Friday, March 6, 2009

fort funston

It's low tide time again this weekend. My love affair with low tides started when I was a tiny person and got to spend hours on the rocky beaches in Seattle, peering under rocks for all the amazingness that lurked underneath (thanks Mom!). To this day one of my favorite places in the entire world is on the beach at a low tide.

So one of the things that has always bummed me out about San Francisco was its lack of rocky shores for exploring. There are truly amazing beaches just north and south of the city but they are, sadly, rather difficult to reach by public transport. (I will say in San Francisco's defense that the only octopus that I have ever seen at a low tide anywhere was in the rock wall off Marina Green. That was great.)

The ocean was calling my name last month at low tide, so I headed out to Fort Funston by way of a long walk down Ocean Beach. And honestly, it was lovely. The sandstone cliffs are beautiful, composed of soft soft rocks from the Merced and Colma Formations. The Merced Formation makes up most of the cliff face and the Colma Formation is the thin sandy layer at the top. The Merced was deposited from about 2 or 3 million years ago to about half a million years ago and the Colma, about 125,000 to 55,000 years ago. In some sections of the cliff face you can see a thick, chalky substance-veins of volcanic ash thrown from Mount Lassen thousands of years ago!

At the base of the cliffs is this thick black sand. I thought this was the sad remnants of the Cosco Busan spill but it turns out that it's a kind of iron ore called magnetite. The magnetite is a part of the cliff walls, and as the sandstone erodes, it leaves the heavier iron-based magnetite on the beach.

So I was relieved that the black stuff wasn't oil. But oh man, was there a lot of other garbage at the high tide line. From a distance I thought this junk was was sad to get up close and realize it was all plastic and styrofoam.

Ah....but there were shells lower on the beach.

Western Sand Dollar
(Dendraster excentricus)

These lovely critters form dense beds in the low intertidal and subtidal zone in the sand just beyond the break zone of coastal areas. Their living body consists of a rigid test (a hard external covering) covered with movable spines, usually a pale gray-lavender to a dark purplish black. The lovely flower-like pattern on the aboral side of the body is composed of pore pairs whose specialized tube feet are used for gas exchange. At the center is the madreporite - a perforated structure that forms the intake for their water-vascular system-a system of water-filled canals that connect with tube feet, which the animal use for locomotion, feeding and breathing. It's fun to stare at the ocean and imagine all the life out there just underneath the surface.

1 comment:

loebus said...

Thanks we were just at Fort Funston today and were wondering about the black sand. We were afraid it could be a product of the runoff pipeline that feeds into the ocean there.