1918.J The N.Z. Journal of Science and Technology. 
145 
RIVER TERRACES IN NEW ZEALAND. 
Fig. 1 . —Entrenched meanders. 
By C. A. Cotton, D.Sc., F.G-.S., Victoria University College, Wellington. 
River terraces are among the most characteristic features of the typical 
New Zealand landscape. Their flat surfaces were developed as valley- 
floors by rivers which, generally speaking, are still in existence, but which 
flow now at lower levels. All river terraces, however, have not the same 
history. 
Terraces developed in connection with 
Entrenchment of Meanders:— Entrenched 
meanders are developed as an accom¬ 
paniment of general rejuvenation of the 
topography. The condition necessary for 
their initiation is the presence, before 
rejuvenation, of broadly opened, flat- • 
floored valleys in which rivers follow 
meandering courses (as in fig. 1, block A). 
When a portion of a valley of this kind 
is uplifted, the younger, new valley, 
which is inevitably developed within 
the older, is guided by the existent 
windings of the river-channel, so that 
the inner trench of the resulting valley- 
in-valley form winds about on the 
earlier flood-plain beneath which it is 
entrenched (fig. 1, block B). 
Free swinging of meanders on the flood-plain must, obviously, be 
checked at once when the stream begins to cut downward ; but there is 
still the same tendency as before to cut into the concave bank, against 
which the strongest current impinges. Thus as the meandering channel 
is deepened, if the stream is a vigorous one, the curves are enlarged and 
each is pushed a short distance down the valley. As a result the slopes 
descending from the ends of spurs 
of the abandoned flood-plain, and 
from their down-stream sides, to 
the stream below are less steep 
than are the slopes descending to 
the stream along its concave banks 
(see fig. 1, B). 
Sometimes the more gently 
sloping convex side of the inner 
valley of an entrenched meander 
descends as a series of terraces, 
which seem to mark a series of 
pauses during discontinuous uplift. 
During each pause the river, if 
eroding weak rock and more or less 
keeping pace in its downcutting with the uplift, swings laterally and 
cuts a narrow flood-plain, only to go on deepening its valley again, and to 
repeat this process after each small movement of uplift. The resulting 
10—Science. 
Fig. 2.— Diagram of terraces on the slip- 
off slope of an entrenched meander of 
the Awatere River. 
