although the sandstone is mildly schistose, sand grains are still apparent. The blueschist minerals in most of these rocks are only visible under the microscope in thin sections cut from the rock. However, in some cases the rocks were more strongly metamorphosed, forming completely recrystallized schist and obliterating the original texture and character of the rock.
Layered blueschist on Angel Island with rock hammer for scale
There are a few outcrops of these schists on Angel Island. The most prominent blueschist outcrop is exposed on the south side of the island, just east of Perle’s Beach. A prominent band of serpentine (derived from the mantle) is also present just east of the blueschist. There is a thin band of poorly-exposed schist on the slope above the northwest shore of the island between Point Ione and Stuart Point. Pebbles of silvery quartz-muscovite schist and dark green actinolite schist can be found on the beach below.
Sparkly crystal green schist
Nancy: While looking for blueschist we also found green actinolite schist. This stuff is amazingly beautiful and has to be seen under a microscope to really be appreciated. Regardless, it’s lovely to look at.
After Ayala Cove we headed back to the put in. It was about a 45 minute paddle each way, although John told me that there is often an ebb tide you can ride back that makes the return quicker. Still, it was a great 6 mile round trip, just enough to make you feel like you got a good work out. Our return was uneventful, and after loading up our boats and gear we drove back to John’s house for a beer and a soak in the hot tub. The best of days!
Have you been to Angel Island? Whatcha know about blueschist? Share your story or any other comment you’d like by clicking below!
