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Chapter II. — Rock Constituents of the Run

page 9

Chapter II.
Rock Constituents of the Run.

The materials of which the station is formed are marl or “papa,” sandstone, sandy marl, limestone, and conglomerate. Except the last, they are obviously of marine origin. Marl is the foundation on which the others lie,—it is the bed-rock of Tutira. Whatever may have been its origin, the remains of an ancient southern continent or not, its constituents seem to have been carried by ocean currents or tidal action. Deposition has been, at any rate, intermittent, not constant. Undulatory lines of sand-grit can be traced on cliffs where flaking is constant and where consequently exposures are clean.1 Although so faint as to be only decipherable in certain lights, like patterns in watered silk, they nevertheless mark brief periods of quiescence as surely as the grosser pelagic accumulations outcropping elsewhere on the run. The immensity of these deluges of mud can be gauged by sand lines sometimes yards apart. Their close recurrence can be inferred by interpolations of sand so thin as to be practically invisible.

The marls of Tutira vary in fertility and mode of weathering, the least fertile being the most homogeneous and compact, the most fertile those that disintegrate in cubes or exfoliate in peelings.2

We can now consider rock formations superposed on this base of marl. To do so it will be convenient that in imagination the reader should as before take his stand on the Maungaharuru range and again traverse the run from west to east.

The great dry bastion on the west is built up of alternate layers

1 Such lines are particularly noticeable on the beach cliffs between Waikari and Mohaka.

2 Through the kindness of Dr Lauder, of the Edinburgh and East of Scotland College of Agriculture, a station sample of marl has been analysed. It “contained 34 per cent of calcium carbonate (chalk), fairly large quantities of the oxides of iron and aluminium; smaller quantities of calcium and magnesium oxides. Phosphates were present, but no potash.”

page 10 of sandstone and limestone, the result, doubtless, of floodings of sand over shell-strewn ocean beds. Each band is of a different thickness. Each slightly differs also in material, the sandstone sometimes less and sometimes more dry, the limestone bands varying in the nature of their shell fragments. Where, as occasionally happens, the topmost sea-floor has been stripped by gales of its earthy coverings, cleavage is revealed in the form of almost exact squares, in a mosaic of grey blocks of limestone set in packed red sand.1

Continuing to traverse the run, moving eastwards, we reach central Tutira. Hereabouts limestone disappears, sandstone and conglomerate resting on the marl, the sand of the former seemingly ground out of gravel, the pebbles of the latter rolled, worn, and of a generally ovoid form. In these conglomerates and sandstones, fossils are rare. In the first I have found but a single specimen, a short length apparently of some knotless tree stem; in the second, one or two kinds of bivalves. Usually the weathering of the alternate bands of a cliff face proceeds evenly. Sometimes, however, it happens that in a loose type of sandstone erosion by frost and wind is rather more pronounced than on the conglomerates above. When that occurs the cliff assumes a protuberant air, a curious rotund or pot-bellied appearance. Normally the conglomerates of the central run cap the sandstones. Lacking their protection, the softer rock has melted into cones, domes, razorbacks, and peaks.

Proceeding once more from the west to the east we reach low lines of hills, eminences hardly more than hummocks. What is visible of their rock material differs but little from the earlier mentioned conglomerates; their colour is rusty-red instead of grey, they do not appear to be so thoroughly set, they can be worked with a pick, almost with a shovel. There is perhaps rather less variation in the size of their pebbles.

Again advancing on our traverse of the run, crossing the lakes we reach the eastern range of Tutira—the eastern edge of the trough. Once again the rock formations change: the typical western facing cliff is built up here of limestones, sandstones, and marls, irregularly superposed one upon another in bands of different depths. Compared with the sandstones and limestones of the west, the sandstones and limestones

1 A very perfect example of such a pattern may be seen on the great wind-blow of the Maungaharuru range.

page 11 of the east show fresher and fuller signs of marine life. Their sand is more truly sea sand. Their limestones contain a larger number of shell-species closely connected, or actually identical with those now existing in the pertingent seas. Above several of the limestone bands are shallow deposits of a highly friable ferruginous matter. On the cliffs of the east, too, there supervenes a new and most important factor. In this region the marl bands interpolated at irregular intervals, rapidly crumbling, sap and undermine the more durable strata; the cliffs are somewhat less sheer, the strata exposed having weathered less evenly. From the sea-floors thus sapped great quadrilaterals—rock cleavage
Sea-floors on eastern Tutira.

Sea-floors on eastern Tutira.

is similar to that of the western limestones—have already dropped out, whilst others projecting into space also appear to be about to break away.

To recapitulate: The base of the whole station is blue marl rock or “papa.” Superposed on it rest the ranges of the west formed of sandstone and limestone, the ranges of the centre formed of compressed sandless pebble-rock and slaty sands, the ranges of the east built up of layers of marl, sandstone, and limestone.

Of these masses of material, the sandstones and conglomerates extend over nine-tenths of the station and are of little value. Such elements of fertility as exist are contained in limestone outcrops, in masses of travertine, but especially in elevated streaks of marl.