Popular Science Monthly/Volume 24/December 1883/The Loess-Deposits of Northern China
|THE LOESS-DEPOSITS OF NORTHERN CHINA.|
SCIENTISTS as well as economists and statesmen are turning with a scrutiny, renewed as each year advances, toward the great region of middle Asia—a territory which, if it supplies society with immigrants much too thrifty for the tastes of our broader-minded Celtic brethren, bids fair in many ways to furnish materials for scientific research that can be compared in interest to no other portion of the world's surface. Without delaying to mention here the recent travelers who are rapidly lessening the bounds of that tract, still confessed to be the least known area of the globe, it is our purpose to direct attention to a geological phenomenon among the most important as well as peculiar of any hitherto brought to light in this field of investigation: we mean the loess-beds covering a great portion of Northern China.
The term loess, now generally accepted, has been used to designate a tertiary deposit appearing in the Rhine Valley, along the Danube, and in several isolated sections of Europe. Its formation has heretofore been ascribed to glaciers, but its enormous extent and thickness in China demand some other origin. The substance is a brownish-colored earth, extremely porous, and, when dry, easily powdered between the fingers, when it becomes an impalpable dust that may be rubbed into the pores of the skin. Its particles are somewhat angular in shape, the lumps varying from the size of a peanut to a foot in length, whose appearance warrants the peculiarly appropriate Chinese name meaning "ginger-stones." After washing, the stuff is readily disintegrated, and spread far and wide by rivers during their times of flood. Mr. Kingsmill, in the "Journal of the Geological Society" (London), states that a number of specimens, which crumbled in the moist air of a Shanghai summer, rearranged themselves afterward in the bottom of a drawer in which they had been placed. Every atom of loess is perforated by small tubes, usually very minute, circulating after the manner of root-fibers, and lined with a thin coating of carbonate of lime. The direction of these canals being always from above downward, cleavage in the loess mass, irrespective of size, is invariably vertical, while, from the same cause, water in falling upon a deposit of this material never collects in the form of puddles or lakes on its surface, but sinks at once to the local water-level.
The loess territory of China begins, at its eastern limit, with the foot-hills of the great alluvial plain—roughly speaking, upon the line drawn from Peking to Kaifung in Honan. From this rises a terrace of from ninety to two hundred and fifty feet in height, consisting entirely of loess; and westward of it, in a nearly north and south line, stretches the Tai-hang Shan, or dividing range between the alluvial land and the hill-districts of Shansi. An almost uninterrupted loess-covered country extends west of this line to Lake Koko-nor and head-waters of the Yellow River. On the north the formation can be traced from the vicinity of Kalgan, along the water-shed of the Mongolian steppes, and into the desert beyond the Ala Shan range. Toward the south its limits are less sharply defined; though covering all the country of the Wei basin (in Shensi), none is found in Sz'chuen, due south of this valley, but it appears in parts of Honan and Eastern Shantung. Excepting occasional spurs and isolated spots, loess may be considered as ending everywhere on the north side of the Yangtse Valley, and, to convey a general notion, as covering the parallelogram between longitudes 99° and 115° east, and latitudes 33° and 41° north. The district within China Proper represents a territory half as large again as that of the German Empire, while outside of the provinces there is reason to believe that loess spreads far to the east and north, possibly in varying thicknesses quite across the desert. Baron von Richthofen observed this deposit in Shansi to a height of 7,200 feet above the sea, and supposes that it may occur at higher levels.
One of the most striking as well as important phenomena of this formation is the perpendicular splitting of its mass—already referred to into sudden and multitudinous clefts that cut up the country in every direction, and render observation as well as travel often exceedingly difficult. The cliffs, caused by erosion, vary from cracks measured by inches to canons half a mile wide and hundreds of feet deep; they branch out in every direction, ramifying through the country after the manner of tree-roots in the soil from each root a rootlet, and from these other small fibers until the system of passages develops into a labyrinth of far-reaching and intermingling lanes. Were the loess throughout of the uniform structure seen in single clefts, such a region would indeed be absolutely impassable, the vertical banks becoming precipices of often more than a thousand feet. The fact, however, that loess exhibits in every locality a terrace formation, renders its surface not only habitable, but highly convenient for agricultural purposes; it has given rise, moreover, to the theory advanced by Kingsmill and some others, of its stratification, and from this a proof of its origin as a marine deposit.
But, since attention was first directed to this formation by Mr. Pumpelly, in 1864, its structure has been more carefully examined by other geologists, whose hypotheses are pretty generally discarded for that of Baron von Richthofen. This gentleman, who may be considered facile princeps among foreign geologists who have visited China, argues that these apparent layers of loess are due to external conditions, as of rocks and débris sliding from surrounding hill-sides upon the loess-dust as it sifted into the basin or valley, thus interrupting the homogeneity of the gradually rising deposit. In the sides of gorges near the mountains are seen layers of coarse débris which, in going toward the valley-bottom, become finer, while the layers themselves are thinner and separated by an increasing vertical distance; along these rubble-beds are numerous calcareous concretions which stand upright. These are, then, the terrace-forming layers which, by their resistance to the action of water, cause the broken chasms and step-like contour of the loess regions. Each bank does, indeed, cleave vertically, sometimes—since the erosion works from below—leaving an overhanging bank; but, meeting with this horizontal layer of marlstones, the abrasion is interrupted, and a ledge is made. Falling clods upon such spaces are gradually spread over their surfaces by natural action, converting them into rich fields. When seen from a height in good seasons, these systems of terraces present an endless succession of green fields and growing crops; viewed from the deep cut of some stream or road-bed, the traveler sees nothing but yellow walls of loam and dusty tiers of loess-ridges. As may be readily imagined, a country of this nature exhibits many landscapes of unrivaled picturesqueness, especially when lofty crags, which some variation in the watercourse has left as giant guardsmen of fertile river-valleys, stand out in bold relief against the green background of neighboring hills and a fruitful alluvial bottom, or when an opening of some ascending pass allows the eye to range over leagues of sharp-cut ridges and teeming crops, the work of the careful cultivator.
The extreme ease with which loess is cut away tends at times to seriously embarrass traffic. Dust made by the cart-wheels on a highway is taken up by strong winds during the dry season and blown over the surrounding lands, much after the manner in which it was originally deposited here. This action, continued over centuries, and assisted by occasional deluges of rain, which find a ready channel in the road-bed, has hollowed the country routes into depressions of often fifty or a hundred feet, where the passenger may ride for miles without obtaining a glimpse of field or landscape. Lieutenant Kreitner, of the danger of departing from the highway when in one of these deep cuts; after scrambling for miles along the broken loess above the road, he only regained it when a further passage was cut off by a precipice on the one side, while a jump of some thirty feet into the beaten track was his only alternative upon the other.exploring expedition (whose pleasant article on Thibet appeared in "The Popular Science Monthly" for August, 1882) illustrates, by a personal experience when in Shansi, the difficulty and
Difficult as may be such a territory for roads and the purposes of trade, its advantages to a farmer are manifold. Wherever this deposit extends, there the husbandman has an assured harvest two and even three times in a year. It is easily worked, exceedingly fertile, and submits to constant tillage, with no other manure than a sprinkling of its own loam dug from the nearest bank. But loess performs still another service to its inhabitants. Caves made at the bases of its straight clefts afford homes to millions of people in the northern provinces. Choosing an escarpment where the consistency of the earth is greatest, the natives cut for themselves rooms and houses, whose partition-walls, cement, beds, and furniture are made in toto from the same loess. Whole villages cluster together in a series of adjoining or superimposed chambers, some of which pierce the soil to a depth of often more than two hundred feet. In costlier dwellings the terrace or succession of terraces thus perforated are faced with brick, as well as the arching of rooms within. The advantages of such habitations consist as well in imperviousness to changes of temperature without as in their durability when constructed in properly selected places—many loess dwellings outlasting six or seven generations. The capabilities of defense in a country such as this, where an invading army must inevitably become lost in the tangle of interlacing ways, and where the defenders may always remain concealed, are very suggestive.
There remains, lastly, a peculiar property of loess which is perhaps more important than all other features when measured by its man-serving efficiency. This is the manner in which it brings forth crops without the aid of manure. From a period more than two thousand years before Christ, to the present day, the province of Shansi has borne the name of "Granary of the Empire," while its fertile soil, hwang-tu, or "yellow earth," is the origin of the imperial color. Spite of this productiveness, which, in the fourteenth century, caused Friar Odoric to admiringly call it "the second country in the world," its present capacity for raising crops seems to be as great as ever. In the nature of this substance lies the reason for this apparently inexhaustible fecundity. Its remarkably porous structure must, indeed, cause it to absorb the gases necessary to plant-life to a much greater degree than other soils, but the stable production of those mineral substances needful to the yearly succession of crops is in the ground itself. The salts contained more or less in solution at the water-level of the region are freed by the capillary action of the loess when rainwater sinks through the spongy mass from above. Surface moisture, following the downward direction of the tiny loess-tubes, establishes a connection with the waters compressed below, when, owing to the law of diffusion, the ingredients, being released, mix with the moisture of the little canals, and are there taken from the lowest to the topmost levels, permeating the ground and furnishing nourishment to the plant-roots at the surface. It is on account of this curious action of loess that a copious rainfall is more necessary in Northern China than elsewhere, for with a dearth of rain the capillary communication from above, below, and vice versa, is interrupted, and vegetation loses both its moisture and manure. Drought and famine are consequently synonymous terms here.
As to the origin of loess, Baron von Richthofen's theory is substantially as follows: The uniform composition of this material over extended areas, coupled with the absence of stratification and of marine or fresh-water organic remains, renders impossible the hypothesis that it is a water-deposit. On the other hand, it contains vast quantities of land-shells and the vestiges of animals (mammalia) at every level both in remarkably perfect condition. Concluding, also, that from the conformation of the neighboring mountain-chains and their peculiar weathering, the glacial theory is inadmissible, he advances the supposition that loess is a subaërial deposit, and that its fields are the drained analogues of the steppe-basins of Central Asia. They date from a geological era of great dryness, before the existence of the Yellow and other rivers of the northern provinces. As the rocks and hills of the highlands disintegrated, the sand was removed, not by water-courses seaward, but by the high winds ranging over a treeless desert landward, until the dust settled in the grass-covered districts of what is at present China Proper. New vegetation was at once nourished, while its roots were raised by the constantly arriving deposit; the decay of old roots produced the lime-lined canals which impart to this material its peculiar characteristics. Any one who has observed the terrible dust-storms of Northern China, when the air is filled with an impalpable yellow powder, which leaves its coating upon everything, and often extends in a fog-like cloud hundreds of miles to sea, will understand the power of this action during many thousand centuries. This deposition received the shells and bones of innumerable animals, while the dissolved solutions contained in its bulk staid therein, or saturated the water of small lakes. By the sinking of mountain-chains in the south, rain-clouds emptied themselves over this region with much greater frequency, and gradually the system became drained, the erosion working backward from the coast, slowly cutting into one basin after another. "With the sinking of its salts to lower levels, unexampled richness was added to the wonderful topography of this singular formation.
Mr. Pumpelly, while accepting this ingenious theory in place of his own (that of a fresh-water lake deposit), adds that the supply of loess might have been materially increased by the vast mers-de-glace of High Asia and the Tien Shan, whose streams have for ages transported the products of glacial attrition into Central Asia and North-western China. Again, he insists that Richthofen has not given importance enough to the parting planes, wrongly considered by his predecessors in the study of Chinese geology as planes of stratification. "These," he says, "account for the marginal layers of débris brought down from the mountains. And the continuous and more abundant growth of grasses at one plane would produce a modification of the soil structurally and chemically, which superincumbent accumulations could never efface. It should seem probable that we have herein, also, the explanation of the calcareous concretions which abound along these planes; for the greater amount of carbonic acid generated by the slow decay of this vegetation would, by forming a bicarbonate, give to the lime the mobility necessary to produce the concretions."
It is hardly within the scope of this article to do more than present in brief outline an exposition of the loess-theory that has made its originator already celebrated throughout Germany. Nor can we follow Baron von Richthofen further into the extension of his postulate, where in one is scarcely surprised at finding a plausible and attractive application of this idea of loess-formation to the entire Europe-Asiatic Continent, to the pampas of the South and prairies of the North American world. While the three or four northwestern provinces of China exhibit undoubtedly the strangest and most picturesque features of this formation, its influence upon the climate of Central Asia, the reactionary effect of this upon the surface configuration of the steppe-lands, and thus on the historical and ethnographical development of the cradle of the human race, are but some of the legitimate generalizations—if not necessary results—coming from this interesting phase of nature.