PHYSIOCjRAPHY OF THE BEAVERDELL MAP-AREA. 
47 
seem to us adequate to explain whole series of such terraces 
occurring universally through the length of the larger valleys. 
That meandering rivers form small flat plains of deposit 
on the inside of their meanders is a matter of observation; also 
that a meandering stream often continues to cut down its chan- 
nel and that it is continually changing its course, the meanders 
moving downstream, or being entirely abandoned by cut-offs. 
The inner flood-plains of any particular meander would, therefore, 
be apt to be left as a tabular surface by the continually changing 
river, and stand a chance of being preserved as terraces of ir- 
regular form. The old meanders would be gradually filled by 
flood deposits until only traces of them remained. Terraces 
formed by meanders should lie first on one side of a river and 
then on the other, and should sometimes be preserved as iso- 
lated remnants in the middle of the valley floor. They should 
not be very far apart vertically if they are to be due to the normal 
down cutting of a swinging stream. In plan many of the ter- 
races on the bottom of the West Fork valley occur in just that 
way and one often lies only 20 or 30 feet over the other. Traces 
of old channels are not unusual in its wider parts. Rorick lake, 
below Beaverdell for instance, lies in part of an old abandoned 
river meander and many older and fainter channels are to be 
found on the valley floor (Figure 3). We cannot explain all 
the terraces in this way, however, for in parts of its courge to-day 
the stream is too straight and is cutting down rapidly, leaving 
narrow terraces rising in steps from its banks. 
A final hypothesis for the origin of these forms is that 
changes of climate, influencing the amount of rainfall and the 
abundance or scarcity of vegetation, would affect the erosive 
power of the streams. Stream cutting varies directly as the 
volume of water and indirectly as the amount of debris carried 
in the water. A change of climate involving cooler conditions, 
but not glaciation, would conserve the precipitation as snow and 
send it down the valley in torrents during the short warm season. 
At the same time the streams would not carry very great loads 
because of the partly frozen ground and the result would be 
downcutting by the great volumes of water rushing down the 
valleys. A return to warmer conditions would result in a more 
