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Show 648 The National Geographic Magazine Preparations for our long trip into northwest China completed, we crossed the border mountains. From the Kunlun mountains of central Asia, which separate the sources of the Yellow and Yangtze Rivers, this natural wall extends eastward and gives out near the coast on the Yellow Sea. South of this mountain wall annual rainfall is 40 inches and more. People there grow and eat rice. Originally the land was covered with dense forests. North of the barrier rainfall is from 30 inches to almost none in the Gobi. There people grow and eat wheat and millet. Originally the land for the most part was a vast grassland, with forests in alluvial valleys and on higher mountains. South of this natural wall no loess was deposited, but north of it lies the largest blanket of loess, or wind-blown soil material, on earth. Leaving the teeming Chengtu Plain, we traveled northward over the old Marco Polo Road, now widened for automobiles. Seldom in any country have I found scenery to equal that I saw in these borderlands. Valleys are narrow and sometimes 1,000 to 1,500 feet deep, so that the road must climb by switchbacks and hairpin turns, up one valley, across a sharp divide, then down again into another valley and up again to get through a maze of broken, deeply incised country. Sometimes the road has actually been niched out of the canyon walls, and the bluff above overhangs as a roof. Astonishingly, the natural timber-growing areas in these rugged borderlands have been cleared and cultivated up to the very summits with Indian corn-a New World gift to the Old. Often the slopes appeared as if tipped up by some giant who had tried to spill out the soil, crop and all. Cultivation is sometimes on such steep slopes that farmers slide or fall off their fields into the canyon. Loess Highly Subject to Erosion Such farming is a gross misuse of land. With billions of kilowatts of hydroelectric power in rushing borderland streams going to waste, I look forward to a time when industry will draw off some of the subsistence farm peoples and let the hillsides be reforested. The mountains would be worth far more for production of timber, wood pulp, and other forest raw materials than for growing farm crops and would provide employment for a much larger population (page 658). Throughout the border mountains fertility is declining, and in some places, as between Mienyang and Kienko, soils have been washed off to bedrock as a result of clearing away the forest for farming. The loess of North China is a remarkable and rich wind-blown deposit, probably contemporaneous with the ice sheets of North America and Europe during the Glacial period. Central Asia was too dry to produce an ice sheet, but high winds eroded a vast area of dry lands bordering the Gobi into clouds of dust that settled on the country to the south and east and left multitudes of dunes behind. A mantle of loess accumulated over a vast dust bowl and smoothed out a formerly rough topography. Some of the material was carried away by streams to form the great delta plain of the Yellow River. This blanket of soil material of talcum-powder fineness is about 500 feet thick near its origin, but is thinned out to the south, west, and east until it disappears as a deposit. Long cultivation has exposed sloping fields in the loess to sheet and gully erosion through centuries. Nowhere else on earth has a beautiful land been so deeply and widely gullied. Modern American Methods to the Rescue Our first work area was in the vicinity of Tienshui, southern Kansu, in the upper Wei River Valley (Plates I and I I I ) . Here we began a series of studies in detail of the condition of the land and measured up hundreds of fields after heavy rains. We put in experimental strip crops of alfalfa, rye grass, and sweet clover in mixture to discover how nearly we could get full absorption of rain by the soil on sloping land. We discovered that there was no runoff on flat lands, for loessal soils in flat fields absorbed all the rain that fell. If all the rain could be absorbed as it falls on sloping land, there would be no soil erosion, no loss of fertilizers, no loss of beneficent rains, no silting of streams; and maximum yields in food crops would be harvested. At the end of the 1943 rainy season, the heaviest since 1927, we found that our demonstration of narrow strip cropping on the contour with a contour channel had prevented any runoff on slopes up to 24 per cent! The steep walls of gullies, large and small, that have cut back into,the slopes through the years are a difficult and a different problem. These raw surfaces are both a loss and a menace. They yield little or no useful crop, but give rise to flashy runoff from rains that cuts and carries great quantities of earth, enlarging gullies and charging streams with enormous burdens of troublesome silt to damage other lands and interfere with flood-control works. |
