A little south of Samarra the stony plateau of Mesopotamia ends, and the alluvial plain of Irak, ancient Babylonia, begins.
The floor of this depression being below baselevel, it has necessarily come to be the seat of the mountain waste brought down by the many streams from the newly uplifted Sierra Nevada on the east and the coast ranges on the west; each stream forms an alluvial fan of very gentle slope; the fans all become laterally confluent, and incline very gently forward to meet in a nearly level axial belt, where the trunk riversthe Sacramento from the north and the San Joaquin from the south-east--wander in braided courses; their tendency to aggradation having been increased in the last half century by the gravels from gold washing; their waters entering San Francisco Bay.
It is evident from the foregoing remarks, that while even the smallest stream may make deposits of alluvial character it is in the flood-plains and deltas of large rivers that the great alluvial deposits are to be found.
Five descents lead from it to the lower town, planted on the alluvial terrace, 30 to 35 ft.
Iridosmine occurs commonly with gold or tin in alluvial drifts.
At the mouth of the Selenga, however, which enters from the south-east, pouring into it the waters and the alluvial deposits from a drainage area of 173,500 sq.
The barrage rests entirely on the alluvial bed of the Nile.
A small part of the northern Colombia, on the lower courses of the Atrato and Magdalena, extending across the country from the Eastern to the Western Cordilleras with a varying width of 100 to 150 m., not including the lower river basins which penetrate much farther inland, also consists of low, alluvial plains, partly covered with swamps and intricate watercourses, densely overgrown with vegetation, but in places admirably adapted to different kinds of tropical agriculture.
There are stony wastes, or alluvial fans, where mountain streams emerge upon the plains, in time of flood, bringing detritus in their torrential courses from the mountain canyons and depositing it along the mountain base.
As stream tin it occurs abundantly in some of the alluvial deposits derived from the crystalline area, especially on the west coast.