Considerable deposits of mud, silt and sand are accumulating in many of the estuaries.
Covering the higher parts of the south-western Palaeozoic area in most places are rolling hills of boulder clay or stony moraines; while the lower levels are plains gently sloping toward the nearest of the Great Lakes and sheeted with silt deposited in more ancient lakes when the St Lawrence outlet was blocked with ice at the end of the glacial period.
On the Coastal Plain the soil is generally sandy, but in nearly all parts of this region more or less marl abounds; south of the Neuse river the soil is mostly a loose sand, north of it there is more loam on the uplands, and in the lowlands the soil is usually compact with clay, silt or peat; toward the western border of the region the sand becomes coarser and some gravel is mixed with it.
In the mechanical analysis of the soil, after separation of the stones and fine gravel by means of sieves, the remainder of the finer earth is subjected to various processes of sifting and deposition from water with a view of determining the relative proportions of sand, silt and clay present in it.
In the valleys of rivers which have overflowed their banks and on level bench lands there is considerable silt and vegetable loam mixed with glacial clay; but on the hills and ridges of western Washington the soil is almost wholly a glacial deposit consisting principally of clay but usually containing some sand and gravel.
The water should be made to run completely off and the ditches should become dry before the influx of the next tide, otherwise the silt will not incrust and the tide not have the same effect.
The load of silt borne down stream by the river finally, after many halts on the way, reaches the waters of the Gulf, where the decrease of velocity, aided by the salinity of the sea water, causes the formation of a remarkable delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river).
About 1880, while the Gediz Chai was throwing its silt unchecked into the Gulf of Smyrna and gradually filling the navigable channel, there was talk of reviving Fokia as a new port for Smyrna, and connecting it with the Cassaba railway.
But while the wisdom of one age thus succeeded in restricting within bounds the tidal water of the river, it was left to the greater wisdom of a succeeding age to improve upon' this arrangement by admitting these muddy waters to lay a fresh coat of rich silt on the exhausted soils.
The first difficulty that presented itself in carrying this out, was that during the months of highest flood the Nile is so charged with alluvial matter that to pond it up then would inevitably lead to a deposit of silt in the reservoir, which would in no great number of years fill it up. It was found, however, that the flood water was comparatively free from deposit by the middle of November, while the river was still so high that, without injuring the irrigation, water might go on being stored up until March.