Art. XLV.—On Changes in the Hataitai Valley.
[Read before the Wellington Philosophical Society, 9th July, 1884.]
As I am rapidly bringing the Hataitai Valley into cultivation, and thereby destroying geological landmarks, it may be as well to put on record what will soon be lost to the eye. At present I refer particularly to the old forest which at one time filled the valley, and whose stumps and logs are apt to smash my ploughs. Doubtless the hills also were at one time covered by forest, but all traces of this seem to have long ago disappeared.
There are a series of sea-formed lakes, detached rocks, etc., showing a depression of the land to the extent of about fifteen feet. This is the point at which I propose to commence, as it would be desirable to fix some approximate date for this occurrence.
The highest level of the land in the Hataitai Valley being now about 14 feet, it is evident that a depression of 15 feet would entirely submerge it. We therefore find the valley to be filled with strata of sand and gravel, the latter generally underneath, but also occurring in bars at various points;
the sand is much mixed with recent marine shells. A rise of the land must now have taken place, for it next appears to have been covered by dense forest of the usual trees of the district, viz., totara, rata, kahikatea, manuka, etc.
The forest has then been destroyed, possibly by the advent of man, and has been replaced by swamps and by a shallow lake (Burnham Water) in the centre. The swamps supported a vegetation of raupo, flax, etc., and before being drained a stick could be easily thrust down for about seven feet, until it struck the hard sand bottom. Burnham Water was evidently formed after the destruction of the forest, because roots of large trees are found in its bed clearly in situ.
How long ago was it that the land was depressed to the extent of 15 feet? To fill the Hataitai Valley with sand, gravel and shells must have taken a considerable time; but this time it must be impossible to arrive at, for the sea might have gone on for ages at the work, grinding away, and shifting, and replacing. For the age of the forest we should probably have to allow some centuries. The swamp era must also have occupied a very long time. The seven feet of soft vegetable matter consolidated into about 5 or 6 inches of peat. How many centuries must we allow for this accumulation of vegetable matter?
I have in a previous paper on wind-formed lakes shown how Burnham Water was formed. Probably the advent of the Maoris occurred during the forest period. Hataitai having been a great settlement from which eventually the South Island was colonized, the ground would naturally be cleared by the natives, and thus the forest would be destroyed. Then the wind would have got its opportunity of scooping out the bed of Burnham Water.
At what period did the moa live on Hataitai? Many remains have been found near the Maori ovens, and I myself found a head in the bed of Burnham Water.
Altogether, I think we might be safe in making a rough guess of ten centuries as the date of depression of the land. It may have been less, it may have been indefinitely more.
As the imbedded timber from the old forest is excellent firewood, and as in clearing the ground for ploughing it must be removed, all traces of its previous existence will disappear during the next few years.