Day 11: Geology
Apologies for the long radio silence, it’s been an expansive stretch of coastline with no computers to check-in on! Although I’ve passed through many colorful communities — Gualala, Manchester, Elk, Albion — most have little more than a grocery store and Post Office, and the one town large enough to have a public library, Point Arena, I happened to pass through on Sunday when everything was closed and quiet. But in the past few days, I made it through Sonoma and into a new county, Mendocino, this afternoon reaching its picturesque eponymous town.
And of course, happy Cinco de Mayo! We Americans (I’m including myself here) always seem to need a reminder that Cinco de Mayo is not a celebration of Mexican independence from Spain, the equivalent of our 4th of July. Instead, it celebrates the improbable victory of the Mexican army over invading French forces at the Battle of Puebla in 1862. Mexican independence — celebrated September 16 — actually happened long before, back in 1821 (when, incidentally, California was part of Mexico).
But enough of history — today’s update is about the remarkable geology I’ve been walking over during the past few days. Zoomed out, the beautiful Mendocino coast appears as a series of prominent marine terraces (or wave-cut benches). Carved at sea level by wind and waves, the terraces consist of a steep cliff at their seaward edge, followed
by a long, flat bench extending toward the foothills. They rise out of the ocean and lead inland like a giant staircase, as though a Godzilla-like monster might climb out of the Pacific in ponderous steps, sheets of water pouring from its body, and start chomping Mendocino cattle from their pastures.
These marine terraces puzzled me as I walked along the edge of the shoremost one, looking down its cliff at the raucous waves below. My guidebook to the California Coastal Trail states that “where the coastline is being pushed upward over time by the collision of two tectonic plates, a series of terraces exists…” But surely, I reasoned, if this coastal uplift is gradual, then you should end up with a gradually sloping coast, not a series of terraces. Why the step-like shape of the Mendocino coast?
Reading on, I discovered a clue: “… each terrace is backed by another terrace around 100,000 years older and 100 feet higher than than the one closer to the coast.” Aha! That special number, 100,000 years, is as familiar to the earth scientist as π = 3.14. It is the period of the glacial cycles — the time between one ice age and the next, throughout the past 2 million years or so of the Pleistocene. So what do ice ages have to do with marine terraces? Ice ages change global sea level, thus changing where the waves eat away at the coast.
During interglacial periods, sea level stands high, where it slowly carves the California coastline into a flat bench. Then, when Earth’s orbit around the sun is right, we move into a glacial period, locking up so much water in the great polar ice sheets that sea level drops, by 100 meters or more. The bench that was formed goes high and dry — and over time rises even higher due to coastal uplift. After 100,000 years go by, we return to a new interglacial period, melting ice and raising sea level — and begin carving a new bench below the first one. Yes! Looking down at the new bench forming in the waves 100 feet below, during our own interglacial (the Holocene), I knew this must be the answer to the marine terrace riddle. For the earth scientist, making connections like this one is a special kind of joy — like finding a fossil underfoot, or feeling the thunder in your chest as a glacier calves — that briefly let us touch the timeless processes at work in our world.
Almost timeless, I corrected myself. The terrace forming in the waves below probably isn’t going to rise up out of the waves and form another coastal stair-step like those that came before it — at least not anytime soon. Although we’re about due for a new glacial period (which would drop sea level and raise this young terrace out of the water, there to be uplifted by tectonic action like its forebears), human alteration of Earth’s climate is leading to rising sea level, by as much as a meter in the next century. This will soon bury the incipient terrace deep beneath the waves. That we could interrupt the geologic processes that formed the very ground I’m standing on — that the town of Mendocino is built on — now that is a remarkable thought.