The rivers which drain the plateau—­the Snake, the Columbia, and their tributaries—­have deeply trenched it, yet their canyons, which reach the depth of several thousand feet, have not been worn to the base of the lava except near the margin and where they cut the summits of mountains drowned beneath the flood.  Here and there the plateau has been deformed.  It has been upbent into great folds, and broken into immense blocks of bedded lava, forming mountain ranges, which run parallel with the Pacific coast line.  On the edges of these tilted blocks the thickness of the lava is seen to be fully five thousand feet.  The plateau has been built, like that of Iceland, of innumerable overlapping sheets of lava.  On the canyon walls they weather back in horizontal terraces and long talus slopes.  One may distinguish each successive flow by its dense central portion, often jointed with large vertical columns, and the upper portion with its mass of confused irregular columns and scoriaceous surface.  The average thickness of the flows seems to be about seventy-five feet.

The plateau was long in building.  Between the layers are found in places old soil beds and forest grounds and the sediments of lakes.  Hence the interval between the flows in any locality was sometimes long enough for clays to gather in the lakes which filled depressions in the surface.  Again and again the surface of the black basalt was reddened by oxidation and decayed to soil, and forests had time to grow upon it before the succeeding inundation sealed the sediments and soils away beneath a sheet of stone.  Near the edges of the lava plain, rivers from the surrounding mountains spread sheets of sand and gravel on the surface of one flow after another.  These pervious sands, interbedded with the lava, become the aquifers of artesian wells.

In places the lavas rest on extensive lake deposits, one thousand feet deep, and Miocene in age as their fossils prove.  It is to the middle Tertiary, then, that the earliest flows and the largest bulk of the great inundation belong.  So ancient are the latest floods in the Columbia basin that they have weathered to a residual yellow clay from thirty to sixty feet in depth and marvelously rich in the mineral substances on which plants feed.

In the Snake River valley the latest lavas are much younger.  Their surfaces are so fresh and undecayed that here the effusive eruptions may well have continued to within the period of human history.  Low lava domes like those of Iceland mark where last the basalt outwelled and spread far and wide before it chilled (Fig. 341).  In places small mounds of scoria show that the eruptions were accompanied to a slight degree by explosions of steam.  So fluid was this superheated lava that recent flows have been traced for more than fifty miles.

The rocks underlying the Columbia lavas, where exposed to view, are seen to be cut by numerous great dikes of dense basalt, which mark the fissures through which the molten rock rose to the surface.