48 
MUSEUM BULLETIN NO* 11. 
even run-off, no great volumes of water at one time, and more 
heavily loaded streams, which would mean that the streams would 
lay down their load and build flood-plains. Into these flood- 
plains the rivers would trench during the next cycle. It is a 
pretty well established fact that climatic changes are cyclic, 
both as regards rainfall and cold, and such conditions as we 
have indicated above are, therefore, not exceptional but rather 
to be expected, especially when the region is far removed from the 
tempering influence of the ocean. An example of such a cycle 
of climatic changes on a large scale is the alternation of glacial 
and interglacial stages which occurred during the Glacial epoch 
in eastern North America. 
A number of hypotheses may thus be put forward to explain 
the formation of these river terraces. Obstructions in the valley 
channels appear to account for some of the terrace fragments, 
and stream meandering may account for others. There remain 
the hypotheses of interrupted uplift and cyclic changes in climate, 
to account for alternations of deposition and downcutting. 
No evidence of uplift was found and the hypothesis of climatic 
changes is, therefore, proposed to account for the greater 
number of terraces. 
Summary. 
We have given evidence for believing that in early Tertiary 
times parts of the southern Interior plateaus like the Beaver- 
dell area were mountainous and rugged, but that just before the 
deposition of the later Tertiary lavas they had been reduced to a 
hilly but not very rugged region; that after the deposition of the 
lavas a cycle was started which progressed until the whole region 
had attained the stage of late maturity ; that it was then uplifted 
and certain of the main valleys deeply dissected. Later on 
the whole country was profoundly glaciated by a continental 
ice sheet, and the main valleys further visited by local valley glac- 
iers. The effect of glaciation was to remould and partly scour 
out the main valleys, disorganize the existing drainage, and 
provide plenty of rock waste for the later streams. After 
glaciation had ceased, a variety of conditions combined to form 
