DENVER & RIO GRANDE WESTERN ROUTE. 11 
A few miles below Tolland the valley changes from a rocky V- 
shaped ravine to a broad valley having a U-shaped cross section, The 
meaning of such a change is shown in figure 4. The mountain valley 
shown in figure 4, A, has been carved only by the stream which occu- 
pies it. The walls slope gradually from the ridge on either side to 
the stream in its bottom, and the form of a section of such a valley, 
if cut directly across, would be a flat V. If after its excavation by 
the stream this same valley had been occupied by a glacier the ice 
would have ground away the projecting spurs on its sides and left 
it in the form shown in figure 4, B. The cross section of a valley is 
a nearly infallible indication whether the valley has been carved by 
running water alone or has been modified by ice. Thus the change 
from a V shape to a U shape a few miles below Tolland marks the 
point of farthest extension of the old glacier that had its source near 
the summit of James Peak and filled this valley with ice to a depth 
Fieurn 4.—Diagrams showing effect of stream and glacial erosion, A, V-shaped valley 
cut by running water; B, same valley after it has been occupied by a glacier and 
reduced to a broad, flat U in cross section. 
of many hundreds of feet if not a thousand feet. Usually the foot 
of a glacier of this magnitude is marked by a terminal moraine— 
a ridge of loose material carried down by the ice—but if such a 
moraine was ever built in this locality it has been washed away by 
the stream swollen with the waters of the melting ice. 
Although the valley at Tolland and for some distance above that 
place is broad and the slopes are smooth, it soon terminates abruptly 
at the foot of the Continental Divide, and no railroad can ascend it 
much farther and succeed in crossing the range. Consequently the 
engineers were forced to turn aside from what seems to be an easy 
pathway up the valley and construct the road to the summit in a 
roundabout way by scaling the valley walls. The train makes this 
climb with many turns and twists, and the traveler is generally 
deeply impressed with the care and precision with which the en- 
gineers fitted the roadbed to the mountain slopes. To the railroad 
engineer no slopes are too steep for railroad construction, provided 
he can find ground sufficiently level to enable the road to curve ~ 
