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People and the Coast Sand Stories Waves and the Moving Sand The Great Wall Rivers, Mountains, and Sea Level Canyons under the Sea Abyssal Catastrophe Few things are more pleasant than walking along the beach barefoot, feeling the sand between the toes. Millions enjoy these simple delights every year. They come and spread their blankets on the sand and watch the children build sand castles , fated to be washed away by the waves of the rising tide. They watch the shorebirds hunting for worms and crabs hidden within the sand. But what is sand? What is the nature of the grains that feel gritty between toes and fingers? What stories do they have to tell? The answer depends on the location, of course. The beaches of southern California generally have mineral grains derived from the weathering of rocks in nearby mountains to the east, mixed with similar grains from the sandy deposits that make up the cliffs rising landward of the beaches. The mountains are largely made of igneous and metamorphic rocks, that is, the minerals they deliver upon weathering were made deep inside the Earth. Material locally derived from cliff erosion is commonly marine sediment, with ground-up shell mixed in. Some beaches hardly have any sand but consist of pebbles. Off Scripps, the beach consists of a layer of well-sorted sand, several feet thick, with pebbles at the base of the sand layer. The beach rests on a terrace cut into the land by waves. The terrace grows as the cliffs keep retreating. In turn, this retreat threatens houses built on the edge of cliffs. Among the mineral grains making up the beach sand, quartz is the most conspicuous. Quartz is typical for many beaches fed from terrestrial sources, for the simple reason that quartz is resistant to abrasion and chemical destruction and outlasts most other types of grains. Sand, then, tells stories about its origin and about its travel to the site where we find it. But the sand is usually not just sitting there—it is in transit. Waves wash the sand, move it seaward and landward according to season, and 125 FIVE The Zen of the Beach MUSINGS ON A RIVER OF SAND move it along the shore, as well. The general direction of travel is south, because winter waves from the north are the most effective in moving the sand along the shore. Eventually, the sand comes up against an obstacle, such as a promontory . It then has no place to go but down into the deep sea, commonly within a canyon carved into shelf and slope. Exactly how this vanishing act is accomplished was a complete mystery for a long time; some elements of the process still are obscure despite much study. The layers formed at the final resting place of the sand—in a basin offshore or at the foot of the continental slope— tell a story of giant muddy floods invading a usually quiet environment. To survive such events, animals living on the bottom subject to episodic flooding have to be able to escape upward through the layer of mud left by a flood. Those that fail to escape make fossils. Beaches on the West Coast, if present at all, tend to be rather narrow, and the beaches of San Diego are no exception. As a thin band of bright sand, they separate the vast ocean to the west from the former wetlands and elevated terraces to the east, now largely developed along a wide coastal strip. No longer a lonely marine biology station as it was in its early years at the beginning of the twentieth century, Scripps is now situated at the northern rim of a thriving coastal metropolis. PEOPLE AND THE COAST More than half—perhaps two-thirds—of the people in the world live within a few tens of miles from the shore.1 The coastal regions of California have always been attractive to people. Ancient shell middens along the shores of California testify that humans lived here for thousands of years, including on the shores of San Diego.2 A coastal location has many advantages, even if seafood is no longer the main motivator. In southern California, the mild climate along the coast, with cool summers and mild winters, is especially appealing. Farther inland, the seasonal contrast increases markedly. Most of the largest cities of the world are on the coast and have large harbors to facilitate trade; those away from the ocean commonly are...

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