Art. XLIV.—On Water-worn Pebbles in the Soil.
[Read before the Wellington Philosophical Society, 9th July, 1884.]
In the soil and subsoil of the Hataitai Peninsula, and I am inclined to believe of all the district surrounding Port Nicholson, water-worn pebbles may be found, in general sparsely distributed, possibly a foot or a yard or more from each other, reminding one of the old story of the school plum-pudding where the drummer-boy had to be called in to beat his instrument to call the plums together.
The occurrence of these pebbles may seem of small import, but in reality they form a very puzzling geological problem, possibly involving great movements of the earth's surface. How they got into their present position I shall try to explain; but I am quite open to conviction if any one can produce a more plausible theory.
An explanation by a subsidence of the land to the extent of 1,000 feet or more is not admissible. There is no appearance of marine strata or action above a height of about 15 feet from present high-water mark.
It may be suggested that these pebbles are the remains of a conglomerate, or of a coarse sandstone rock, which has undergone decomposition, but no traces are found of any such rock.
Some years ago I endeavoured to point out that Port Nicholson had formerly been a fresh-water lake. Does not the occurrence of these pebbles support that theory, supply direct evidence of lacustrine deposit which I previously admitted we had not found, and show that the boundaries of the lake were far outside those of the present harbour?
I have reason to suppose that there are deposits of gravel high up on the hills surrounding Port Nicholson. These may show the margin of the ancient lake, while the sparsely deposited pebbles may show where these were gradually distributed by waves or currents over the lake bottom.
The theory involves great movements in the land, and a great sinking in the direction of the Straits, but I do not see any other way out of the difficulty.
To obtain further proof of lacustrine deposit may be difficult, and will at all events take much time. Should these deposits occur in the harbour they must be overlaid by those of marine origin, and could only be found by boring, but there are strata at Karori and Johnsonville, etc., lying on the old rocks, which it might be well to study to see whether they are lacustrine or marine. The talus on which that part of Wellington called Thorndon is built might also yield some information, as also the lower parts of Te Aro.