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Chapter VI. Surface Slips

page 39

Chapter VI. Surface Slips.

The deluges that from time to time pass over Tutira have been mentioned. Readers will have, therefore, no difficulty in picturing their effects on steep marl slopes. Although on the station itself there is only a small proportion of land of this type, yet speaking broadly of the “papa” country of the east coast of the North Island, it is being flattened towards the sea by a mighty melting process, most marked and most discernible in the soft marls of Poverty Bay. As, however, it is the history of Tutira I am writing—limited as is the acreage affected—I shall cull my facts from local sources.

During heavy rainfalls on eastern Tutira the numerous oozes, leakages, and “damps,” consequent on alternate bands of marl and limestone, become surcharged with water. The supersaturated subsoils burst with their weight of wet, chasms of many feet in depth are created, the hillsides spew forth mud; under-runners become gulches, or, choked with debris, spill on the hillsides their streams of silt, torn turf, and curious rough-rolled balls of clay.

Eastern Tutira, indeed, after a violent “buster,” appears to have been weeping mud. From the edges of all ancient slips the water-sudden fringes drip with clay; new red-raw wounds smear the green slopes, scalp-shaped patches detach themselves, slipping downward in slash and turf. Sometimes a whole hillside will wrinkle and slide like snow melting off a roof, its huge corrugations smothering and smashing the wretched sheep, half or wholly burying them in every posture. Sometimes a slip rushing down a steep incline will temporarily block the creek below, piling itself up until again washed away, and leaving on the opposite slope, yards above the stream, a curious plaster mark of dirt. Gluey streams, hardly moving faster than glaciers, from whose page 40 tenacious mud bogged sheep have to be extricated hoof by hoof, make the hillsides a terror to shepherds.1 After a “southerly buster” or a “black nor'-easter” of three or four days' uninterrupted torrential rain, I have counted on a two-mile stretch of hillside over two hundred slips great and small, new or newly scoured out. Seven or eight times since '82 the grasses and sedges of the valleys around the lake have been overlaid by mud varying in depth from six inches to a couple or three feet. Huge masses of solid hill have slid on to the larger flats. Fencing is buried, roads and bridges washed away, culverts
“Like snow sliding off a roof.”

Like snow sliding off a roof.

destroyed, stock bogged or caught and buried in the displaced masses of earth.
Besides earth avalanches there remain after every great storm, here and there, fissures on the hillside nine inches or a foot in depth. Sometimes they are mere longitudinal cracks, but more often of an irregularly ovoid shape. They mark areas, often of great extent, where the surface has slid a few inches. They can be detected further by trees slightly out of the natural angle of growth, by the bulging and bellying of fence page 40a
East Tutira—Landslips after Flood.

East Tutira—Landslips after Flood.

page 41 lines, by the dry overlapping of turf over turf, by surface wrinklings, by bursting of gate fastenings. Even, however, when thus started on its downward path, the progress of a landcreep is by no means always sustained. Sometimes for years the gaping rents remain unwidened, sometimes they fill with dust and debris. They play, nevertheless, an important part in the promotion of the earth avalanches already described. Water lodging in them penetrates to the marl, greases the base on which the upper soils rest, and expedites the slip.

I believe that even during my brief span on Tutira scarcely a rood of marl in the eastern run has not been affected in some degree by the great rainfall—has not slid seaward, perhaps a few inches, perhaps a few feet.

Landslips and landcreeps may, in fact, be considered complementary to the earlier processes of subcutaneous erosion. In the valley of “Newton” paddock we have an example of surface wear and tear arrested from lack of sufficient fall. There the local stream, blocked and barred with limestone debris, still runs several hundred feet above sea-level. The original upper soils too, torn and patchy, have not yet been completely sloughed.

In the valley of the Maungahinahina, however, where the fall nearly reaches sea-level, and where, moreover, the mouth of the great gap abuts directly on to the Waikoau river, we get an almost perfectly completed bit of water sculpture five or six hundred feet deep and nearly half a mile in width. The fibrous, rooty humus, the pumice grit, the red sands, the clays are gone, the great scoop they used to hide is wholly revealed. Percolation and soakage has developed into the under-runner system, that into the open gulch, the gulch into multitudinous lateral gorges, until the loose heterogeneous mixture of soils that once filled the huge interstice to the brim has been scoured out and the marl basis of the gap exposed. Lastly, unable to cope with and carry off the vast quantity of limestone fragments,—portions of the original rock-cap slid into it from either side,—the little stream has finally left them piled and prominent in a sort of moraine at the mouth of the gap.

Nevertheless, although thus buffeted by deluges and sapped by earthslips, the remaining portion of the rock-cap of eastern Tutira is likely to endure for an almost incalculable period. Attrition is enormously slow.

During my ownership three only of the great grey squares into which page 42 the limestone sea-floors split themselves have perceptibly shifted their sites. In 1905 a landslip of some quarter of a mile in length started from the lower part of the Racecourse flat, overwhelmed the road near the Waikoau crossing, swept it out of existence, smashed like matches trees of three and four feet in circumference, finally depositing two great boulders in the Waikoau river. There to this day they stand, monumentally white in their unlichened youth.

Valley of the Maungahinahina.

Valley of the Maungahinahina.

In 1911 another vast rock moved, not after rain, but after a long spell of particularly dry weather, and on a day so calm as to forbid suspicion of earth tremors. This enormous fragment of limestone cliff broke away from the highest sea-floor of the Racecourse paddock. The sound of the mass moving, the clouds of dust raised, were perceptible half a mile off. Viewed more nearly, it had ploughed a deep chasm into the earthy slope below, parting it as a battleship breasts page 43 the water at her bow. The weight of the mass had been so great that notwithstanding its drop—or rather precipitous slide—of ten or fifteen feet, the grasses and flax on its top had perfectly retained their natural angle of growth. Although, however, only three limestone quadrilaterals have thus been detected in motion, I believe that the numberless boulders already broken from the cap and deeply embedded in the hillsides really never cease to move,—that they are being slowly sucked downhill, perhaps an inch or so every year, sapped by the action of innumerable under-runners.

Alterations in the positions of the large-sized river-bed boulders have hardly been more conspicuous. Certain very noticeable fragments have moved a foot or two oceanwards. The ford of a river is more closely scanned than any other portion of its bed: that of the Waikoau has scarcely changed in forty years. We cross now—or did until the bridge was built—within a few yards of where we crossed in '82.

Two minor processes of erosion yet remain unchronicled, the more important I believe responsible for the circular pits found over the relics of the ancient plateau caps of eastern Tutira. Sometimes these funnelshaped cavities are still skinned over by turf, sometimes the turf has broken through and they are open at bottom. Most of them are of moderate depth. One pit, however, within a short distance of the Tutira boundary, was, until opened up by spade-work, a death-trap for animals. At its base remains of sheep and pigs used often to be visible,—the former presumably tempted over the edge by succulent weeds or trapped by mere bad luck, the latter induced to slide down by the bait of the former and then unable to escape. These pits, great and small, have been probably worn by the action of the carbonic acid of rain-water affecting the limestone rock-cap. Its substance is dissolved and borne away as travertine, masses of which accumulate about the sides of the streams. Consequent on the chemical dissolution of the rock-cap beneath, the unsupported soils slide downwards towards the centre of weakness, thus forming rudely circular pits. Withdrawal of matter from beneath may also be held responsible for the almost perfectly moulded funnels alongside of one of the streams of this part of the station. It runs over a jumble of squares, cubes, and slabs—sections that have broken away from the limestone cap and been carried violently by earth avalanches or mined by under-runners into page 44 the valley bottom. Amongst them no doubt are endless cracks and gaps through which water escapes—perhaps to reappear at lower levels of the stream itself, perhaps to reissue elsewhere as fountains. There are also in its bed minute intermittent whirlpools that alternately suck and cease to suck its waters down.

One other form of erosion remains to be described. The sum-total of its effects is so puny that perhaps I should apologise for its inclusion, yet there is a fascination in its strange rapid action. It is an operation readily to be appreciated by those who have attempted to water a steeply-sloping garden bed, dust-dry and in finest tilth. About the bases of bare scarps—the unhealed scars of hillside slips—quantities of the finest dust accumulate in dry seasons. On these minature skrees of powdered soil fall the first great drops of a western shower. The dust slope can neither retain the drops nor instantaneously absorb them. Striking the slope they gather earth particles in their downward course. While thus in motion, as if by miracle, they change from liquid to solid. Metamorphosed first into ashen-grey, and then into brown balls, these earthen pilules, preserving their shape but changing their substance, race madly downhill, bound downhill, no longer clear drops from heaven, but minute circular solid globes of soil. With a faster fall of raindrops the process ends perforce; the dust-heap becomes a mud torrent.

“No longer clear drops from heaven, but minute circular solid globes of soil.”

No longer clear drops from heaven, but minute circular solid globes of soil.

Frost and wind have played but minor parts in the transfiguration of the run. The former on wintry mornings has accelerated the weathering of the come area by elevation of their sand surfaces on upright spicules of ice the latter by blowing abroad the desiccated dust. Doubtless also frost has contributed to the disintegration of other surfaces; speaking generally, nevertheless, it is water which has moulded the run to its present shape.

Shrinkage of the station has been compared to the decay of page 45 dead beast: the softer parts dissolve, the skin, shrivelled and sagged, endures. So has it been over the vast bulk of Tutira. Its skin also remains as of yore, shrunk and wrinkled indeed with age, but intact. In spite of torrential rainfalls, the surface of the station remains at this date as it was ages ago. It is now, as it was then, blanketed with a dark, porous, unfertile, rooty humus. So vast a change by processes of internal waste, of subcutaneous dissolution, is perhaps unique in the annals of geology.

1 Returning directly after the Armistice, I was amused to find that recollections of the flood of the previous year had been adapted to the needs of the nursery. My three nieces had invented the new game of “bogged sheep.” There is no necessity to give the exact rules as framed by the little shepherdesses. Suffice it to say that the game can be played as most convenient on carpet or grass, that some of the players are “sheep,” others “shepherds,” the object of the latter of course being to rescue the entombed animal by dragging it leg by leg out of the mud. Should it bleat piteously during the operation, again fall back into the mire or, best of all, should its cold cramped legs refuse their office without pastoral support, by as much more is the game quickened.