defective germ plasm shrivels one pea in almost every pod. I ain’t so pretty myself.
S A N D A few years ago, I grew interested in sand. Why is there sand in deserts? Where does it come from? I thought ocean waves made sand on seashores: waves pounded continents’ rock and shattered it to stone, gravel, and finally sand. This, I learned, is only slightly true.
Lichens, and ice and salt crystals, make more sand than ocean waves do. On mountaintops and on hillsides you see cracked rock faces and boulders. Lichens grow on them, inrings or tufts. “The still explosions on the rocks/the lichens grow in gray, concentric shocks,” wrote Elizabeth Bishop. These explosions blast the rocks; lichens secrete acids, which break minerals. Lichens widen rocks’ cracks, growing salt crystals split them further, and freezing water shatters them.
Glaciers make some sand; their bottoms pluck boulders and stones that scour all the land in their paths. When glaciers melt, they leave in outwash plains boulders, rocks, gravels, sand, and clays—the sand ground to floury powder. Winds lift the sand and bear it aloft.
Mostly, the continents’ streams and rivers make sand. Streams, especially, and fast rivers bear bouncing rocks that knock the earth, and break themselves into sharp chips of sand. The sand grains leap—saltate—downstream. So the banks and bottoms of most streams are sandy. Look in any small stream in the woods or mountains, as far inland as you like. That stream is making sand, and sand lies on its bed. Caddis-fly larvae use it as stones for their odd masonry houses.
Rivers bear sand to the sea. As rivers slow, they drop their sand, and harbors silt up and deltas spread. If the land’s rock is fresh lava, as it is in Tahiti and on the Caribbean coast of Costa Rica, the sand the streams bear down to the beaches is black. If the inland rock is basaltic, like the Columbia River plateau’s, the sand the river carries to beaches is dark andfine. If the rock is granite, as it is in the eastern United States, the sand is pale quartz and feldspar, granite’s parts.
When Los Angeles and Orange Counties dammed their intermittent streams, all the beaches from Los Angeles to Newport Beach lost their sand supply. Those weak hillside streams, which had never even flowed year-round, had supplied all that sand. Now beach towns buy dredged harbor sand to ship and dump.
Coastal currents smear sand round the continents’ edges. So there is sand on ocean beaches. Ocean waves do not make stony sand except where waves beat cliffs. Mostly, waves and longshore currents spread river sand coastwise, and waves fling it back at the continents’ feet. Ocean waves crumble dead coral reefs. And parrotfish eat coral polyps. The fish do not digest the corals’ limey bits, but instead defecate them in dribbles, making that grand white sand we prize on tropical beaches and shallow sea floors. Little or no sand lies under the deep oceans.
Why is there sand in deserts? Because windblown sand collects in every low place, and deserts are low, like beaches. However far you live from the sea, however high your altitude, you will find sand in ditches, in roadside drains, and in cracks between rocks and sidewalks.
Sand collects in flat places too, like high-altitude deserts. During interglacials, such as the one in which we live now,soils dry. Clay particles clump and lie low; sand grains part and blow about. Winds drop sand by weight, as one drops anything when it gets too heavy for one’s strength. Winds carry light stone dust—loess—far afield. Wherever they drop it, it stays put in only a few places: in the rich prairies in central North America, and in precious flat basins in China and Russia.
C H I N A Teilhard had glimpsed the Gobi Desert from muleback on his 1923 Ordos expedition. It was the biggest desert on earth: five hundred thousand square miles of sandstorms and ravaged
S A N D A few years ago, I grew interested in sand. Why is there sand in deserts? Where does it come from? I thought ocean waves made sand on seashores: waves pounded continents’ rock and shattered it to stone, gravel, and finally sand. This, I learned, is only slightly true.
Lichens, and ice and salt crystals, make more sand than ocean waves do. On mountaintops and on hillsides you see cracked rock faces and boulders. Lichens grow on them, inrings or tufts. “The still explosions on the rocks/the lichens grow in gray, concentric shocks,” wrote Elizabeth Bishop. These explosions blast the rocks; lichens secrete acids, which break minerals. Lichens widen rocks’ cracks, growing salt crystals split them further, and freezing water shatters them.
Glaciers make some sand; their bottoms pluck boulders and stones that scour all the land in their paths. When glaciers melt, they leave in outwash plains boulders, rocks, gravels, sand, and clays—the sand ground to floury powder. Winds lift the sand and bear it aloft.
Mostly, the continents’ streams and rivers make sand. Streams, especially, and fast rivers bear bouncing rocks that knock the earth, and break themselves into sharp chips of sand. The sand grains leap—saltate—downstream. So the banks and bottoms of most streams are sandy. Look in any small stream in the woods or mountains, as far inland as you like. That stream is making sand, and sand lies on its bed. Caddis-fly larvae use it as stones for their odd masonry houses.
Rivers bear sand to the sea. As rivers slow, they drop their sand, and harbors silt up and deltas spread. If the land’s rock is fresh lava, as it is in Tahiti and on the Caribbean coast of Costa Rica, the sand the streams bear down to the beaches is black. If the inland rock is basaltic, like the Columbia River plateau’s, the sand the river carries to beaches is dark andfine. If the rock is granite, as it is in the eastern United States, the sand is pale quartz and feldspar, granite’s parts.
When Los Angeles and Orange Counties dammed their intermittent streams, all the beaches from Los Angeles to Newport Beach lost their sand supply. Those weak hillside streams, which had never even flowed year-round, had supplied all that sand. Now beach towns buy dredged harbor sand to ship and dump.
Coastal currents smear sand round the continents’ edges. So there is sand on ocean beaches. Ocean waves do not make stony sand except where waves beat cliffs. Mostly, waves and longshore currents spread river sand coastwise, and waves fling it back at the continents’ feet. Ocean waves crumble dead coral reefs. And parrotfish eat coral polyps. The fish do not digest the corals’ limey bits, but instead defecate them in dribbles, making that grand white sand we prize on tropical beaches and shallow sea floors. Little or no sand lies under the deep oceans.
Why is there sand in deserts? Because windblown sand collects in every low place, and deserts are low, like beaches. However far you live from the sea, however high your altitude, you will find sand in ditches, in roadside drains, and in cracks between rocks and sidewalks.
Sand collects in flat places too, like high-altitude deserts. During interglacials, such as the one in which we live now,soils dry. Clay particles clump and lie low; sand grains part and blow about. Winds drop sand by weight, as one drops anything when it gets too heavy for one’s strength. Winds carry light stone dust—loess—far afield. Wherever they drop it, it stays put in only a few places: in the rich prairies in central North America, and in precious flat basins in China and Russia.
C H I N A Teilhard had glimpsed the Gobi Desert from muleback on his 1923 Ordos expedition. It was the biggest desert on earth: five hundred thousand square miles of sandstorms and ravaged
Similar Books
