STRUCTURE OF THE VALLEY DEPOSITS 281 



borders are remarkably straight, not infrequently have outliers which 

 lie a little way removed toward the centers of the troughs. These are 

 of the harder rocks, such as are found in a region of varied structure. 

 It is a noticeable fact that scarcely any of these isolated masses occur 

 far out from the main ranges. Very seldom in valleys 10 miles in width 

 do they appear at more than a mile from the upper margin of the valley 

 slopes. Now, it may be assumed that in any normal process of valley 

 erosion producing basins 10 to 20 miles in width there must have been 

 many outlying eminences left on its floor, of which the summits would 

 have remained at a relatively great height above the bottom of the ex- 

 cavation. We should, for instance, in vales such as these were before 

 they were in large part filled, expect to find, even near their central parts, 

 outlying elevations some thousands of feet in height above the level of 

 the drainage channel. As above remarked, except along the margins of 

 these wide expanses, we do not find an}^ trace of these normal remain- 

 ders of erosion where it acts on rocks of diverse resistance, such as are 

 found in almost all parts of the region we are considering. These topo- 

 graphic outliers may be seen in all- conditions of occlusion b)'' the valley 

 beds, from the state in which the rocks connecting them with the main 

 ranges are still uncovered to that where only the ver}^ tips of the sum- 

 mits break the surface of the detrital slopes. Although this kind of 

 evidence clearly indicates the very great original depth of these valleys 

 before the process of obstruction began, it does not afford any sufficient 

 basis on which to reckon their original depth in terms of feet or even in 

 miles. A study of the facts in the field, however, brings the observer to 

 the hypothesis that in the average of these vales — as, for instance, that 

 of the Jefferson or Gallatin river — the amount of the infilling must 

 amount to not less than 5,000 feet. 



The evidence goes to show that the erosion of the valleys of the Rocky 

 mountains was accomplished with exceptional rapidity. This is well 

 indicated by the fact that the remains of ancient river channels, now 

 filled with gravels, lie at a remarkable height above the present levels 

 of the main streams. Thus, on the mountain known as " Old Baldy," 

 in Madison county, Montana, there are the fragments of several deep 

 troughs, which were originally the seat of large mountain torrents, which 

 are filled with river gravels to thi? depth of over 200 feet. These gravels 

 are of so recent an aspect that they have been mapped on the United 

 States Geological Survey sheets as glacial drift, a mistake which was 

 most natural for the reason that it is onl}'' recently that extensive exca- 

 vations, in searching for placer gold, have revealed their true character. 

 The unmetamorphosed condition of these gravels makes it seem likely 

 that they are of no older than Tertiary age ; yet since they were deposited 



