Archive for October, 2009

The Rocks of Leona Quarry

31 October 2009

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

leona quarry

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.

leona quarry

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

leona quarry

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

leona quarry

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

leona quarry

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.

leona quarry

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

25 October 2009

rainbow trout

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 don’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.


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