PART I.. The Origin and Formation of Soils.

THE ORIGIN AND FORMATION OF SOILS.

CHAPTER I.

THE PHYSICAL PROCESSES OF SOIL FORMATION.

Since the physical and mechanical effects of the agencies mentioned above usually precede, in time, the chemical changes, which are materially facilitated by the previous pulverization of the rocks, the former should be first considered.

Effects of heat and cold on rocks.—Most rocks are aggregates of several simple minerals; a few only (limestone, quartzite and a few others) expand or contract alike in all their parts. Of the minerals composing the compound rocks, scarcely any expand to exactly the same extent under the influence of the sun’s heat, especially when their colors differ; nor, in the great majority of cases, does one and the same mineral expand alike in all three directions. It follows that at each change of temperature there is a tendency to the formation of minute fissures between adjacent crystals or masses of different simple minerals; and especially in the case of large crystals of certain kinds, this action alone will gradually result in the disruption of the rock surface, so that individual crystals may be detached with little difficulty. In any case, the cracks so formed are gradually widened by a frequent repetition of the changes of temperature, coupled with access of air, water, dust, and the rootlets of plants; all of which brings about a gradually increasing rate of surface crumbling. This is especially conspicuous at the higher elevations of mountains, where the temperature changes are very great and abrupt; and also in the clear atmosphere of deserts, where owing to the extent and suddenness of temperature-changes between day and night, caused by the free radiation of heat into the clear sky, even homogeneous pebbles are known to be almost explosively disrupted in the mornings and evenings of clear days.

Such effects may often be strikingly observed on small surfaces of compound crystalline rocks, such as granite, exposed on glaciers, where the daily changes of temperature are often extreme, viz., from below the freezing point to as much as 130 degrees Fahr. (54.4 degrees C). In such cases one may sometimes scoop off the disintegrated rock by the handful, while yet the mineral surfaces are almost perfectly fresh.

On a larger scale, the disruption and scaling off of huge slabs of granite, and rocks of similar structure, may be observed in southern California on the southwestern side of rock exposures, where slabs from a few inches to ten and more feet in length and eight or ten inches thick, have slid off, perhaps still leaning against the parent rock, which has been rounded off by a succession of such events into the domelike form so characteristic of granite mountains. Merrill reports similar exfoliations to occur especially on the peninsula of California, on Stone Mountain in Georgia, and elsewhere.

See Rocks, Rock-weathering, and Soils, page 246; also paper on Domes and Dome Structure, by G. K. Gilbert, in Bulletins of the Geol. Society Am., Vol. 15, pp. 29-36.

A striking exemplification of the effects of frequent and rapid changes of temperature on rocks, and of humid and dry climates as well, is seen in the case of the great monoliths of Egypt, one of which now stands in the Central Park, New York. In the quarries of Syene in Upper Egypt, where most of these monoliths were obtained, the rough blocks that were in progress of quarrying when the work was abandoned, quite two thousand years ago, still show an almost perfectly fresh surface; and the same is true of the finished obelisks in Lower Egypt, where both the changes of temperature and the rainfall are somewhat greater. It is a matter of public note that one of “Cleopatra’s Needles” which was set up in Central Park nearly thirty years ago, but originally erected at Heliopolis on the Nile, is in great danger of destruction from the influence of a totally different climate, in which both the temperature changes and the rainfall are much more frequent and severe than in Egypt. The large crystals of feldspar and quartz which compose the (syenite) rock material have had fine fissures formed between them by often-repeated expansion and contraction; which when filled with water and subsequently changed to ice, the latter’s expansion in freezing (see below) has still farther enlarged them and caused a scaling-off, which threatens to obliterate the hieroglyphic inscriptions. Thus temperature-changes and a rain followed by freezing may in a few days produce a greater effect than a thousand years of Egyptian climate.

Cleavage of rocks.—Many kinds of rocks have definite directions of ready cleavage. The most common and obvious cases of this kind are schists, slates and shales, cleaving readily into plates or irregular flat or lens-shaped fragments. Such structure greatly favors disintegration, especially when the layers are on edge at steep angles. But there are other apparently structureless, massive rocks, particularly basalts and other eruptive rocks related to them, as well as many sandstones and claystones, that have a strong tendency to cleave into more or less definite forms when struck; such as columns or prisms, square, six-sided or diamond-shaped blocks, etc. Similar forms are naturally produced in them under the influence of changes of temperature; by the formation of minute cracks at first, then enlargement of these by the several agencies already mentioned.

Effects of freezing water.—The irresistible force exerted by the expansion of water in freezing, amounting to about 9 per cent of its bulk, is a powerful factor in widening and deepening fissures and cracks of rocks; not uncommonly, whole masses of rock are rent into fragments by this agency, which is one of the most common causes of “rock falls” on the brink of precipices. By the freezing process cracks and crevices are enlarged, and the surfaces exposed to weathering are still farther increased; and the rock fragments or soil particles are loosened and rendered more liable to be removed from the original site, whether by gravity, wind or water.

Glaciers.—Ice in the form of the glaciers that descend from mountain chains (see figure 1), and of the moving ice sheets that have covered large portions of North America and Europe in past ages and now cover Greenland and the South Polar continent, exerts a most potent action in abrading and grinding even the hardest rocks; not so much by the direct friction of the moving ice itself, as by the cutting, scoring, grinding and crushing action which the stones imbedded in the ice, or carried and shoved by it, exert upon the rocky channels in which the ice stream moves, as well as upon each other. The product of this grinding process is largely very fine (hence “glacier flour”), so that it remains suspended in the water of the glacier-streams until their velocity is permanently checked when reaching a plain or lake. This suspended stone-flour imparts to the glacier streams their distinctive character of “white rivers,” as contradistinguished from the clear, dark “green rivers” that have their origin outside of glaciated areas. This difference can be readily observed in traveling along any of the glacier-bearing mountain chains of the world, and is frequently expressed in the names of the streams.

The physical analysis of mud from the foot of Muir glacier, Alaska, at its sea front, made by Professor Loughridge, shows the prevalent fineness of the materials brought down by the glacier waters.

Collected by Dr. W. E. Ritter of the University of California.

PHYSICAL COMPOSITION OF GLACIER MUD.

============+====================+=============== MATERIAL. | DIAMETER. | PER CENT. ------------+--------------------+--------------- Clay | ? | 16.57 } 70.31 Fine silt | .0023 -- .016 mm. | 53.74 } Fine silt | .016 to .025 mm. | 4.38 Medium silt | .025 to .036 mm. | 7.06 Coarse silt | .036 to .047 mm. | 5.91 Coarse silt | .047 to .072 mm. | 3.76 Fine sand | .072 to .12 mm. | 1.14 Medium sand | .12 to .16 mm. | 1.56 | | ----- Total | | 94.12 ------------+--------------------+---------------

It will be noted that over 70 per cent of this mud consists of extremely fine, wholly impalpable materials; but little of which is true clay.

The fineness of the glacier flour renders it peculiarly suitable for the rapid conversion into soil, and such soils are usually excellent and remarkably durable. The great and lasting fertility of the soils of southern Sweden is traced directly to this mode of origin, and doubtless the great American ice sheet of glacier times is similarly concerned in the high quality of the soil of our “north central” states, from the Ohio to the Great Lakes and the Missouri.

The accumulations of rocks and debris of all sizes in the “moraines” or detrital deposits of glaciers and ice-sheets form another class of glacier-made lands which cover extensive and important agricultural areas (drift areas), both in the old and new worlds. Such lands are undulating or slightly hilly, and the soil usually contains imbedded in it stones of a great variety of kinds and sizes, partly angular, partly rounded and polished by friction. Of course the frequent and violent changes of temperature occurring on the surface of a glacier, aid materially in reducing the rocks carried by it to the condition in which we find the material of the moraines; which commonly form lateral or cross ridges in valleys formerly occupied by glaciers.

Action of flowing water.—The action of flowing water is doubtless at this time the most potent mechanical agency of soil formation. From the sculpturing of the original simple forms in which geological agencies left the earth’s surface into the complex ones of modern mountain chains, to the formation of valleys, plains, and basins out of the materials so carried away, its effects are prodigious. The torrents and streams in carrying silt, sand, gravel and bowlders, according to velocity and volume, do not merely displace these materials; the rock fragments of all sizes not only score and abrade the bed of the rill or stream, but by their mutual attrition produce more or less of fine powder similar to that formed by glacier action; usually more mixed in its ingredients than the former, because derived from a wider range of drainage surface. In the glacier stream itself, it is easy to trace the gradual transition from the sharp stone fragments lying in the water as it issues from the terminal ice cave at the lower end of the glacier, to the rounded shingle found a few miles below.

On slopes where water flows only during rain or the melting of snow, the same erosive effects may be seen as between the heads of ravines and their outlets. (See figure 2,) It is there too that the surprisingly rapid cutting-out of channels by the aid of water charged with rock fragments or gravel, can readily be observed, and the enormous power of water erosion convincingly shown. In the United States the stupendous gorges of the Columbia and Colorado rivers, the former cut to a depth of over 2000 feet into hard basalt rock, the latter to over 5,000 feet, partly into softer materials, partly into granite, are perhaps the most striking examples of this power; the manifestations of which can, however, be as convincingly seen in thousands of minor rivers and streams.

All the materials so carried off from the higher slopes are finally deposited on a lower level; whether only a short distance away on a lower slope (colluvial soils), or farther away in the flood plain of streams, rivers, or lakes (alluvial soils). Other things being equal, the finest materials are of course, carried farthest, and often into the sea; in which, however, they cannot long remain suspended, but are quickly thrown down, forming river bars, flood plains, and deltas. The fineness of the material of delta soils, like that of those made from glacier flour, insures them the same advantage, viz. great fertility and durability.

It is calculated that the Mississippi River carries into the Gulf of Mexico annually some 7469 millions of cubic feet of earthy deposits, which would fill one square mile of surface to the height of 268 feet, or would cover that number of square miles to the depth of one foot.

Wave-Action.—The powerful effects of the beating of waves upon abrupt shores of seas or lakes are in evidence all over the world, and these effects are so characteristic that they can be recognized even where no sea or lake exists at present. Gravel and sand are carried in the surf and serve as grinding materials, wearing even the hardest rocks into grooves, rills, channels and caves, defining sharply the varying degrees of hardness or tough resistance in different parts of rocky cliffs; frequently undermining them and causing extensive rock falls. The latter then serve for a time to break the violence of the waves’ onset, and may even cause permanent shore deposits to be formed under their lee.

Such deposits are very generally formed on gently sloping beaches, and as the water gradually recedes, sometimes by elevation of the ground, beach lines or beach-terraces are left, which indicate the successive levels of the lake or sea. Such old beach lines or terraces and level-surfaced “buttes” in the Great Basin country, and “bench lands” elsewhere, show in their structure the characteristic lines of wave-deposition.

Effects of Winds.—The action of winds in transporting soil particles (dust and sand) is familiar; and the accumulations that may be formed under the influence of regular, continuous winds are sufficiently obvious on lee shores having sandy beaches, inland of which the formation of sand dunes at times assumes a threatening magnitude. Where winds are irregular, frequently reversing their direction, of course the local effects will be less obvious, and the transportation of material actually occurring will often not be noticed. Yet there can be no doubt of the importance of wind action in soil formation, and there are cases in which no other agency can explain the facts observed over widely extended areas. This is especially true with regard to the soil masses of the high plains or plateaus of the dry continental interiors, where not only the regularity of the prevailing winds, but also the structure (or absence of structure) and pulverulent character of the soil itself, renders this the only rational mode of accounting for its presence where we find it.

The effects that may be exerted by regular winds are well illustrated in the plains and deserts of Africa as well as those of central Asia. Here we find a distinct subdivision of the desert (rainless) areas into the stony, from which the wind has swept all but the bedrock and gravel and where scarcely any natural growth, and certainly no cultivation is possible in the almost total absence of soil. The next subdivision is the sandy desert, to leeward of the stony area, where the winds are less violent and regular, and where, therefore, the sand has been dropped and is waited back and forth by “sand storms,” the surface being covered with moving sand dunes. Still farther to leeward we find the region in which the finer portions of the desert surface has been deposited; here we have “dust storms” so long as the land is not irrigated: but the application of water renders the soil abundantly fruitful. Such is the case of the Oases and fertile border-lands of the Sahara and Libyan deserts.

In the cultivated portions of the Mojave and Colorado deserts in California, plowing of the land during a dry time is not uncommonly followed by a bodily removal of the loosened soil to neighboring fields, sometimes leaving a gravel surface behind. Such “blown-out lands” exist naturally at numerous points in the Colorado desert.

Sven Hedin (Central Asia and Tibet, Vol. II.) shows that from the effects of the violent storms that prevail in the Gobi or Takla Makan desert, Lop-nor lake, the sink of the Tarim river, has in the course of time shifted its bed as much as fifty miles in consequence of the excavation of the southern part of the desert by the wind; while the sand so blown out, together with the deposits from the rivers, now tends to fill up the present (southern) lake, which is gradually returning northward toward its original site, now a desert, but around which formerly a dense population existed.

The great plains of North America, the pampas of South America, the plateaus of Mongolia and especially the fertile loess region of northwestern China, are also cases in point. The dense dust storms of these regions are familiar and unpleasant phenomena, which are often observed even by vessels at sea off the east coast of South America, where the dust-laden “pamperos” at times compel them to proceed with the same precautions as in a fog; and the same is true of the northeast winds blowing off the Sahara desert on the west coast of Africa.

The effects of windstorms carrying sand in the erosion of rocks are very obvious and striking in many parts of the world; nowhere probably as much so as on the great plains of western North America, where the geological composition of the “bad lands” is frequently impressed upon the rock surfaces very prominently. The strikingly grotesque forms are frequently brought out in this way, especially in the case of “mushroom” rocks, where a hard stratum has remained as a covering while softer layers underneath have been worn away. The illustration annexed shows such a case on the plains of Wyoming as figured in the Report of the U. S. Geological Survey, on the Central Great Plains, by N. H. Darton. Striking examples of the same effects are seen on the shores of Lake Michigan in the Grand Traverse region, where the rocky cliffs are visibly worn away and carved under the influence of the regular “sand-blasts” of northwest winds. On a smaller scale the effects of these sand-blasts may be noted in the cobble-deserts, where we frequently find the cobbles worn away on the windward side in a very characteristic manner; the lee side remaining rounded and smooth, while the structure of the rock is strongly outlined on the windward side.

CLASSIFICATION OF SOILS.

The physical Constituents of soils are thus, in the most general terms, first, rock powder (“sand”) more or less changed by weathering; second, clay, as one of the chief results of the weathering process of silicate minerals; and thirdly, humus, the dark-colored remnant of vegetable decay. According to the obvious predominance of one or the other of these primary ingredients, soils are popularly, in the most general sense, classed as “heavy” and “light”; the former term corresponding as a rule to those in which clay forms a prominent ingredient, while sandy and humous or “mold” soils usually fall under the latter designation, because of their easy tillage. For practical purposes these subdivisions are both convenient and important, and they form the ordinary basis of land classification. Beyond these, the degree of fineness of the rock debris, and their physical and chemical constitution, determine distinctions such as gravelly, sandy, silty, loamy, calcareous, siliceous, magnesian, ferruginous, and others of less general application, though locally often of considerable importance.

For the purposes of discussion and definition, however, another basis of classification is needed, which essentially concerns both the origin and the adaptations of lands.

1. Sedentary Soils.—When soils have been formed without removal from the site of the original rock, by simple weathering, they are designated as sedentary, or residual soils, or “soils in place.” In the case of these, the original rock underlies the soil or subsoil at a greater or less depth, according to the intensity and duration of the weathering process, and is usually more or less softened and decomposed at the surface where it meets the soil layer. The latter of course bears some of the distinctive characters of the parent rock, and its composition and adaptations may, in a measure, be directly inferred from that origin. Such soils usually contain, especially in their lower portions, some angular fragments of the parent rock. In some cases sedentary soils may have been partially derived from rocks that have been removed from above the present country rock by erosion, and in that case fragments of such vanished rock may also be present.

Sedentary soils are most commonly found on rock plateaus and on slopes or plains underlaid by rock strata of but slight inclination, where the velocity of the “runoff” rainfall is not sufficient to dislodge the rock debris. Extended areas of such soils exist in the granitic areas of the southern Alleghenies, in the “black prairies” of the Cotton States, and on the “basaltic” plateaus of the Pacific Northwest.

2. Colluvial Soils.—When the soil mass formed by weathering has been removed from the original site to such a degree as to cause it to intermingle with the materials of other rocks or layers, as is usually the case on hillsides, and in undulating uplands generally, as the result of rolling or sliding down, washing of rains, sweeping of wind, etc., the mixed soil, which will usually be found to contain angular fragments of various rocks, and is destitute of any definite structure, is designated as a colluvial one. Colluvial soil masses are frequently subject to disturbance from landslides, which are usually the result of water penetrating underneath, between the soil mass and the underlying rock, or sometimes simply of complete saturation of the former with water. Aside from such catastrophic action, they commonly have a slow downward movement in mass (creep), which ordinarily becomes perceptible only in the course of years; most quickly where there are heavy frosts in winter, which act both by direct expansion, and by the state of extreme looseness in which the soil mass is left on thawing. Colluvial soils form a large portion of rolling and hilly uplands, and are of very varying degrees of productiveness.

The term “overplaced,” used for such soils in late memoirs of the U. S. Geological Survey, is at least superfluous, in view of the perfectly understood term already in general use, and does not seem to commend itself for adoption by any special or superior fitness; nor does the suggestion of Shaler (The Origin and Nature of Soils, 12th Rep. U. S. Geol. Survey) to include the colluvial soils within the alluvial class, commend itself either from a theoretical or practical point of view, since but few useful generalizations can apply to both classes.

3. Alluvial Soils.—When soils are the result of deposition by streams, the material having been gathered along the course of the stream from various sources and carried to a distance before being deposited, the soil is designated as alluvial. These are the soils of the valleys, flood-plains, and sea- and lake-borders, past and present. Being of mixed origin, their general character may vary from one extreme to the other, both as regards physical and chemical composition. Since, moreover, they represent the finer portions of the soils of the regions drained by the water-courses, alluvial soils are as a rule of a fine texture; and as representing the most advanced decomposition products of the parent rocks, they are usually preëminently fertile. This is proverbially true of the flood-plains of rivers, and still more of their deltas—the bodies of lands formed near their outlets into seas or lakes.

Character of these soil-classes.—Sedentary soils are as a matter of course, other things being equal, dependent entirely on the parent rock for their specific character; and taking into consideration the various rocks (usually one or few) from which they may have been derived, nearly the same is true of colluvial soils, except that a portion of the clay and finest pulverulent matters may in their case be carried down on the lower slopes and into the valleys and streams, by the hillside rills.

According to the calculation of Merrill (Rocks, Rock-weathering, and Soils, p. 188) granite when transformed into soil without loss would increase in weight by 88%; more than doubling its bulk. More usually, the leaching process diminishes their volume as compared with the parent rock.

Alluvial soils are also of course to a certain extent dependent upon the character of the rocks and surface deposits occurring within the drainage area of the depositing stream. As a rule their composition is much more generalized; but their character as to the relative proportions of sand and clay is essentially dependent upon the velocity of the water current. Thus in the upper portions of valleys, where the slope is relatively steep and the velocity therefore high, a large proportion of cobbles and gravel is often present in the deposits, sometimes to the extent of rendering cultivation impracticable, or at least unprofitable. As the slope and velocity decrease, first coarse and then fine sand will be the prominent component of the deposited soil; while still lower down, in the region of slack water, the finest sand or silt, together with clay, will predominate. According to Hopkins, flowing water will, at a velocity of three inches per second, carry in suspension only fine clay (and silt); at eight inches it will carry sand as large as linseed. At one and one-third inches, it will move pebbles one inch in diameter; and at a velocity of two inches per second, pebbles of egg size are moved along the stream bed. Since the velocity of streams subject to freshets will vary greatly from time to time, deposits of very different grain will in such cases be found alternating with one another in the soil stratum of the flood plain. In fact, this alternation and the more or less stratified structure resulting therefrom, is the distinguishing mark of alluvial soils as such. It is true that this peculiarity is also sometimes found in the case of lands now lying far above the flood-plains of present rivers; but this is due to the elevation of the land or the depression of the river channels at a former period, prior to which such lands (commonly known as river terraces, benches or second bottoms) were formed. The same is true of lake terraces (“mesas”), which cover enormous areas in some parts of the world, more particularly in western North America. It must nevertheless be remembered that such alluvial terrace or bench soils differ in some respects from the modern alluvials, on account of their long exposure to atmospheric action alone; one result of which is that they are usually much poorer in humus, and therefore of lighter tints, than the more modern soils of alluvial origin. Other differences will be adverted to hereafter.

Geikie, “Text-book of Geology,” 3d ed.

As a matter of course the above distinctions, especially between colluvial and alluvial soils, cannot be rigorously maintained in all cases. There are transitions from one class to the other, so that it is sometimes optional with the observer to which of the two classes a particular soil may be considered as belonging. On the lower slopes of the hills bordering alluvial valleys the colluvial slope-soil may often be found alternating with the alluvial deposits, or bodily washed away to be redeposited as alluvium at a greater or less distance.

One characteristic of the flood-plain lands of all the larger rivers, and more or less of all streams subject to periodic overflows, is that the land immediately adjoining the banks is both higher and more sandy than are the lands farther back from the stream. The cause of this phenomenon is that as lateral overflow diminishes the velocity of that flow, its coarser portions are deposited near the river banks, while the finer particles are carried farther away, until finally only the finest—clay-substance—reach the lagoons or lakes filled with the overflow or backwater, and are there in the course of time deposited as heavy clay “swamp” soils. The same occurs where rivers empty into lakes or the sea; and these slack-water or delta lands are, as a rule, the most productive on the river’s course. The continued productiveness of alluvial soils is moreover in many cases assured by the deposition, during overflows, of fresh soil-material brought down from the head waters of the streams. The Nile, and the Colorado river of the West, illustrate this point.

Lowering of the land-surface by soil formation.—It is evident that the soil-forming agencies must in the course of time materially affect both the surface conformation and the absolute level of the land. The sharp pinnacles and crests of rock are abraded into the rounded forms now characterizing our uplands and lower ranges of hills and mountains; and it is estimated that, e. g., the general level of the drainage basin of the Mississippi river is lowered about one foot in 7,000 years, the material being carried into the lowlands and the sea.