42 
GEOLOGY—ORIGIN OF CANONS. 
Still later, having crossed the main ridge north of the Three Sisters, we noticed the same 
phenomena extending down to the altitude of 4,459 feet, where they terminated in a deep 
canon, through which a stream flowed into the Willamette valley. This canon led down from 
Mount Jefferson, and was joined by another, which came from the Three Sisters. At their 
point of junction they had a depth of more than a thousand feet. Here, as before, the furrows 
in the rock pointed to the Three Sisters, hearing from us a little south of east. 
TRAP-LEDGES, EXHIBITING MARKS OF GLACIAL ACTION. 
We had evidence in these scratched and furrowed rocks of the former existence in these 
mountains of glaciers, which extended down at least 2,500 feet below the present line of per¬ 
petual snow. I suspect, indeed, that they descended much lower, and that they filled the 
canons of which I have spoken, which, in the regularity of their outline, and the accurate slopes 
of their sides seem to have been formed by some such cause. The area over which these marks 
of glacical action extend is probably very large, including the slopes of the Three Sisters, 
Mount Jefferson, and all the line of peaks which mark the crest of the chain. And there is 
little doubt that all this surface was once covered, not simply by lines of ice following the valleys, 
but by a continuous sheet which, on the west, reached down to the base of the next and lower 
line of mountains, and that the sides and summits of these subordinate peaks, rising high 
above the lowest point where I noticed the grooves and scratches, were also capped and covered 
by masses of ice. These glacial grooves do not seem to have attracted the attention of those 
who have crossed the Cascade Range further north ; but we can hardly suppose that while 
here the evidences remain of glaciers, so wide, and extending to so low a level, that they could 
have been produced by the operation of local causes ; or that upon more careful examination 
the traces of their former existence will not be found on the flanks of Mount Hood, the loftiest 
peak in the chain, of Mount Rainier, and Mount Adams north of the Columbia, and of the 
many other lower but still elevated summits. 
Origin of canons .—The indications of the existence, in former times, of glaciers extending 
over large surfaces in the Cascade mountains is closely connected with the formation of the 
deep canons, through which the streams which drain these mountains and the plateau of the 
“Great Basin” beyond uniformly flow. I think we have evidence in the magnitude of the 
excavation—often in the most resistant material—as compared with their present volume, that 
the amount of precipitation which formerly supplied them was much greater than now. 
The Golden Gate, the Straits of Carquines, the canon of Pit river—which, following its 
tortuous course is, for nearly a hundred miles, cut through a succession of walls of volcanic 
rock—the many canons of Klamath river, those of the Des Chutes and its tributaries—to which 
I shall soon have occasion to refer—and the gorge of the Columbia, the most stupendous of all, 
