Art. XIX.—On an Ice-striated Rock-surface on the Shore of Circle Cove, Lake Manapouri.
[Read before the Otago Institute, 9th November, 1920, received by Editor, 31st December, 1920; issued separately, 4th July, 1921.]
On the 16th May, 1919, in the stratification of a series of low-lying bush-clad hills known as “The Peninsula,” Mr. Guy Murrell and the writer discovered a series of rounded, though fairly flat, rocks on the shore of Circle Cove, which is the first little inlet to the left after rounding Stony Point. On these rocks were marks quite different from anything that could be attributed to jointing or fracturing, and which there was no difficulty in attributing to ice-action. The marks run parallel for the length of the rock-exposure, and follow all the undulations. In places they are lost from view, as the hollows between the exposed parts either dip below water-level or are filled with gravel which has drifted into them.
The striated shelf extends along the shore about 150 yards, and is about 20 yards in width, at the summer level of the lake.
The marks are of all sizes, from sharply cut narrow lines to a trough about 2 ft. deep and 30 ft. or 40 ft. long, the bottom of which is polished as smooth as glass. The freshness of the marks is very noticeable.
The rocks as seen on the shore consist of beds of conglomerate, sandstone, fine silt, and thin seams of lignite. The conglomerate consists of granitic boulders set in a matrix of exactly the same material, so that when freshly broken it looks like homogeneous rock, but where weathered its components show out. The ice-marked shelf seems to be simply the lower stratum of the shore-rock, from which the overlying beds have been eroded. The scratches are sometimes in the conglomerate and sometimes on the other strata, according as the contour rises or falls. All, however, are polished so smooth that it is only where a face appears that the different layers can be distinguished.