Art. XXV—An Ancient Buried Forest near Riccarton: its Bearing on the Mode of Formation of the Canterbury Plains.
[Read before the Philosophical Institute of Canterbury, 6th December, 1916; received by Editors, 30th December, 1916; issued separately, 30th October, 1917.]
As the mode of formation of the Canterbury Plains is a matter of especial scientific interest, I venture to submit the following short note, based on the existence of an ancient buried forest, in support of the idea first advanced by Haast (1, 2), and endorsed by the surveys of Doyne (3, 4) and Dobson, that the plains have been formed almost entirely by the action of aggrading streams, and that the form of their surface is due to the overlapping and coalescence of the fans of glacier-fed rivers. The original hypothesis has received support from most geologists of standing, notably from Professor W. M Davis, who after an examination of their salient features has expressed to me privately his substantial adherence to Haast's theory. This opinion was, however, strongly opposed by Hutton (5, 6), who regarded the plains as a marine deposit built up from material brought down by rivers, the upper surface being a plain of marine denudation.
In two somewhat recent papers (7, 8) I have urged that after being built up in the way suggested by Haast, or while they were actually in process of formation, they were subjected to a gradual sinking of the land—that is, instead of the chief recent movement being one of elevation, as demanded by Hutton, it has been one of subsidence This is proved almost conclusively by the records of the bores of the artesian-well sinkers in the Christchurch area On the margin of the plains, especially just north of Banks Peninsula, there is an intermingling of land and marine beds, the latter with marine shells overlying beds of terrestrial origin There has been a struggle between the agents responsible for building up the land and those causing depression, in all probability the latter getting the better of it; but the effect of this struggle is not clearly manifest on the inland part of the plains owing to the enormous depth of the gravel deposit and the absence of sections showing the whole sequence. The records of the bore being put down at Chertsey for the purpose of prospecting the plains for petroleum show up to the present a thickness of 1,250 ft. of shingle and clay deposit, with no clear sign of a change in its character from that occurring near the surface.
When the earliest colonists arrived in Canterbury in the early “forties” there were patches of forest growing on the low-lying and swampy areas in the neighbourhood of Riccarton, Papanui, Rangiora, and Temuka; while near Oxford, Alford Forest, Mount Peel, and Geraldine the eastern foothills of the Southern Alps were clad with submontane forest, tongues of which stretched out into the plains for a varying distance, that from Oxford following the line of the River Eyre and almost junctioning with the patch on the plains near Rangiora and Woodend; also from the north-western
slopes of the Port Hills a few trees formed a straggling connection with the Riccarton Bush In addition to this standing timber, fallen logs, mostly of totara (Podocarpus totara), lay commonly among the tussocks till they were gradually destroyed by grass-fires, thus indicating a former wider extension of these forests In the swampy areas round Lake Ellesmere and on the low-lying eastern fringe of the plain there existed a great amount of timber, chiefly white pine, or kahikatea (Podocarpus dacrydiodes), and manuka (Leptospermum scoparium), some of which lay prone in the bogs, while some formed the stumps of trees in position Since kahikatea is a timber which soon perishes on exposure to the weather, it is evident that the destruction of these forests occurred somewhat recently This was attributed by the early settlers to the action of fire, but it is difficult to understand how a water-logged swamp could have been thoroughly burnt out, and it is extremely probable that this part of the forest of the plains disappeared largely owing to the killing of the trees by water-logging due either to depression of the land on to the flooding of land owing to the changing course of rivers, itself perhaps partly due to depression, of which there is entirely independent evidence This is supported by the statement of Mr Dudley Dobson, Christchurch City Engineer, in connection with the sinking of the sump for the Christchurch water-supply pumping-station at the foot of the Cashmere Hills While making this excavation he came across, at a depth of 25 ft beneath the surface, a fine specimen of a totaıa log, 5 ft in diameter In close proximity to this was the stump of the tree with its roots in proper position, extending 16 ft across, and penetrating a layer of clay, evidently the old land surface on which the trees grew The level of the roots was almost exactly that of high-tide mark, thus affording a clear indication of the depression of the land My attention has been drawn by Mr W. Wilson, of Auckland University College, to stumps in a similar position near Pareora, south of Timaru, thus undoubtedly confirming the existence of a downward movement of the land in the area immediately south of the plains Also, during heavy storms, pieces of submerged trees are frequently washed out of the beds on shore just north of Timaru, but whether these are from drift material or from trees which are in position it is impossible to say This evidence of depression is supported as far as this locality is concerned by the form of the shallow valleys cut in the dolerite capping of the Timaru Downs, with their lower reaches occupied by lagoons of brackish water ponded behind shingle bars, which were built up by the pronounced northerly drift of shore material along the coast The drowned valleys of Banks Peninsula are evidence that a similar downward movement of the land went on farther north
The instance cited on the authority of Mr Dudley Dobson serves to emphasize the existence of buried forests of large trees on the former surface of the plains The occurrence of timber, either drift or in position at great depth beneath the surface is widespread in this area Not only is it found in the artesian bores at all depths up to 450 ft, but it is met with in making excavations for foundations in Christchurch itself, and also in the gravel-pits opened on the plains both within and just without the artesian area In a pit at Hornby, about six miles west of Christchurch and 90 it above sea-level, numerous pieces of the stems and roots of trees are found 40 ft beneath the surface—that is, to the depth of the excavation These are evidently of drift material, but they show that the totara-trees from which they were derived flourished at higher levels on the plains when the bottom of the pit formed the surface of the ground Throughout
the whole depth of this pit river-gravels and occasional irregular sandy layers form the material of the beds, and there is an absence of the stratification which would be evident were the beds of marine origin. Further, they appear beyond the limit of the area where there is the interstratification of pervious and impervious beds necessary for artesian conditions at shallow depths, although this undoubtedly occurs at lower levels in the same locality, as is indicated by the records of the wells at Islington (8). There are similar occurrences of timber in an excavation for gravel close to the Heathcote River, in Spreydon, at the foot of the Port Hills, as well as in gravel-pits near the Harewood Road
A more interesting pit, however, is found in the Riccarton district, at Sockburn, close to the Paparua County Council's office. This is about a mile and a half nearer Christchurch than the Hornby pit. It is about 63 ft. above the sea, and has been excavated to a depth of about 12 ft., at which level a bed of sandy clay forms an impervious layer on which water lies except in summer and autumn The pit has an area of over an acre, and all over this the stumps of trees, mostly of totara, are in position. (See Plate XXIII, fig. 1) Fully thirty large trees are thus represented, some up to 5 ft in diameter At my first visit a large tree, 40 ft. in length and over 4 ft in diameter, lay exposed on the floor of the excavation, having apparently broken off from one of the stumps: this was subsequently cut up for timber During a recent visit to the spot in company with Dr. Cockayne I noticed another large tree, fully 5 ft in thickness, which was uncovered for 20 ft or more. There is no doubt that we have exposed here an old forest which was growing on the floor of the pit when that formed the land surface, and which has been subsequently buried by the pouring-in of sediment from adjacent rivers It is clear that the standing trees were broken off from their stumps before they were buried up, otherwise the trunks would not be separated from their roots as they now are. What caused the breaking-off is not apparent, but in all probability the trees were killed while in the standing position and then the stems were snapped off near the ground, or perhaps while the roots were covered up with river-gravels This process can be seen on many of the shingle fans and river-beds in the mountain region of the province. Living trees of mountain-beech or wild-irishman can be observed standing partly buried in a waste of moving shingle, maintaining a precarious existence, while alongside stand dead and partially buried standing trunks, and farther downstream the fallen stems lie at all angles and are buried up completely or exposed as the stream washes out a deep channel on the surface of the fan The occurrence in the gravel-pit at Sockburn is exactly analogous to the second and third of these conditions
No doubt the growth and destruction of the forests on the plains went on simultaneously in adjacent areas and went on alternately in the same area, the destroying gravel forming the substratum on which the next forest growth was based Stretches of land with good soil would occur in certain parts, usually low-lying alluvial flats, and on these the forests would establish themselves and maintain their footing for a time; but these parts would be especially susceptible to inroads of gravel, since ultimately the river would use them for a dumping-ground, and the forest would be destroyed or be compelled to remove to an adjacent area. The fact that the forest must have been established on a land surface definitely supports Haast's original idea of the formation of the plains, and of itself would negative the idea that they had been formed of waste carried to sea by
rivers, spread on a sea-bottom, and levelled finally by the action of the waves, even if there were not strong and independent evidence to the contrary.
The pit under consideration affords no evidence of the downward movement of the land, since the floor on which the trees grew is 50 ft above the present high-water mark, but there is no evidence which is opposed to that hypothesis
It is impossible to say definitely how long ago this forest grew, but since its existence at least 12 ft of gravel has been deposited on the area, and, judging from the present rate of deposit in the Canterbury area, the time since it grew must be considered in hundreds if not in thousands of years The sound condition of the totara draws attention to the extreme durability of that wood even under conditions which are not altogether favourable for its preservation. On the clay substratum on which it lies it would be exposed to continual alternations of drying and wetting, and the loose texture of the gravel would allow free access of air, so that the present sound condition of the timber is really surprising. (See Plate XXIII, fig. 2.)
The occurrence of the forest serves to draw attention to another point. In an article on the “Post-glacial Climate of Canterbury” (Trans N Z. Inst., vol 43, 1911, p 408) I put forward the hypothesis that these old forests were established during a pluvial climate, and that their disappearance was not due to fire, but to a gradual desiccation of the region. We have here a forest whose destruction was certainly not due to the former of these agencies, so there were conditions previous to the arrival of man which were inimical to the free growth of these totara forests, and the destruction of the patch which covered the site of the old gravel-pit may have been due to a widespread cause of regional and climatic character, and not due to a local accident destroying a mere fragment of a once much more extended forest formation
1 Haast, Julius Report on the Formation of the Canterbury Plains, Canterbury Provincial Report, 1864
2 Haast, Julius Von Geology of Canterbury and Westland, 1879, p 396.
3 Doyne, W J Report on the Plains and Rivers of Canterbury, Canterbury Provincial Report, 1864.
4 —— Second Report upon the River Waımakarırı and Lower Plains of Canterbury, Canterbury Provincial Report, 1864
5. Hutton, F W. Rep. Geol Explor. dur 1873–74, 1877, p. 56.
6 —— Formation of the Canterbury Plains, Trans N Z Inst, vol. 16, 1884, p 449, and vol 37, 1905, p. 465
7. Speight, R. Some Aspects of the Terrace-development in the Valleys of the Canterbury Rivers, Trans N Z. Inst., vol 40, 1908, p. 16.
8 —— Preliminary Account of the Geological Features of the Christchurch Artesian Area, Trans. N Z. Inst., vol 43, 1911, p. 420
Fig 1 —Showing bottom of gravel-pit, with stump of tree 5 ft in diameter in position, as well as drift-wood and remains of other standing trees which have been uncovered
Fig 2 —Bottom of gravel-pit, showing accumulations of surface water (the picture was taken in the month of November) A large stump is also shown in position, as well as other remains of trees.