Oakland has three different bodies of conglomerate: from youngest to oldest they’re in the Orinda Formation (of Miocene age, maybe 10 million years old), the Oakland Conglomerate (Late Cretaceous, maybe 80 Ma) and the Knoxville Formation (Late Jurassic, about 165 Ma). Recently I’ve been getting fixated on the last one. I hope these photos from upper Arroyo Viejo will help you understand why.
Above I-580, Arroyo Viejo runs along the road as you drive up Golf Links Road and into the woods of Knowland Park. Halfway through, it takes a sneaky dogleg to the left, then another left turn through this tunnel under Elysian Fields Road. Beyond is the stream’s source, now in the Sequoyah Country Club golf course.
Between those two left turns, the stream valley exposes the Knoxville Formation. I haven’t walked the whole section yet, just visited both ends. The lower end has wonderful conglomerate outcrops.
High on the valley walls, the rock is heavily vegetated. It gets that way because between the pebbles and cobbles of the conglomerate, the matrix is a fine-grained sediment that supplies nutrients to plants and offers space to their roots.
As you get close to it, the conglomerate reveals the abundant well-rounded cobbles beneath its green coat. Geologists like their rocks clean, but nature prefers them this way. You have to admire them.
For the best exposures, look down into the streambed.
A little bit downstream is a section of creek choked with boulders of this stuff, some as big as sofas, that I showed you a few months ago. Those monster rocks weren’t carved out of the streambed by the creek. Instead, they rolled down the valley walls, which are very steep (35 to 40 degrees) as most are in Oakland.
The valley does have some landslide scars, so we know that those happen. But mostly I blame earthquakes.
Both events provoke the stream into washing the boulders away. Landslides create instant dams, which build up the water pressure in the lakes that pond above them. When the stream bursts through, usually in a matter of days, it makes short work of the rock pile. Earthquakes, for their part, give the whole underground a shakedown and cause a weeks-long surge of water afterward that likewise gives the boulders a good head start.
In between these disturbances, maybe once in a century, rare cloudbursts pour enough water into the watershed to roll the biggest rocks downstream and grind down the streambed an inch or so at a time.
Given enough time — and geology always provides that — the boulders break down into pebbles and clay and wash out to sea, eventually to become new rocks. The Knoxville Formation conglomerate has waited some 165 million years to start that journey. The Earth is almost thirty times older.