9 
1918.] The N.Z. Journal of Science and Technology. 
New Zealand’s Niagara. 
By C. A. Cotton, D.Sc., F.G.S., Victoria University College, Wellington. 
The Wairua Falls, in North Auckland, form an excellent example of the 
■class of which the Falls of Niagara are the type. Though they are far 
surpassed both in height and in volume of water by numerous falls in 
various parts of New Zealand, they are still of great interest as an example 
of sharp-edged, rapidly retreating falls in horizontal strata. The scenic 
beauty of the falls may be impaired as their water is utilized for the genera¬ 
tion of electrical power, but their value as a physiographic example still 
remains. 
Falls are features characteristic of the yourg stage of valley-formation 
—of the stage, that is to say, in which the river, having recently taken 
Fig. 1 . —Wairua Falls, from the side. 
its present course, has not yet succeeded in smoothing out its bed to a 
regular, gradual, or graded slope. Without entering upon a discussion of 
the reasons for and the method of this activity, the reader may personify 
the river and think of it as labouring eagerly at the task of evening out 
its channel. While some rivers resort to filling in the deeper parts of their 
courses, the majority do their “ grading ” by cutting down in those parts 
of their beds in which they flow initially at a level above that of the coveted 
graded slope. The details of topographic form developed during down¬ 
cutting depend to a large extent upon the geological structure encountered ; 
but one type only of geological structure here concerns us—namely, hori¬ 
zontal stratification. The most clear-cut falls-—-those of the true Niagara 
type—are determined by the occurrence of a horizontal resistant stratum 
overlying one of weaker rock. 
The Niagara Biver, with which the humbler Wairua may be compared, 
came into existence when Lake Erie overflowed ; and the infant river took 
a course which led it down over a cliff at the edge of a horizontal sheet of 
limestone : thus the Niagara Falls had their beginning. The Wairua, on 
