Tutira
Chapter V. — Subcutaneous Erosion
Chapter V.
Subcutaneous Erosion.
The original appearance of Tutira has been described. We have yet to trace the agencies by which its contours have been modified. It will be remembered that the station was pictured in the beginning as a series of plateaux afterwards tilted towards the east, these tilted terraces or canted plateaux smooth and unbroken by fissures. The evolution of the comb system, the development of these fissures, has yet to be described. It has, I think, been brought to light by a very remarkable process of subcutaneous erosion, a process akin to the dissolution of a dead beast when first the flesh decays, then the skin shrinks and shrivels, whilst only at the last do the bones protrude. In this transformation water has been the chief agent, but before proceeding to state what water has done it will simplify our task to state what water has not done.
Water has not created the dry cliff system. Water has not scoured out the solid rock of these strange valleys walled in by precipice. Both sense and sight forbid the supposition. There is no lateral soakage whatever into them. In hundreds of instances, except the drops that actually fall on to their surfaces from the sky, no extraneous water has ever reached them. Rock erosion by scour has not occurred, because water has been able to pass away from the contiguous lands on either side through beds of grit and sand. The junction of their double walls of cliff is not lanceolate but horse-shoe shaped. It bears no resemblance to the typical commencement of a water-worn ravine. The difference between the width of the valleys as measured from cliff to cliff at two, twenty, and two hundred yards from their beginnings is out of all proportion to the necessities of scour, supposing such a process had ever been at work. Finally, at the mouths of these valleys there exist page 29 no indications of the rock material which must have been deposited had water erosion been responsible for their formation. As regards these chasms, this is, I think, a fair statement of what water has not done.
The task of water has been to remove by infinitesimal quantities the material deposited between the rock walls; to flush, if indeed such a verb can be applied to an enormously tardy process, and scour out material already deposited in the valleys; to render visible what a mightier force had already accomplished. Erosion, in fact, has brought to light, not created, the rock-bound valleys of Tutira. When the plateaux which I have supposed to have been the original shape of the run canted to the east, making the station a series of tilted terraces, these deep interstices had been already moulded.
About the nature of the force that has severed the sea-floors upon which Tutira sheep now run, yet incompletely severed them,—that has severed them, yet not severed them in parallel lines,—any suggestions I can offer are little likely to be of value. There seems, however, to have been a twofold motion—the one cracking them in lines running north and south, the other incompletely parting these strips or oblongs of country into numerous short unparallel asymmetrical gaps east and west. The fissurings extending north and south are attributable probably to the effects of subsidence as segment after segment canted towards the east. The gaps east and west, with their broad horse-shoe beginnings, are less easy to account for. They may be due to shrinkage by evaporation whilst their rocky material was still plastic; their shape forbids the idea of cracking or fissuring.
Although no satisfactory solution can be offered as to their origin, much can be said as to the manner in which they have been brought to view. Each range shows its own slight modification of the general geological plan. In order, therefore, not to confuse the reader and darken counsel with a multiplicity of detail, I propose to work stage by stage from past to present conditions. The reader is invited to contemplate an ideal section of a hill range in the conglomerates of central Tutira before erosion had begun its work.
Imaginary section of central Tutira prior to erosion, the dotted lines showing yet hidden interstices.
Our first hypothetical block was selected from the conglomerates of central Tutira, where conditions are most simple and where the valleys are invariably dry except for such rain as reaches them from the sky. A further development of the subcutaneous drainage system will again be made easily comprehensible if once more a conjectural block be visualised, taken this time from the limestone ranges of the west. As before, we must imagine the slope towards the east, the skin of matted humus covering grit and pumice sands, the subterranean soakage system, the right-angle rivulet at base, the western precipice; finally, the different stages of sag as exemplified in the history of the conjectural conglomerate block of central Tutira. We can start, in fact, now where we concluded then—that is, with a deep sag between cliffs of 10 or 20 feet. Our western valley, however, to begin with, is many times the length and width of the central valley. Owing to its greater area, there is a quite important rainfall reaching it from the skies. It stands at a higher elevation above sea-level: there is a greater fall for its drainage system. Shrinkage in the sag becomes more and more pronounced, until the rock walls stand up 80 and 100 feet on either side, until there comes a time when the centre of the fold undermined by the soakage system, by the flushing action of springs, which now first come into account, and by rain-water falling within the gorge, changes from a deep lap to an angled incline—from a U, in fact, to a V. The extreme point, the apex of the inverted V of humus skin, is now for the first time in our story directly exposed to water. It is finally worn through by the action of the running water of springs supplemented by soakage of heavy rainfalls; a brook trickles over the lower portion of the sag; a normal valley, in fact, has been formed save for the impossibility of lateral expansion.
“Sag deepened from a U to a V—a normal valley formed save for the impossibility of lateral expansion.”
There have now been traced three stages of development in the underground drainage system of Tutira: the first, erosion by percolation of rain-water only, together with a deepening of the valley within spright walls; the second, erosion by percolation of rain-water plus erosion by springs, together with a deepening of the valley within page 36 upright walls; the third, erosion by percolation of rain-water plus erosion by springs, and supplemented further by a certain slight slow widening as well as by a deepening of the valley no longer within exactly perpendicular walls.
These conclusions have been reached by working from considerable heights above sea-level downwards. They can, I think, be proved, to use an arithmetical term, by ascent. The reader has but to trace the course of the main streams, to follow up their tributaries, lastly, to mark the sources from which the tributaries themselves are fed. Our conclusions too have been reached by deduction. Corroboration by the dry light of the inductive method is easy. We can drop ideal sections and consider actual conditions.
Shrinkage shows itself in every stage; there are endless modifications, but although details differ, the general principle is unmistakable, the pattern clear. Beginning with instances where the sagging is still in its preliminary stage, a fold can be instanced on the Heru-o-Tureia block parallel with and to the south of the steep horse-trail known as the “Zigzag.” Although close to the enormous gorges of the highest range on Tutira, this particular narrow fold remains but a fold; on the other hand, in the “Waterfall” paddock the cliffs are so hummocky as to have remained innominate. The “comb” pattern is hardly recognisable; teeth and interstices alike are so little in evidence that the plough has passed over both. Again, the laps of the “Second Range” have sagged so little that only the outlines of the cliffs show beneath the humus covering; the teeth have not yet broken through the gums.
On the “Sand Hills” the interstices are extraordinarily wide, whilst the back of the “comb” is less emphasised than usual. On the “Tutu Faces,” where the “teeth” are set particularly near to one another, folds are to be found varying from those hardly noticeable to others enclosed by cliffs from ten to fifteen feet high. The “Nobbies” range, a duplicate in miniature of the Heru-o-Tureia, is gapped in lines more nearly parallel to one another than elsewhere. Everywhere, however, erosion has taken place subterraneously, subcutaneously; be the sags deep or shallow, wide or narrow, salient or unseen, the ancient original humus still blankets the surface.
Subcutaneous erosion has played as curious a part about the bases of these solitaries—these erratics, if I may so call them—as on the slopes of the terrace system. However probable it might have seemed that their dusty weatherings would have been deposited on the surface, no such boon has blessed the land. Everywhere the ocean robs the upland farmer, but nowhere more brazenly than on Tutira. Stuff urgently needed for the amelioration of the surface of the run is borne page 38 off by under-runners to the sea—that vast, barren, grassless flat which does not carry a sheep to ten million acres. About the sandstone formations of the run—especially about the softer sandstones—their ramifications are most highly developed. The “Dome” and “Dead Man's Hill”1 in central Tutira exemplify in an extreme degree this network of tunnellings: their steep slopes are everywhere honeycombed with hollows. Hardly a grain of sand-weathering is deposited on the surface. In rain-storms it is washed directly off the surface of the melting cone into tunnels, whose circular, open, funnel-shaped mouths seem actually to gape for it. As on the marls of the east a water sandwich is formed, so here again similar conditions are re-enacted with the substitution of sandstone rock for marl. Tutira remains unfertilised, constituents that might be supporting grass and sheep are rushed to the hungry ocean, the old original sin of worthless humus persists almost to the rounded sandstone cones. Although the land surrounding these rain-scoured, wind-blown, melting solitaries has sunk scores, even hundreds of feet, yet always the worst soil—the dusty humus—has contrived to remain on top.
1 So called from the discovery of a human skull and bones scattered by pig, but evidently when first found those of a man but recently dead. We surmise that the poor chap may have at first missed his way on the high tops, may have in an exhausted state seen the lake, and in making for it become trapped in the gorges of the central run. At any rate, a few yards back from the edge of one of these precipices lay the bones. The remains of two other men have been in my day discovered on Tutira. In the one case they were those of a European, in the other those of a Maori. Near the skeleton of the latter lay a fragment of fire-bleached greenstone.