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In part the loess seems to have been washed from glacial waste and spread in sluggish glacial waters, and in part to have been distributed by the wind from plains of aggrading glacial streams. In some regions the ancient valleys were completely filled with drift.
It has been estimated that each inundation of the Nile leaves a layer of fertilizing silt three hundredths of an inch thick over the flood plain of Egypt. Flood-plain deposits of great thickness may be built by aggrading rivers even in valleys whose rock floors have never been thus widened.
TERRACES. While aggrading streams thus tend to shift their channels, degrading streams, on the contrary, become more and more deeply intrenched in their valleys. It often occurs that a stream, after having built a flood plain, ceases to aggrade its bed because of a lessened load or for other reasons, such as an uplift of the region, and begins instead to degrade it.
On the other hand, if its load is diminished it has energy to spare, and resuming its work of erosion, DEGRADES its bed. In either case the stream continues aggrading or degrading until a new gradient is found where the velocity is just sufficient to move the load, and here again it reaches grade.
These aggrading rivers, which have channels but no valleys, spread their muddy floods which in the case of the Oxus sometimes equal the average volume of the Mississippi far and wide over the plain, washing the bases of the desert dunes. PLAYAS. In arid interior basins the central depressions may be occupied by playas, plains of fine mud washed forward from the margins.
The aggrading streams by which flood plains are constructed gradually build their immediate banks and beds to higher and higher levels, and therefore find it easy at times of great floods to break their natural embankments and take new courses over the plain. In this way they aggrade each portion of it in turn by means of their shifting channels,
As a matter of fact we do not know when it was abandoned. Further investigation might point to its having been deserted when the Spanish village of San Geronimo was founded. However, I believe students of agriculture will agree with me that deforestation, increased erosion, and aggrading gravel banks probably drove the folk out of Saylla.
At Lucknow an artesian well was sunk to one thousand feet below sea level without reaching the bottom of these river-laid sands and silts, proving a slow subsidence with which the aggrading rivers have kept pace. WARPED VALLEYS. It is not necessary that an area should sink below sea level in order to be filled with stream-swept waste.
BRAIDED CHANNELS. A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels.
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