this stage in the process, or even
before it is attained, the valley is likely to be submerged beneath
the sea, where it is buried beneath the deposits formed on the floor;
or a further uplift of the land may occur with the result that the
stream is rejuvenated; or once more endowed with the power to create
torrents, build alluvial plains, and do the other interesting work of
a normal river.

It rarely, if ever, happens that a river valley attains old age before
it has sunk beneath the sea or been refreshed by further upliftings.
In the unstable conditions of the continents, one or the other of
these processes, sometimes in different places both together, is apt
to be going on. Thus if we take the case of the Mississippi and its
principal tributaries, the Ohio and Missouri, we find that for many
geological ages the mountains about their sources have frequently, if
not constantly, grown upward, so that their torrent sections, though
they have worn down tens of thousands of feet, are still high above
the sea level, perhaps on the average as high as they have ever been.
At the same time the slight up-and-down swayings of the shore lands,
amounting in general to less than five hundred feet, have greatly
affected the channels of the main river and its tributaries in their
lower parts. Not long ago the Mississippi between Cairo and the Gulf
flowed in a rather steep-sided valley probably some hundreds of feet
in depth, which had a width of many miles. Then at the close of the
last Glacial period the region sank down so that the sea flooded the
valley to a point above the present junction of the Ohio River with
the main stream. Since then alluvial plains have filled this estuary
to even beyond the original mouth. In many other of our Southern
rivers, as along the shore from the Mississippi to the Hudson, the
streams have not brought in enough detritus to fill their drowned
valleys, which have now the name of bays, of which the Delaware and
Chesapeake on the Atlantic coast, and Mobile Bay on the Gulf of
Mexico, are good examples. The failure of Chesapeake and Delaware Bays
to fill with _debris_ in the measure exhibited by the more southern
valleys is due to the fact that the streams which flow into them to a
great extent drain from a region thickly covered with glacial waste, a
mass which holds the flood waters, yielding the supply but slowly to
the torrents, which there have but a slight cutting power.

In our sketch of river