San Leandro Creek (2)

San Leandro Creek is Oakland’s largest watercourse. Before Anthony Chabot dammed its upper reaches and the Bay shore was filled in, the creek was navigable up to East 14th Street. Where you enter the M. L. King Regional Shoreline at Hegenberger Road, San Leandro Creek looks quite capacious.

Today the creek no longer has a lazy, sinuous course but instead runs down a straight ditch to its mouth near the Coliseum. The water looks wholesome, there’s wildlife all around, and the fishing seems to be good. Today’s paper showed men pulling sardines out of the bay here.

An elaborate observation tower offers a view of San Leandro Bay, an unsung body of water between Alameda Island, Bay Farm Island and the mainland. My visit was near high tide; the map shows it as almost all mudflats.

The spear of grassy marshland is Arrowhead Marsh. One story has it that the marsh was created in 1879 when Chabot’s dam construction, accomplished with hydraulic hoses, washed an enormous quantity of sediment down to the Bay. If so, that would be just another item in the long list of damages done to California during early statehood. But just as likely is that it was always there, along with 2000 more acres of marsh stretching across today’s airport and in a wide fringe around San Leandro Bay.

Here’s a view of the creek’s course across the East Oakland flats.

Treasure!

Where the subsurface is inaccessible, you learn to prize the opportunities to sample it. A house on the crest of Montgomery ridge is upgrading its foundation, and the contractor has helpfully kept the tailings in this handy bin. It’s mostly sand and mud, but there’s a good deal of the Franciscan sandstone in it as well, plus hunks of the lovely red chert that seems to be part of the youngest detrital beds all over this hill. I restricted myself to collecting just one chert piece to add to the pile in the front yard.

I keep thinking that a concerted effort by enough Oakland citizens could help ensure that these ad-hoc trenching studies are properly exploited for their scientific value.

Rocks of Leona Quarry Con’d.

Along the entryway to the Leona Quarry development is a tumble of big decorative boulders, ready for deployment in the landscaping. When I drove by I made a mental note to check them out. I assumed they were chunks from the quarry itself. No such luck; they’re exotic blocks full of shell fossils, which are unheard of in Oakland. This is a view of the shell faces.

This boulder is almost what geologists call a shell hash or coquina, but the shells are mostly intact. I might call it a shell marl instead, but the default name would be fossiliferous limestone. I have no idea where it’s from—somewhere over the hills, or maybe in the Santa Cruz Mountains, but not Oakland.

Here’s a closeup of another boulder showing a cross section of the shells.

This boulder is closer to a shell hash; it’s mixed with sedimentary rock clasts. A few of the darker clasts have released iron, presumably from sulfide minerals. This formed in a very active coastal setting.

It’s a fascinating set of boulders, but an arbitrary one. It also includes big hunks of shale that have disintegrated in the sun and rain here. Who picked them, and why?

The Rocks of Leona Quarry

A few days ago I visited the former Leona Quarry, now slowly filling with townhomes. The view was great.

The rock slope on the north side looks rugged but is heavily engineered with many setbacks and retaining walls. But I was here to look at the rocks. There’s no trespassing on the high slopes, which is just as well because there is little bedrock to be seen. The whole hill is mapped as quartz keratophyre at the very bottom of the Great Valley sequence, of Jurassic age (no younger than 140 million years). It’s lava flows from an oceanic volcanic arc, like modern-day Japan or the Philippines, that have been heavily altered. It’s typically light colored with strong rust stains and full of fractures. You’ll see several different examples below.

Some outcrops show bits of less-altered gray lava, although this example is still full of veins.

This small boulder displays slickensides. The white mineralization includes quartz as well as carbonate minerals, sometimes layers of each in the same vein.

It’s pretty sterile, but life is taking hold amid the stones.

I took a couple hand specimens of the hardest, most pristine looking rocks. Even these have a greenish tinge from metamorphism. This specimen has faint banding in it, probably metamorphic rather than from flowing of the lava.

This kind of rock is almost impossible to study without using thin sections and a petrographic microscope. I don’t know what I would call it if the map didn’t say—probably just “altered lava.” Quartz keratophyre is specifically a metamorphosed trachyte, a highly alkaline high-silica lava. That’s meaningful to petrologists in a way that would put most of us to sleep if they were to tell us about it. I don’t mean that unkindly. But I’d hazard a guess that here it reflects some input from subducted continental sediments as well as the usual melted seafloor.

Rock from the Leona Quarry appears all around town in rustic walls, but I suspect that nearly all of its output went to crushed stone.

The trout ladder

The original rainbow trout come from Oakland; did you know that? The species Salmo iridia was first described, in 1855, from San Leandro Creek. Redwood Creek is a branch of San Leandro Creek that still contains good spawning grounds for rainbow trout, and this nicely maintained fish ladder is here to help them upstream.

The rainbow trout has been spread all over the world, of course. This wasn’t the only locality in the Coast Ranges they came from. The Oakland fish aren’t superior to other local strains. Their only distinction is artificial: they’re merely the first to come to scientific attention. Naturalists name species simply by picking an individual and describing it in detail in the literature. It’s arbitrary, but the only way to begin.

That’s as arbitrary as the creationist’s crude notion that the species is simply something uttered by God and named by Adam. Creationists doesn’t care to ask why populations vary in their genetic makeup—or if they do they regard it as the necessary decay from perfection of all earthly things. Variation is useless in their theory. But to naturalists, the reality of the species includes precisely the variation that the creationist downplays. Variation is the first rung in the ladder of evidence showing how the tree of life fits together as the result of evolution. The creationist’s interest in that project is only to suppress it.

Today we lump Salmo iridia as a subspecies under the larger species Oncorhynchus mykiss. Maybe God isn’t clear, or maybe our ideas need work. Maybe both statements mean the same thing.

A glimpse below

The new Christ the Light Cathedral is a beautiful structure, designed to guide the mind toward bliss, to allow the susceptible a glimpse of heaven. In early 2005 I sought a high place of my own type—a parking structure—to have a look at the cathedral’s construction site.

The site was at one time a glamorous high-Deco car dealership. Before that I don’t know, but the Oakland geologic map shows it as half fill and half “marine coastal terrace” deposits. The fill half would be on the lakeshore side, naturally. The terrace is basically a shelf of sediments deposited in San Francisco Bay during the last interglacial, more than 70,000 years ago, when the sea was a good five or ten meters higher than today. Only small, subtle bits of it are around today. The pit looks like it’s floored with nice clean golden sand. That might be aboriginal sediment, or it might be dirt from downtown hauled here to fill in the swampy lake shore, as it was around almost the whole lake. The downtown dirt is Merritt Sand, a widespread sheet of ancient windblown dune sand much like what underlies western San Francisco. That sand came here at the height of the last ice age, when the seas were very low, the weather was cold and the winds blew fine sand from the wide, exposed continental shelves onto the coastal hills.

If I had an hour to poke around these excavations! But only the geotechnical engineers get to do that, and maybe a touring group of their fellow professionals, all in hardhats. If any of those fine specialists are reading this, my email is geology at about dot com.