Chapter III. — The Lakes
Examination of the general physical features of the run has shown a series of parallel hill chains running north and south. Traversing the run from its inland boundary, the reader has crossed them one by one and seen the comb formation, though becoming less and less marked in height, persist through the west and throughout the trough of the run. A continuity of these features might have been looked for, especially as after a brief interruption they reappear in eastern Tutira. Instead we come upon a sheet of water still, after the depositions of centuries of alluvium, ninety feet in depth, two miles in length, and half a mile in width. In lieu of the normal narrow gorge there occur a series of immense hollows, one of which, the “Big swamp,” is already filled with alluvium; whilst others still exist as lakes Tutira, Waikopiro, and Orakai.
It is at the base of the Newton range, where the conglomerates of the central run cease and the limestones of eastern Tutira begin, that this change in the plan of the run, this interpolation of a new pattern, occurs. Its presence is an anomaly; it is an extraneous feature to the great general scheme of the station; it can, I think, be accounted for only by processes unlike any yet considered.
Hollows where waters lie may be attributed to erosion, to lodgment of waters in craters, to the accumulation of material forming barriers or dams, by subsidence of the crust of the earth.
Except the last, that of erosion is the only theory which might at first seem to fit the facts. The great trough passing through the centre of Tutira,—a trough extending scores of miles north and south of the station,—marked throughout by extensive beds of conglomerate and sandstone, has been described. These deposits, sharply separated from page 13 one another, have presumably been lodged by water action of some sort. In the site of this trough or long hollow a great river might have been imagined to have run at some period, a river which might have left the “Big swamp”—once a sheet of water—and the present lakes Tutira, Waikopiro, Orakai, Opouahi, and Temaru, further to the north Waikaremoana and Waikareiti, further to the south Waipukurau, Te Roto-a-Tara and Wairarapa, as evidences of its former course. It might have been imagined, in fact, that this chain of lakes had been scooped out by some vast old-world river.
There are, however, difficulties in the acceptance of this theory. The sands and conglomerates are not mixed, they are sharply distinct; certainly the latter do not contain that proportion of sand which might be expected in a river-bed. The shape, moreover, of the pebbles suggests neither the grinding of a shelving beach nor the erosion of a running river. In each conglomerate band lie horizontal seams varying in size of stone. Inspection of these seams suggests that their pebbles have been sown in a vast top-dressing, rocked into settlement rather than rushed into position by chance of currents. Perhaps these stones, originally cubes frost-fractured, have been ground by a more remarkable trituration; perhaps their smooth, ovoid form has been acquired by volcanic boilings or tossings. At any rate they are perfectly different from stones of the same material gathered from the top of the Newton range—stones evidently shaped by some little stream that ages ago must have flowed there.
Alternate layers of sand and conglomerate seem in fact to have been laid down very much as on eastern Tutira sands, limestones, and marls have been superposed one on another. Perhaps, indeed, a more profound likeness may be traced—the depths of strata conforming or corresponding to one another on east and west.
There is another difficulty also in regard to the fashioning of the Tutira lake basins by river action: the existence of the coupling spurs already mentioned, spurs here and there linking one range to another. Two of them traverse the breadth of the lakes, one of them at either end of Tutira lake proper, a third barring the way at a slightly higher elevation south of Waikopiro. These coupling spurs, as elsewhere on the run, cannot be reckoned as enormous afterthoughts, as avalanches of rock and soil solidified. They are marl—basic, homogeneous parts of the original scheme of things. Their presence at right angles to the length of the page 14 lakes precludes the possibility of river action. Scour sufficiently violent to have scooped out the lake basins must have worn to an equal depth these barriers of solid marl. Their surfaces, now submerged, must moreover have been subject, during a comparatively recent geological period, to superterranean influences such as now obtain elsewhere on the run. They bear evidence, too, that the drainage system ran then as it continues to run. They are mere relics, in fact, of former shelves and terraces, whose material has been worn away when levels were other than they are now. Previous to subsidence they had been sculptured by processes similar to those which have created spurs of kindred shape on the ranges of the west, centre, and east. What, moreover, is true locally of these comparatively small water areas is true of the whole terrain included in the great dip which runs throughout and beyond the length of the Hawke's Bay province. If, indeed, there has been a river flowing at any time north or south along the trough of the run, no signs now remain. It is safe to affirm that since Tutira assumed approximately its modern form the drainage system has been west and east, never north and south.
If, then, the creation of the lake basins is due neither to scour nor to blockage by earthfall of waters once freely escaping, a single possibility remains. Their presence, I believe, is due to subsidence of the crust of the earth—a movement sympathetic with that of the great outside subsidence of what is at present the bay of the province.
Although too much stress need not be placed on local phenomena, it is nevertheless certain that many small facts countenance the belief that Tutira is situated on a line of seismic partiality. During the 'eighties, when the famous Pink and White Terraces were destroyed by the eruption of Tarawera, the waters of Orakai became a dull brownish - green colour, and for many weeks continuously gave forth a strong smell of sulphuretted hydrogen; it has done so on several occasions for shorter periods since that date. The reek of sulphur is distinct, too, in many parts of the gorge of the Waikari. In the Waterfall paddock there is a spring of sulphur water; there is a tepid runnel in the bed of one of the Waikoau tributaries. Earth tremors are frequent, though perhaps not more so than elsewhere in the district. The eastward fall, moreover, of the Newton range seems to be rather less pronounced than elsewhere, as if its page 15 base, in close proximity to the lake, had somewhat sunk in local sympathy—that, in fact, the cant of the range had been in some degree readjusted by the subsidence which created the lake basins.
Local weakness in the earth's crust, break in continuity of geological plan, proof in the exposed ocean-floors of many changes of elevation, difference possibly in degree of eastward cant of the range contiguous to the lakes, makes subsidence at any rate a tenable theory in regard to the lakes of Tutira.
Their original depths may be approximately calculated from the present appearance of the ranges on either side and from examination of the cachment area of the Papakiri. The valleys gouged out, the sand and grit borne down, minus that percentage drained off in the form of muddy water, rest at the bottom of one or other of the lake basins. The area covered by Tutira lake has been in past times considerably greater than at present, for the stream that drains away its surplus water has eaten through a bank of sandy marl to the depth of some twenty feet. There remain, nevertheless, no signs whatever of this period—a fact explicable only on the assumption that erosion of the channel of escape must have been at first exceedingly rapid; that it must have been worn through not in years, but weeks, perhaps days.
Had flood - water, containing in suspension mud carried down in landslips from the fertile marls of the Newton range, been deposited on the receptive pumiceous lands which must have been then under water, a distinct vegetation would have arisen. The effect of such submersion would have been apparent still, for after heavy floods the waters of the lake remain wan for months with comminuted clay.1 Such a top-dressing would, I am confident, have remained distinguishable in its effects to this day. If in no other way, it would have been recognisable by bracken of a deeper green, by manuka of a taller growth.
1 After the deluge of 1917, when more than 20 inches fell in four sequent days—the rainguage twice overflowing—the lake had not regained its usual blue twenty-four months later.
At present the lake is drained from its nor'-west corner by the stream Tutira. This stream, after a tortuous course of half a mile through level flax swamp, reaches the old native crossing Maheawha. Immediately below begins a series of overfalls and waterfalls culminating in a leap of over a hundred and fifty feet. This drop is distant some forty chains from the lake, a distance lessened every year by erosion. I imagine that the fall has receded lakewards some two yards since the 'eighties. Exact accuracy is impossible, as the landmarks by which I page 18 have tried to gauge wear and tear have themselves moved. There is, however, growing on the stream's edge immediately above the fall, a certain aged kowhai tree whose bole is, I believe, five or six feet farther from the chasm's rim than thirty-seven years ago; the rim has receded that distance. At all events there can be no doubt that the fall is slowly retreating lakewards. Attrition is at present almost imperceptible, yet there are reasons to suppose that under certain circumstances it might become rapid, and that then the alluvial deposits of the lake-basin accumulated during centuries might be washed away in weeks. Because there has been almost no movement for years, it does not follow that such conditions will continue.
In droughts, or when nor'-westers press its waters south, for a few days or hours every year, Tutira lake will remain landlocked.1
1 The pressure of wind upon water even on so small a surface as that of Tutira is very noticeable. Upon cessation of violent nor'-west gales I have seen water forced over the low gut separating in calm weather the twin sheets of water, Tutira and Waikopiro, pouring back to the depth of many inches; there has in fact occurred, in a small way, what happens in the Red Sea on an immense scale.