224 
The N.Z. Journal of Science and Technology. 
[Nov. 
DEPARTMENTAL REPORT. 
Geological Features disclosed by Excavations at the 
Proposed Dam Site at Arapuni, Waikato River. 
By J. Henderson, New Zealand Geological Survey. 
The Arapuni Gorge and the nature of the rocks through which it is cut 
have already been described by Sealy* and by the present writer,*)* and only 
a few details, obtained in the course of a recent visit, can here be added 
to those accounts. Briefly, the gorge is cut through deposits of pumiceous 
tuff and breccia, chiefly of subaqueous origin. At the dam-site these rocks 
were laid down as three deposits, here termed respectively the A, B, and C 
tuffs, and separated by two irregular surfaces of erosion. 
The A beds, which form the upper 30 ft. or 35 ft. of the cliff on the 
eastern side of the river at the dam-site, consist of massive light-coloured 
tuff somewhat weathered but well consolidated. Their base, a rather 
soft 6 ft. layer of sand and silt bands, rests on an irregular surface of the 
underlying beds. These silts, which are cut through by A shaft and No. 8 
adit,J are exposed at the cliff-face at the mouth of the adit. The under¬ 
lying surface may be traced southward, in which direction it forms a wide 
uneven shelf, prominent along the cliffs on both sides of the river. 
On the eastern side of the gorge the B beds form the lower 40 ft. of a 
cliff of which the base is concealed by a talus slope reaching almost to the 
water’s edge. This part of the B beds, which has been explored by the 
adits 9 and 1 and by the rises A and P, consists of strong light-coloured tuff 
and breccia. The lower part of the B beds, concealed by the talus, has 
been explored by adits 2 and 3, and consists of dark friable tuff and soft 
sandstone. The overlying light-coloured tuff exposed at the top of rise D 
grades downward into the dark friable tuff. Eastward along No. 2 adit 
the friable tuff passes into soft pumiceous sandstone, and a like gradation 
must occur between adits 2 and 3. At the base of the soft sandstone are 
bands of soft silt and pumiceous grit stained by iron oxides and resting on 
an irregular surface of the underlying C beds. Exploratory excavations 
from adits 2 and 3 show that the soft beds decrease in thickness as followed 
eastward along the proposed course of the dam, and that the surface of the 
underlying beds rises in the same direction. Thus at shaft U, 220 ft. from 
the face of the cliff, the soft sandstone (no dark friable tuff is present) is 
about 15 ft. thick, whereas at the cliff-face the soft beds are about 50 ft. 
thick, with their base 70 ft. lower, near normal water-level. On the western 
side of the river the same erosion surface, 65 ft. above river-level, is over- 
lain by beds of well-consolidated pumiceous grit and sandstone about 20 ft. 
thick, passing upward into strong light-coloured tuff and breccia. This 
basal layer is exposed in No. 6 adit and in the rise from its end. 
The thickness of the C beds, as shown in adits 3, 4, and 5, in rises and 
winzes therefrom, and in the level under the river, is at least 130 ft. As far 
as explored these beds consist of strong, well-consolidated breccia, darker 
* C. B. Sealy, Some Notes on the Engineering Survey of the Proposed Arapuni 
Dam Site, N.Z. Jour. Sci. & Tech., vol. 3, pp. 72-79, 1920. 
t J. Henderson, Notes on the Geology of the Waikato Valley near Maungatautari, 
N.Z. Journ. Sci. <0 Tech., vol. 1, pp. 56-60, 1918. 
J A plan and section showing the position of these excavations and others mentioned 
below accompanies the paper by C. B. Sealy. 
