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Art. LXVIII.—On Bidwill's Front Hills.
[Read before the Wellington Philosophical Society, 10th January, 1880.]
There is a low range of hills, about seven or eight miles in length by about one mile in breadth, down which the road runs from the Featherston and Waihenga Ferry road to Mr. Bidwill's house, which forms an interesting meter of the immense work which the action of rain and rivers has performed in the Wairarapa Valley.
This range separates the low part of the valley to the westward towards Featherston, from the equally low part on the east containing the river flats of the Ruamahunga and the Wharekaka Plains. The height of these low
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plains is not much above the sea-level, let us say 20 or 30 feet, while the height of the ridge in question, I should estimate, in its highest part, to be fully 300 feet above the sea. Now this is higher than the level of Greytown or Carterton, and the corresponding level will probably be found in the middle of the Taratahi Plain.
At first sight there is nothing extraordinary in finding a spur running out from the mountains into a plain, but when we examine the structure of this ridge we find much upon which to speculate. I am aware that there are some remains of old rocks, apparently belonging to the Rimutaka series, in the core of the ridge in question, because years ago I found them myself, but the mass of the range appears to be entirely composed of river-borne gravels and clays (perhaps the latter would be better described as muds). The cuttings for the formation of roads bring this point more clearly to view than formerly.
Now this ridge having been formed of river-borne gravels and clays, the water at the time of its formation must have stood or run at a higher level than it, the ridge, stands at present, and either great denudation of the lower valley must since have taken place on both sides of the ridge, or the slope of the ridge must show a tendency to a termination of the deposit, either as a talus of materials dropped into a lake or as a finish-off of the deposit, when the rivers stood at a level of several hundred feet higher than they do now.
Bidwill's range is clearly a continuation of the deposits of gravels and clays which are found on the front ranges of the Huangaroa station, and through which the Ruamahunga has burst, leaving the former range isolated.
It is conceivable that the Ruamahunga and its tributaries ran originally along the ranges on the western side of the Wairarapa, until, by the increase and spread of their deposits, they gradually forced the bed of the main river to the eastern side of the valley and against the eastern ranges. If at this time we suppose the river to have had a channel over three hundred feet above its present level, we can account for the formation of Bidwill's range. Afterwards the river would cut its channel through its own deposits and carry its material further down the valley.
I am more disposed to believe in the existence of a former barrier and a great extension of the present lake, than to suppose the immense denudation of previous deposits of gravel and clay which is probably involved in any other supposition. The general evenness of the Wairarapa Plain seems to point to a spreading of the surface materials under lake water, and I think the appearance of the lower valley, approaching the lake, and including Bidwill's range, gives a similar impression. We might call in
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earthquake-action to account for the elevations and depressions, but I do not see any evidence to warrant it.
The amount of work which has been done by the Wairarapa rivers seems utterly out of proportion to their present strength, and either an enormous period of time must be allowed, or the amount of water must have formerly been vastly greater. The old deposits are precisely similar in character to those which are brought down at the present day. The Ruamahunga and its tributaries of the right bank, or from the Rimutaka, run clear and chiefly bring down gravel. The Taueru, coming from tertiary rocks, brings down softer materials, viz.,—sand, clay, and mud; and, after its junction with the Ruamahunga, a difference in the deposits from those in the upper valley may be observed, both in old and new deposits. We may, therefore, conclude that the same rivers which are at work now, have done the work formerly. Possibly by shutting up the Manawatu Gorge, that river might find its way, by some low saddle, into the basin of the Ruamahunga; or some change of level near the sources may have induced it to cut a channel to the West Coast.
Even supposing we bring the Manawatu into the Wairarapa Valley, we have apparently a small supply of force for the quantity of work done. To cover a valley, say sixty miles long and ten broad, with gravels and clays several hundred feet thick is an immense operation, which has been perhaps only exceeded in recent times in New Zealand by the rivers of the Canterbury Plain; but these rivers are much larger, and run from much higher mountains. There are other parts of the Wairarapa which may probably answer as texts as well as Bidwill's front hills, but I have neither time nor opportunity to take the levels, make the necessary observations, and draw the conclusions, and must therefore leave it to others to work up this interesting subject.