Art. LXX.—On a Deposit of Moa Bones near Motanau, North Canterbury.
[Read before the Wellington Philosophical Society, 21st January, 1882.]
During the past winter a flood occurred in Northern Canterbury which laid bare a deposit of moa bones near Motanau. This is exposed in the banks of a small creek which forms the boundary between the properties of Mr. Arkle and the Hon. William Robinson. Mr. Robinson having reported the discovery to Dr. Hector, I was sent to examine and collect from the deposit, and am now permitted to lay before the Society an account of what I saw and did.
Where exposed the bones form a closely compacted bed, varying from a few inches to 18 or 20 inches in thickness, and within a distance of 40 feet they occur in gravel, sandy clay, plastic clay, and peaty lignite, the progression as stated being from east to west, there being a slight dip of the bone-bed in that direction. In the banks of the creek the bone-bed is overlaid by a deposit of gravel, sand, clay, and loam, to a depth of 10 or 12 feet, and has at one time been covered to a much greater depth, as in the immediate vicinity there is evidence that the bones must since their deposition have been covered by at least some 50 or 60 feet of a deposit similar in character to that which at the present time is seen to overlie them. Mr. Arkle informed me that for many years past he had noticed bones in the bed of this creek, which he now recognizes to be moa bones; and to this gentleman is due the discovery of the bone-bed, which he observed immediately subsequent to the flood of last winter. Part of the deposit lying within his property, Mr. Arkle kindly gave me permission to make such excavations as I should deem necessary, for which I take this opportunity of thanking him.
The thickest part of the deposit appearing to lie on Mr. Robinson's side of the creek, I opened a paddock on that side. In doing so 5 or 6 feet of sandy loam was first passed through, in the lower part of which stumps of trees were encountered of which no indications appeared at the surface. Below this for the next 2 to 3 feet, a sandy bed with patches of gravel was encountered, after which a bed of variable composition (quickly changing from sand to plastic clay) overlaid the stratum in which the moa bones occur. The bone-bed over the area excavated varied from 1 foot to nearly 2 feet in thickness, and consisted of a closely compacted layer of bones, the interspaces between which were filled by a soft tough clay. The whole rested on a bed of well rounded gravel, the thickness of which could not, at this place, be ascertained. All the bones, as far as the bed was excavated,
appeared to have been disarticulated before being finally deposited, as no two bones were found which lay in such a position as to warrant their reference to the same individual moa. Most of the tibial bones were not found lying in a horizontal position, but inclined at all angles and so locked in the deposit one by the other, or by the other larger bones of the leg, that the extraction of one of them almost invariably involved the destruction of one or more lying contiguous to it.
Although at first sight the bones appeared as a confused mass, yet there proved to be some order in the mode of their occurrence. In the eastern end of the paddock most of the metatarsal bones were found. Tibiæ were most abundant in the middle part, and femora at the western end. Where the bed was thickest the pelvic bones formed its upper part and were universally in a crushed and highly decomposed condition. Vertebræ, toe bones, and occasionally bones of the head, were found from top to bottom of the bed. In the clay bed which at the western end of the paddock overlaid the bones, vertebræ and toe bones were of frequent occurrence, and in this horizon was found a metatarsus with the toe bones complete. The metatarsus was lying horizontally in the clay bed, while the toes were sunk in the clay in a vertical position.
In the comparatively small area of this paddock, which was less than 30 square feet, there must have been present skeletons or portions of skeletons of no less than thirty birds. To all appearance the deposit is a most extensive one, the thickest part of it extending north-east from the northern bank of the creek; how far it extends in this or in the opposite direction on the southern bank of the creek, indications at the surface afford no means of determining.
On the southern bank of the creek the bone-bed is not so thick, and, following the water-course upwards, it passes into a bed of peaty lignite without bones, but a few feet beyond this bones are present in the lignite where it crosses the water channel, and is exposed at the base of a low cliff bounding the upper end of a deep gulch, which is rapidly being cut back towards the source of the creek. Followed till disappearing under the northern bank, the bones in the lignite increase from an occasional one till they form a bed about 8 inches thick, the lignite increasing to about double that thickness, but being without bones in its upper part. In the peaty lignite the bones proved quite as much crushed, and much more fragile than where they were imbedded in the clay; in fact, the vertebræ and other bones of open texture were little more than discernible in the lignite, while the leg bones were, though apparently in good condition, so brittle that scarcely any could be got out without breakage. As there appeared to be no difference in the species imbedded in the lignite and in
the clay, I excavated but a small paddock in the lignite, and getting nothing more than I had already obtained in a better condition from the first paddock, I did not think it necessary to continue the work of excavation.
Of the moa bones collected, all of them seem to be referable to not more than two species; and of these the bones of Dinornis elephantopus certainly constitute nine-tenths of the whole.
The remains of other birds were very rare in the bone-bed, belonging, with the exception of a few fragments, to Harpagornis moorei. All the bones of this species that were strong enough to resist the pressure of the overlying deposit are beautifully preserved. The only part which has suffered damage from this cause is the skull, which, occupying an interspace between two large moa bones, managed to escape total destruction.
A curious feature in the mode of occurrence of these Harpagornis bones is that all those of the leg and wing were found with their greatest length vertical in the bone-bed. This was also noticeable in the case of most of the immature moa bones.
Finding that there was but small probability of finding the skeleton of an individual moa by itself, and equally little being the hope of securing the material to construct one, I had to be contented with making a selection of the larger leg bones and such vertebræ ribs and toe bones as were met with during the progress of the excavation.
I have already mentioned that the bone-bed was covered to some depth by a deposit of gravel, clay, and loamy soil, and that it rested on a bed of well-worn gravel the thickness of which could not be ascertained at the place where the bones were found.
From a little south of the Motanau River to Stonyhurst, a distance of seven miles, these gravels overlying tertiary strata form between the coast range and the shore line a table-land elevated 200 to 300 feet above the sea. On the seaward side this is bounded by a line of high cliffs washed by the tide at high water. Besides the Motanau River, there are three smaller streams which rising on the western break through the eastern ridge of the coast range and flow across these flats in narrow channels, which are now so deeply cut that until an elevation of the coast-line takes place, they have no power to cut them deeper. A number of smaller streams rising on the flats or commencing from the slopes of the neighbouring ranges, have near the coast-line cut deep channels quite to the base of the sea cliffs. The deep narrow gulches thus formed do not as yet extend across the whole breadth of the flats, but terminate abruptly in a cliff beyond which there is no defined water-course, and as a rule no permanent stream. North of Boundary Creek, which reaches the sea three miles north of Motanau, the
surface of the flats undulates in low rolling downs, and one or two isolated hills stand above the general level. Close past the southern side of one of these hills runs the little creek in the banks of which the moa bones are found. Just abreast of the little hill the creek breaks the surface of the flat and plunges into the deeper channel, which it cuts through the gravels, and further down its course into the underlying tertiary strata. Along this and other creeks numerous sections show that a heavy deposit of well-rounded gravel of uniform size overlies the tertiary marine beds. The gravels are parted 20 or 30 feet from their base by sandy clays, which at many places contain trunks of trees or pass into beds of impure peaty lignite. This is the horizon of the moa bones. Above this lies an indefinite thickness of gravel and silt, variable on account of having been unequally denuded in different localities.
Professor Haast's researches at Glenmark appear to have led him to the conclusion that there the moa bones occurred in three different horizons. These in descending order are:—
The turbary deposits near Glenmark homestead, from which the great bulk of the collections in the Canterbury Museum were made.
The alluvial deposits of Glenmark Creek and the Omihi Valley.
Pleistocene deposits occurring in Glenmark Creek, one mile above the home station.
In trying to correlate the Motanau bone-bed with either of these it is at once apparent that this can only be done with No. 2 or 3.
The character of the deposits forming No. 2 agrees well, both as regards the material and the sequence, with the Motanau beds; but the alluvial deposits of the Omihi Valley show not the least sign of having been denuded further than by the excavation of the present creek beds, while the Motanau flats, especially towards the northern end, have been so far denuded that the surface forms low rolling downs with here and there a low isolated hill, of which an example stands close to the locality where the bones are found.
It is true that at Motanau these gravels are isolated from the fringe of gravels skirting the coast line near the mouth of the Waipara, and also from an extensive development of gravels on both banks of the Hurunui, and near Gore Bay, Cheviot Hills. At. Gore Bay these gravels are in their lower beds alternations of silt and angular gravels, in which large angular blocks are of frequent occurrence. The upper beds are well rounded gravels, clay, and loam, as at Motanau, but here the total thickness is much greater, ranging from 300 to 500 feet.
Marine shells are found in the lower beds, while from about the middle of the beds I obtained broken moa bones and fragments of moa egg-shell. I have little doubt but that these gravels are the same as those in which the bones are found at Motanau. In the latter locality the presence of the lignite bed may indicate an unconformity between the higher and lower parts of the gravel deposit. This may be so and yet the younger upper beds may be of greater age than the alluvial deposits of the Omihi Valley. The Motanau moa-bone beds would therefore belong to the older beds in Glenmark Creek already referred to. These Glenmark beds belong either to the gravels of the plain south of the Waipara, or to the Upper Miocene beds forming gravels extending south from the Weka Pass to Mount Grey Downs, and forming part of the hills between Brown's Bridge and the mouth of the Waipara River. I should say they belong to the younger beds, and as in character they agree with the gravels of Gore Bay, Cheviot Hills, we might thus find a reason for correlating them with the Motanau moa-bone beds.