The Elements of Geology

rms which they assume may be treated under three heads:

Y DEP

along the channel, while a shallow sheet of water moves slowly over the flood plain, spreading upon it a thin layer of sediment. It has been

eing widened by the lateral erosion of a graded stream (Fig. 60). Flood-plain deposits of great thick

y sides. These wide natural embankments are due to the fact that the river deposit is heavier near the bank, wh

od plain are often ill-drained and swampy, and here vegetal

pi or the Missouri (Fig. 77), shows that the courses of th

and higher levels, and therefore find it easy at times of great floods to break their natural embankments and take

t course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and build

ause of a lessened load or for other reasons, such as an uplift of the region, and begins instead to degrade it. It leaves the original flood plain out of reach of even the highest floods. When again it reaches grade at a lower level it produces a new floo

he flood plain of either river or creek we may find examples of the successive stages in the development of the meander, from its beginning in the slight initial bend sufficient to deflect the current against the outer side. Eroding here and depositing on the inner side of the bend, it gradually reaches first the open bend whose width and length are not far from equal, and later that of the horse

it deposits on the inner bank scroll-like flood-plain patches. For a while the valley bluffs do not give its growing meanders room to develop to their normal size, but as pla

o have a widened and mature valley over its lower course, while its young headwaters are still busily eroding their beds. Its ungraded branches may thus bring down to its lower course mo

ivered to the river. It now has energy to spare and again degrades its valley, excavating its former f

The cause is obvious: the young branch has not been able as yet to wear its bed to accordant level with the already deepened valley of the master stream. It therefore

the western side of the Sierra Nevada Mountains have spread fans with a radius of as much as forty miles and a slope too slight to be det

hannels called DISTRIBUTARIES, since they lead off the water from the main stream

y flow out upon the thirsty lowlands and are therefore compelled to lay down a large portion of their load. In humid climates mountain-born streams are usually competent to carry their loads of waste on to the se

s proved by the remains which it contains of land animals and plants of species now extinct. That it was laid by rivers and not by fresh-water lakes is shown by its structure. Wide stretches of flat-lying, clays and sands are interrupted by long, narrow belts of gravel which mark the channels of the ancient streams. Gravels,

mense thickness, and such deposits of past geological ages have not infrequently been preserved, with all their records of the times in which they were built, by being carried b

close it,-the Sierra Nevada on the east and the Coast Range on the west. On the waste slopes at the foot of the bordering hills coarse gravels and even bowlders are left, while over the interior the slow-flowing st

. These are land deposits, and the absence of any marine deposits among them proves that the region has not been invaded by the sea since the accumulation began. It has therefore been slowly subsidi

aries which issue from the mountains on the north. Large areas are subject to overflow each season of flood, and still larger tracts mark abandoned flood plains below which the rivers have now cut their beds. The plain is built of far- stretching beds of clay, penetrated by streaks of sand, and also of gravel near the mountains. Beds of i

blockaded by rising mountain folds athwart them. Where the deformation is rapid enough, the river may be ponded and the valley filled with lake-laid sediments. Even when the river is able to mainta

ave brought in from the surrounding mountains. Their present flood plains lie as much as some hundreds of feet below wide alluvial terraces which mark their former levels. Indeed, the horizontal beds of the Hundes Valley have been trenched to the depth of nearly three thousand feet by the Sutlej River. These deposits are recent or subrecent, for there have been fou

f the shifting channels. Occasional beds of muck mark the sites of shallow lakelets or fresh-water swamps. The various strata also contain some remains of the countless myriads of animals and plants which live upon the surface of the plain as it is in process of building. River shells such as the mussel, land shells such as those of snails, the bones of fishes and of suc

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ely. The rivers of the surrounding mountains, fed by the rains and melting snows of these comparatively moist elevations, dry and soak away as they come down upon the arid plains. The

s mouth in Aral Lake the Oxus receives no tributaries, since even the larger branches of its system are lost in a network of distributaries and choked with desert sands before they reach their master stream. These aggradin

the playa is covered with a thin sheet of muddy water, a playa lake, supplied usually by some stream at flood. In the dry season the lake evaporates, the

enty miles wide. In summer it disappears; the Quinn River, which feeds it, shrinks back one

d gravel are found along; their margins, being brought in by streams and worn by waves from the shore, but there are no tidal or other strong currents to sweep coarse waste out from shore to any considerable distance. Where fine clays are now found on

lets are not infrequent on the surface of the younger drift sheets of Michigan and northern Indiana, where their beds of marl-sometimes as much as forty feet thick-are utilized in the manufacture of Portland cement. The deposit results from the de

plants of the surface of the bog lies a spongy mass composed of dead vegetable tissue, which passes downward gradually into PEAT,-a dense, dark brown carbonaceous deposit in which, to the

ed and returned to it in its original form of carbon dioxide. But decomposing in the presence of water, as in a

ry have not yet been drained. One tenth of the surface of Ireland is said to be covered with peat, and small bogs abound in the drift-covered area of New England and the states lyi

na is forty miles across. It is covered with a dense growth of water-loving trees such as the cypress and black gum. The center of the swamp is occupied by

salt and the sulphates and carbonates of lime, of soda, and of potash, and these are thrown down from solution one after another as the point of saturation for each mineral is reached. Carbonate of lime, the least soluble and often the most abundant mineral brought in, is the first to be precipitated. As concentration goes on, gypsum, which is insoluble in a strong brine, is deposited, and afterwards common salt. As the saltness of the lake varies with the seasons and with clima

their mountainous shores at the different levels at which they stood, and in the deposits of their beds. At its highest stage Lake Bonneville, then one thousand feet deep, overflowed to the north and was a fresh-water lake. As it shrank below the out

with the salts absorbed by them as the briny water of the lake dried away. These sedimentary clays are in two divisions, the upper and lower, each being about one hundred feet thick. They are separated by heavy deposits of well-rounded, cross-bedded grav

LT

being taken up and swept farther on downstream. Although these land forms may long persist, the particles which compose them are ever changing. We may therefore thin

or filled, and their deposits, if above sea level, resume their journey to their final goa

nts, and to dispose of it in ways which we shall study later. But often the load is so large, or the tides are so weak, that much of the waste which the

in the velocity of the river, compelling a deposit of the load; both are triangular in outline, the

t of waste brought down by the river and redistributed and spread by waves and tides over the sea bottom adjacent to th

ar to those of the flood plain, of which indeed the subaerial delta is the prolongation. Here extended deposits of peat may accumulate in s

ss. Layers of peat, old soils, and forest grounds with the stumps of trees are discovered hundreds of feet below sea level. In the Nile delta some eight layers

to the Gulf at a maximum rate of a mile in sixteen years. Muddy shoals surround its front, shallow lakes, e.g. lakes Pontchartrain and Borgne, are for

the highest tides. The submarine portion of the delta has been built to near sea level over so wide a belt o

ooded with salt water when the sea is brought in by strong southwesterly monsoon winds, and the climate during the remainder of the year is hot and dry. By the evaporation of sea water the soil is thus left so salty that no vegetatio

ided. The entire thickness of the Ganges delta, for example, so far as it has been sounded, consists of deposits laid in open air. In other cases interbedded limes

osion until a movement of the earth's crust in the opposite direction lifts them to form part of the land. We shall read later in the hard rocks of our

waters of lakes. The shore lines of extinct lakes, such as Lake Agassiz and Lakes Bonneville

ed chiefly of material thus worn from still more ancient lands-lands with their hills and valleys like those of to-day-and carried by their rivers to the ocean. In all geological times, as at the present, the work of streams has been to destroy the lands, and in so doing to furnish to the ocean th