A few weeks ago I managed to get on the list for a field trip with the Northern California Geological Society (where I serve as webmaster/social media chief). We visited four memorable spots on the Sonoma coast. The first one was Bodega Head, where I’d been once a long time ago. The second was Shell Beach, my third visit. The third was Goat Rock, which I’d photographed long ago but where I’d never actually stood. And the last one, which had seemed like an afterthought, was an illuminating surprise.

The wave-polished diorite at Bodega Head was beautiful. These rocks are a former part of the southernmost Sierra Nevada, pulled up here along the San Andreas fault. They match granitoids in the San Emigdio Mountains south of Bakersfield.

Shell Beach displayed its wonderful assortment of boulders eroded from Franciscan melange, the lithologic scrapple formed beneath the deep-sea trench of the former California subduction zone.

Goat Rock, I learned, is a single large block of high-grade metamorphosed mudstone floating in the melange. Like the sea stacks nearby, the hard blocks in the melange withstand the sea better than the shaly matrix.

Then we went a short way up the left bank of the Russian River to an exposure of conglomerate that has come far from its birthplace during the Cretaceous Period.

The Franciscan Complex is such an intricate mess of melange and ordinary rocks that to see anything else on a geologic map, you need to gray it out, as in this map from a paper on the major faults up there. The place we visited, at the upper left corner, is the small dark-blue finger representing rocks assigned to the Great Valley Sequence (GVS). It’s almost a hundred kilometers from the main belt of the Great Valley Sequence, a gigantic set of upturned strata that lines the whole west side of the Central Valley.

^{Geologic map adapted from McLaughlin et al. (2012)}

It closely resembles the finger of Great Valley Sequence conglomerate in the Oakland Hills that’s formally named the Oakland Conglomerate.

It has a matrix of fine-grained sand, shot through and through with iron staining, that holds well-rounded cobbles — the remains of high mountains with a wide mixture of rocks.

Just for comparison, here’s our own Oakland Conglomerate.

Rocks like this, separated from their place of origin, are hard to date because our best dating tools don’t work in them. This kind of conglomerate forms well offshore in the deep sea where submarine canyons deliver persistent large landslides of sediment from a continent. Fossils (yielding biostratigraphic ages) don’t survive those conditions, nor do volcanic ash beds (radiometric ages).

The best geologists can do is provenance studies — examining the mixture of cobbles to work out where they likely came from. Also, some of the cobbles could yield radiometric dates that establish a maximum age. Provenance studies are a painstaking exercise that when done right involves thousands of stones, and even after that the results are a small set of tables in a small published paper.

Just looking at these rocks, in Oakland and Sonoma, I might well correlate them and consider them the same. But California in Cretaceous time was a big domain, about the size of the coast of Peru, and it endured over 100 million years of geologic time, so the odds are rather small. In that time and that geologic setting, though, the same set of circumstances surely occurred many times: a large body of coarse river gravel gradually built up, then was tipped into a submarine canyon over a short period of time, a few thousands of years or so, building up these remarkably consistent mixtures.

Today geologists think of California as a dynamic place where the big active fault systems are ripping apart and rearranging the landscape sideways, but Cretaceous California was not like that. It was a straightforward subduction zone much like Cascadia, its shrinking remnant, and western South American are today. There were some lateral movements of continental crust as incoming oceanic plates changed directions, but not like what’s been happening lately. The big thing that happened over and over in the Cretaceous was rising and falling sea levels, not today’s short ice-age cycles but larger, slower fluctuations caused by global changes in the formation and destruction of oceanic crust.

Somehow those changes gently assembled large piles of cobbles and sand, like ingredients in a bakery, then expertly mixed and poured them offshore to make these widespread, consistent conglomerate deposits. The products look alike because they were made the same way, in the same domain, over a long period of time. Later, the San Andreas fault system came along, pulled them apart and left scattered pieces up and down the Coast Range in Oakland, in Sonoma and elsewhere. How they once fit together, we may never know, but we can try.

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