482 GEOLOGY OF CENTRAL WASHINGTON TERRITORY. 
would necessarily assume in the narrow valleys between parallel chains of mountains. But 
these existing lakes are the few and scattered remains of a vast system which stretched its arms 
into every recess long after the general elevation of the continent, and which has, in the course of 
time, been drained by the successive bursting of the barriers which confined them. Sufficient 
observations have not been made to determine their exact relations to each other, and to the 
expanse of water which seems to have occupied the country on the Columbia and Snake rivers, 
from the Dalles upward ; but the records, it is believed, exist for establishing with an extraordi- 
nary degree of accuracy the limits of each. The barometrical observations of the expedition 
were made with a view to the more direct objects of the survey only, and do not present any 
connected series of results bearing upon the former hydrographical face of the country. Such 
facts as have been collected will, however, serve at least to point the way for future observa- 
tions. In conducting any regular geographical survey of the country, and especially in running 
and marking the boundary, lines of elevation having reference.to the comparative heights of 
these terraces would have peculiar interest. 
Besides the terraces which line the valleys, another class, not characterized by the same 
straight lines, but arranged in amphitheatres, or otherwise conforming to the make of the elevations 
against which they rest, appear on the summits of the ranges dividing different branches of the 
Columbia. In crossing from Fort Okinakane to the Methow river, and again from the Okinakane 
lake to Fort Colville, such terraces were noticed at heights too great to suppose them to have 
been deposited, except at a period when the elevation of these mountains was in its early stage. 
As regards the others, however, they bear every mark of a much later date. 
As the route of the expedition met the Columbia above the lower obstructions through which 
it has forced its way, the point of the first barrier cannot be given with certainty. It is supposed 
to have been about the Priest’s rapids, and another very probably intervened between that and 
the Pisquouse. Upon this supposition, the Grand Coulée, which is said not to have been a 
continuous channel, may have been an arm or strait cutting off an island or peninsula. Numerous 
other coulées, which once separated islands, abound among these mountains, the terraced banks 
remaining at each end towards the river, while the intermediate level now constitutes an elevated 
valley or ‘land strait.” Such was the pass by which the expedition passed from the Columbia 
to Lake Chelan. The destruction of the barriers upon the rivers seems, in some cases, to have 
been violent, and others gradual. In the former case it would probably result that a mass of 
earth and rock, in proportion to the body of water liberated, would be swept down from the shore 
in the neighborhood of the ruptured dam. This may have been the cause of the removal of the 
large masses of gneiss which have been torn from their bed below the Pisquouse, as already 
mentioned, and scattered along the shore of the Columbia. These evidently originated in the 
immediate neighborhood, as they are not worn by transportation, and the strata above them 
seem to have been disrupted. Under any circumstances where a rapid current prevailed, but 
more particularly where its egress was sudden and violent, if the valley were circuitous, the 
terraces, consisting of sand and loose stone, would be swept away, in a great measure, unless 
protected by the bends of the river, in which case they would appear alternately on the different 
sides of the stream. Where, on the other hand, the fall was slower, as from the subsidence of 
the water by gradual or steady drainage, the terraces would remain perfect on both sides. So, 
too, if the subsidence were altogether checked, from time to time, for a considerable period, 
or occurred merely at irregular intervals, successive steps would be formed of greater or less 
height according to the amount of water escaped, and of greater or less width according to the 
length of the intervale, the diminution of the entire body of water, or increase in the amount of 
detritus brought down. The lower terraces in a valley will naturally be widest, because an 
amount of material, brought down during higher stages of water, will have been directly pre- 
cipitated or spread out, forming a level floor at the bottom of the valley, and its erosion will take 
place only to an extent proportioned to the size of the usual stream. 
