See sea stars in the intertidal during one of the upcoming super-low tides.

QUEST blogger Andrew Alden’s recent post about Bay Area Tides got me thinking about pulling on my rubber boots and heading out to the intertidal during an upcoming low tide. In the next few weeks, we’ll get some really low tides during daylight hours—a great opportunity to see the organisms that live on the narrow edge between the land and the ocean.

Tides are caused by the gravitational pull of the moon and the sun. (See Science on the Spot: Watching the Tides for a nice, clear explanation.) The moon is a lot closer to the Earth than the sun is, so the moon’s influence on the tides is far greater than the sun’s. But sometimes, the sun and the moon can join gravitational forces and all that gravitational pull can create some really high (and really low) tides. Each year around January 2, Earth, in its elliptical orbit, is closest to the sun. Here, the sun’s gravitational pull on Earth (and Earth’s water) is strongest. The gravitational pull of the moon combines with the gravitational pull of the (relatively) nearby sun when the moon’s position is such that the Earth, sun, and moon are aligned in a straight line. This creates the highest high tides and the lowest low tides of the year. The exact dates vary each year, because it depends on where the moon is in its orbit. Usually we get these super high/super low sun-plus-moon tides, also called King Tides, in December and January. (When the earth is at the point in its orbit that is farthest from the sun, around July 2, and the moon is aligned just right, we also get super high and super low tides.) Super high tides can give us a preview of sea level rise and help us identify areas that are prone to submergence. And when the tide goes out, super low tides are a great opportunity to go tidepooling!

There are quite a few great tidepooling spots in the Bay Area, including Fitzgerald Marine Reserve, near Half Moon Bay. QUEST producer Joshua Cassidy made a fantastic short film about intertidal life in the Reserve. Natural Bridges State Beach in Santa Cruz is another great intertidal area. To see photos of some of its marine life, check out the QUEST Natural Bridges Tidepools Exploration (and see a fun audio slide show I made while I was an intern at QUEST).

My personal favorite place for tidepooling is Duxbury Reef, which is part of Point Reyes National Seashore and is close to Bolinas Lagoon. It has a really flat, rocky bench, and at low tide you can walk way out. Check out the turban snails (there seem to be zillions at this site), and the different species of seaweed (my favorite intertidal inhabitants). If you’re into identifying things and learning about intertidal ecology, there are a lot of great books out there: Seashore Life of the Northern Pacific Coast has nice color pictures, and Between Pacific Tides is a classic.

Wear rubber boots with a rugged sole (that seaweed is slippery) and maybe bring a magnifying glass or hand lens. Keep your wits about you, and look up and look around often so the tide doesn’t sneak in on you.

To choose a good day to go tidepooling, you need to look at a tide table, which lists the predicted times and tidal heights of all the high and low tides throughout the year. You can often get a tide table for your area at a local surf shop or bait shop. Or, check out NOAA’s Tide Predictions, which has tide tables for tidal stations throughout the country. (Tidal stations are places where the height of the water is measured regularly—often along with weather data. San Francisco’s tidal station is the oldest continuously operating tidal station in the western hemisphere, a fun fact I learned in the Watching the Tides video!) Find the tidal station closest to your tidepooling spot on NOAA’s map. My favorite, Duxbury Reef, is closest to the Bolinas Lagoon station. From the Bolinas Lagoon Station’s tide predictions page, download the station’s tide table—click on the Published Tide Tables Formats on the top right. Look for dates with a nice, low tide—something below 1.0 feet is generally pretty good, depending on the site. To time your visit, it is helpful to look at graph of the predicted height of the tide throughout the day. You can generate a graph for any day this year. In Northern California, wintertime low tides occur in the evening; find out what time the sun sets and plan to finish your intertidal excursion before it gets dark. We have some great low tides coming up on December 9, 23, 24, and 25—with heights at -0.9 feet—so ask for rubber boots for Christmas!

All photos by Andrew Alden

Pillar Point, topped by its military base, is the bulwark of Half Moon Bay on the San Mateo County coast. You may be familiar with it as the nearest place to the great Mavericks surf break. Its abrupt appearance along the shoreline, and the linear hill extending from it to the north, are geomorphic oddities that gain significance when the geologist checks them out in detail. Here's what the area looks like on the Geologic Map of San Mateo County:

The pink area is granite and related rocks of the Montara Mountain block, which I showed you in August. Those rocks are something like 80 million years old. The ridge between Pillar Point and the town of Moss Beach is made of something else, though: the much younger Purisima Formation, shown in beige. Between them is a thick black line denoting a good exposure of the San Gregorio fault, which we saw a few months ago at its other end, at Point Año Nuevo. The units labeled "Q" are geologically recent sediments and not rocks at all—Qmt represents marine terraces like those at Point Año Nuevo and elsewhere.

The coastline here and parts of the ridge are part of the Fitzgerald Marine Reserve, a county park with a dedicated group of supporters at fitzgeraldreserve.org. When you visit, leave your rock hammer at home. Oh, and check the tides first, too.

At the reserve's south end is a trail leading over the ridge to the beach. Here we get our first look at the extensive wave-cut platform, exposed at low tides, that makes Fitzgerald such a destination for tidepoolers as well as surfing fans.

The cliffs here are a good place to become acquainted with the Purisima Formation, a mixture of shale and mudstone with occasional large concretions along certain bedding planes. You can find a few fossils here if you look carefully, but there are better places farther along.

The middle portion of the Fitzgerald Reserve is on the east side of the ridge facing the Half Moon Bay airport. From Airport Road you can see where the San Gregorio fault runs along its base.

The north end of the reserve is where the tidepoolers all go. The fault comes out here, pointing straight at Stinson Beach in Marin County where it merges with the San Andreas fault.

Before you go out on the rocks, turn around and look at the east side of the entrance, which is typical marine terrace sand.

Then go to the west side and see the contrast presented by the Purisima Formation.

The Purisima in the tidepools is sanded by the surf to bring out its tilted bedding layers, which are punctuated by layers of dropstones and shell hash.

Shell hash is pretty much what it sounds like, a layer rich in shell fragments.

Farther along the north side of the beach, the rocks turn rapidly into coarse granite conglomerate, some layers containing outright boulders. It was an extremely vigorous environment that produced rocks like this—perhaps a high sea level and large landslides produced by major earthquakes. Perhaps something like Devils Slide today.

But at some point, be sure and climb to a higher vantage point to see why every geology class in the Bay Area comes here to practice mapping.

It is a magnificant syncline, or folded trough, where fault motion has bent the rock layers and the sea has planed them off into this dramatic arc.

Flickr photo community member jalbersmead (John Albers-Mead) will be collaborating with KQED staff on our next 2-minute YPOQ segment for broadcast and web distribution. It will air on November 18, 2008.

His wonderful set of tidepool images from Fitzgerald Marine Reserve in Moss Beach wowed our KQED QUEST editorial staff. His winning submission did a wonderful job of expressing a sense of locale, with a passion for nature, via a process that captures something unexpected and essential.

In his own words:

"Going to tide pools is like a treasure hunt that changes by the minute. The colors, textures, smells, movement, and general feeling of life makes this one of the places where I feel connected with the world."

This was a very difficult decision to make for us – we hope to do more YPOQ calls in the future. If you wish, you may leave your submissions open and we will consider them again in the next round. Sincere thanks to all who participated.

For those of you who are interested in entering the future, sign up for our email newsletter to get an announcement for the next submission call, or head on over to our Flickr photo group for KQED QUEST.

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