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Chapter V. — Subcutaneous Erosion

page 28

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.

In the beginning, then, when the main rivers of Tutira ran hundreds of feet above their present levels, the surface of our imaginary section seemed whole, unwrinkled, ungapped, and inclining gently page 30 towards the east. It was coated with a matted humus overlying æolian pumice grit, overlying disintegrated red sand. Filtering through the loose surface mould on to the layer of grit and sand, rain-water did not so much penetrate downwards as follow subcutaneously the trend of the slopes. It passed away in an unseen filtration; I imagine it lipping from grain to grain of grit, moving as evenly, as diffusedly, and with as little current, as water spilled on an inclined sheet of blotting - paper. Finally, at the base of our imaginary slope the accumulated soakage would gather against the cliff of the next “comb,”—against, that is, the next western-facing precipice—the next segment of canted plateau. There, unable to press further east-ward, it would be temporarily held. Eventually, at the base of our
Imaginary section of central Tutira prior to erosion, the dotted lines showing yet hidden interstices.

Imaginary section of central Tutira prior to erosion, the dotted lines showing yet hidden interstices.

block, there would develop as an essential factor in our scheme of erosion, a tiny runnel or rivulet escaping north or south towards one or other of the main rivers, towards the Waikoau, the Matahorua, or the Waikari. Up to this point our imaginary section of hill-slope has seemed a homogeneous whole, but now, with the deepening of the rivulet—a deepening sympathetic with that of the whole drainage system of the station—two different qualities of surface would begin to reveal themselves. The little stream would ooze at the earliest date along the humus; as the deepening process continued, it would trickle alongside the layer of volcanic grit, and later along beds of disintegrated sands. With a prolongation of the process our rivulet, cutting still deeper, would at last begin to skirt alternately two different qualities of material, hard and soft—the “teeth” and “interstices” respectively of page 31 the yet hidden “comb.” Erosion of the teeth—conglomerate—would be infinitesimal; of the interstices—grit and disintegrated sands—perceptible. The mass of the one would endure; of the other, perish. As this rivulet deepened, the soft material of each interstice tapped would be drawn upon and would sympathetically shrink, whilst the hard would remain unaltered. In process of time this subcutaneous drainage, this subterranean withdrawal of matter from certain portions of our block, would begin to affect the evenness of its surface. The humus skin of the interstices would slightly sag; instead of an unwrinkled soil - sheet over the whole of our imaginary slope, dips and hummocks would alternate. As the sag deepened with the continued drain from beneath of its internal material, the height of the hummocks would seem to rise. As a tooth wears through the gums, the hard edges of the rock would appear to shove through the
“In proportion to the depth of sag the height of the rock walls would seem to rise.”

In proportion to the depth of sag the height of the rock walls would seem to rise.

covering of humus. Unceasing shrinkage of the surface would later reveal low, bare, perpendicular rock walls. Later again, the whole of our once even slope would be an alternation of portions still even and of other portions shrunk and sagged. Uninterrupted subcutaneous erosion of the material of the interstices would year by year continue to deepen the lap, whilst in proportion to the depth of sag the height of the rock walls would seem to rise. At last our imagined block of “comb” country in central Tutira would exist as it does actually exist at this present time,—a slope falling towards the east with gorges of perpendicular cliff, between each of whose walls lies a dry sagging lap or fold still unwrinkled on the surface, showing no sign whatever externally of water action. Enormous rainfalls have never gouged waterways in this porous soil-sheet; no watercourses have forrowed it. Except for pig - rootings and hummocks of overblown page 32 trees, there are to this day no inequalities to vary the monotony of its even blanket of dark fibrous humus.

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.

Proceeding once more from the less simple to the least simple, the reader is invited to picture a third conjectural section, on this occasion from east Tutira. Once more, then, a block must be imagined showing the typical eastern cant, the even upper covering of humus grit and red sand, the drainage system at right angles to the tilt of the slope. As, however, we find differences between the central and western sections, so again we discover others now. The interstices of the east are wider than elsewhere, the cachment area larger, the rainfall heavier, page 33 the presence of clay marked, the number of springs greater, the marls nearer to the surface, in immediate contact moreover with the superposed limestones. These different conditions produce different effects. At first, however, soakage passes down the slope exactly as in the two instances already given. As before, the percolated water banks up at the base of the succeeding western cliff; as before, it soaks and oozes away until an incipient water-channel is established; as before, this
“Sag deepened from a U to a V—a normal valley 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.

water-channel deepens until a scour is created; as before, this scour begins to eat out the topmost constituents of the hidden interstice; as before, a sag which deepens into a lap appears; as before, the lap develops into a shallow U. Now, however, occurs a change. The base of the U rests not on previous sands, as on our conjectural blocks of the centre and west, but on a stiffer material, marl. Rain passing off between a top spit of fibre and grit and an impervious subsoil, forms a page 34 sort of water sandwich, the sands and grit being carried off particle by particle between the floor of marl and the ceiling of humus. In process of time the sharp pumice grit chisels out of the former a minute irregular bed. It deepens into a tiny hidden runnel; at last there is created a subterranean stream, or, in shepherd's phrase, an “under-runner.” Its course is at first unseen, then, as through process of time its channel is gouged out, and as the carpet of humus fibre gives way at irregular intervals, great rents and holes betray its presence. The bed continues to deepen, the carpet of turf falls in more and more, until finally the under-runner becomes an open rivulet. Ancillary effects now become
Under-runner, the roof of which has here and there given way.

Under-runner, the roof of which has here and there given way.

prominent, along each edge of the rivulet a secondary process of subcutaneous attrition is set up. Subsidiary under-runners become established at right angles to the stream, and in time laterals also to the latter. The stream increases in depth, the angle of inclination of the slope on either side grows more acute. In time this triple process of erosion, still subcutaneous, still veiled by the carpet of rooty humus, saps up to the containing walls of the interstice, and now for the first time on Tutira we see a possibility of a widening as well as of a deepening of the gap. The containing walls are not of conglomerate as in central Tutira, nor are they of limestone and sandstone as in west Tutira. They page 35 are composed of alternate layers of limestone and marl. The latter substance crumbles and flakes, the limestone rock cap is undermined and breaks away fragment after fragment; the upper portions of the containing walls, thus exposed to air, frost, and rain, continue no longer perfectly perpendicular—they tend to become exceedingly acute slopes. Shielded by soil only, the unexposed portion of the cliffs, where there is no possibility of wear and tear, remains absolutely perpendicular.
Limestone rock cap undermined.

Limestone rock cap undermined.

At length our conjectural section of once even eastward-sloping range is left an open gorge stripped of its original contents, exposing a slope studded with fragments of undermined limestone.

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.

Where the “comb” system is distinctly marked on Tutira, there is little more to say of its peculiar system of underground drainage, page 37 but there are considerable tracts of country where the subcutaneous processes described are subject to modification, where the different appearance of the countryside itself deserves comment. There is in central Tutira a considerable area of peaked country—the “Dome,” the “Conical Hill,” the “Razorback,” “Mata-te-Rangi,” the “Pa Hill,” and other solitaries. These hills are of sandstone formation, more or less weathered into points, as their names imply. Each of them is a relic of a small fragment of tilted terrace from which the conglomerate cap has gone, whose uncrowned top has been exposed to the elements; they are the scattered “teeth” of dislocated “comb” systems. Unroofed by the action of rain, and in a minor degree by that of frost and wind,
Fragment of terrace still rock-capped.

Fragment of terrace still rock-capped.

Fragment from which cap has slid, melting into a cone.

Fragment from which cap has slid, melting into a cone.

they have been melted and moulded to cone and dome and razorback. Peak formation in fact represents, on the east coast of the North Island of New Zealand, the intermediate stage between the plateau of the far past and the plain of the remote future.

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.