The Indo-Gangetic plain, spread by the Brahmaputra, the Ganges, and the Indus river systems, stretches for sixteen hundred miles along the southern base of the Himalaya Mountains and occupies an area of three hundred thousand square miles (Fig.342).  It consists of the flood plains of the master streams and the confluent fans of the tributaries 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 impure peat occur in it, and it contains fresh-water shells and the bones of land animals of species now living in northern India.  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.  High valleys among growing mountain ranges may suffer warping, or may be 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 maintain its right of way it may yet have its declivity so lessened that it is compelled to aggrade its course continually, filling the valley with river deposits which may grow to an enormous thickness.

Behind the outer ranges of the Himalaya Mountains lie several waste-filled valleys, the largest of which are Kashmir and Nepal, the former being an alluvial plain about as large as the state of Delaware.  The rivers which drain these plains have already cut down their outlet gorges sufficiently to begin the task of the removal of the broad accumulations which they have 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 found at various levels the remains of land plants and land and fresh-water shells, and in some the bones of such animals as the hyena and the goat, of species or of genera now living.  Such soft deposits cannot be expected to endure through any considerable length of future time the rapid erosion to which their great height above the level of the sea will subject them.