148 The N.Z. Journal of Science and Technology. [May 
The terraces shown in figs. 2, 3, 4, 6, and 7 are composed of rock : 
gravel terraces are common in New Zealand also, probably attaining their 
greatest development on the sides of the valleys which trench the Canter¬ 
bury Plains. 
Terraces of a peculiar kind are to be seen in some valleys in Otago, 
which, after being aggraded, were widened by lateral corrasion in such a 
way that, when later the streams in them began to cut downward again, 
they were flowing in some places on solid rock to one side of the filled 
valleys. Such portions of valleys are now bordered by terraces presenting 
a front of solid rock to the river ; but the solid rock is only a narrow bar 
between the present trench and the filled valley, and the terrace is under¬ 
lain some distance back by a great depth of alluvium. This state of 
affairs has been revealed in some cases by mining operations, the gold- 
bearing alluvial filling of the aggraded valleys having been in part removed 
[C. A. Cotton, photo. 
Fig. 7.—Front of a rock terrace in the Clarence Valley, Marlborough.. 
by sluicing. This is the case in the Shotover Valley, which is represented 
in the block diagram, fig. 8, and sketch, fig. 9. Deep gravel underlies the 
terraces which appear to the right in fig. 9 and on the distant bank of the 
river in fig. 8; and between the aggraded valley and the narrow trench 
now occupied by the river is a barrier of schist rock. At one point the 
rock barrier has been pierced by a tunnel to carry away the tailings from 
a sluicing claim. 
Terraces developed during Continuous Valley-excavation .—A degrading 
river may be constrained to deepen its channel very slowly, perhaps owing 
to very slow uplift, but more frequently owing to the fact that it crosses 
a barrier of hard rock. Up-stream from the barrier the river may flow over 
weak material across which it maintains with ease a graded condition 
while the hard-rock ledge—a local base-level—is gradually lowered. Such 
conditions exist where transverse rivers cross outcrops of alternating 
weak and resistant strata. They occur also in somewhat winding valleys 
