Ndreke-ni-wai, which signifies “the hollow of the water,” is the name of a broad tidal estuary, opening into Savu-savu Bay, which is formed by the union, about half a mile above its mouth, of two rivers, the Mbale-mbale River flowing from the north-west past a village of that name, and the Vatu-kawa River, the largest, flowing from the eastward, which I have also named after a village on its banks. The valleys of these two rivers are separated by a mountainous dividing-ridge connected by a saddle with the main range. Its highest peaks rise to 2,100 feet above the sea, the elevation of this “divide” rapidly decreasing as it approaches the coast, where, within a mile of the beach, it terminates in some low hills 200 or 300 feet in height.
It may be observed here that a mouth of the river was originally situated 700 or 800 yards to the west of its present site. This old mouth is now represented by a lagoon communicating with the Mbale-mbale River above, but closed by the sand-mound of the beach at its lower end, which, however, is occasionally broken through when the rivers are in flood. This lagoon is shown in the view facing page [153].
The valley of the Mbale-mbale River, which is much the smaller of the two rivers, is bounded on the north by the precipitous slopes of the Koro-tini Range, which rise to over 2,000 feet, and on the south and west by the lofty Va-lili Range. The valley, above the village of Mbale-mbale, is broad and low-lying; and one can ascend it to the vicinity of Waisali, three to four miles from the river’s mouth, without attaining an elevation of 100 feet above the sea. The main stream, which flows down from Waisali, is joined near Mbale-mbale by a more impetuous stream that descends the steep mountain-sides just to the east of the Koro-tini Bluff.
The valley of the Vatu-kawa River is bounded by lofty mountain-ranges that rise to elevations varying from 2,000 to 3,500 feet. On the south side lies the Mariko Range, on the east lies Mount Mbatini, the most elevated peak of the island, whilst on the north rise up the steep slopes of the Koro-tini Range and of the mountainous “divide.” The valley has such a gentle gradient that one can follow it inland for five or six miles from the estuary to the vicinity of Nukumbolo without exceeding an elevation of 100 feet above the sea. Below Na Salia the valley is confined between the hills that approach the river; but above that village it is very broad; and on account of its slight fall the river here often changes its course, so that the floor of the valley is strewn with water-worn blocks and pebbles marking the old channels.
The Vatu-kawa River, which rises on the west slopes of Mbatini, flows with a placid current past Nukumbolo and Na Salia, until it reaches the village of Vatu-kawa, where it is joined by its impetuous tributary, the Wai-ni-ngio, “the river of the shark.” This affluent, after descending the steep slopes of the Koro-tini mountains, bursts through the dividing range that separates the Mbale-mbale and the Vatu-kawa valleys. It would seem that the Wai-ni-ngio without any great effort on its part might become a tributary of the Mbale-mbale River.
The great character of these two valleys, as shown above, is their little elevation above the sea. For miles inland the level does not attain 100 feet, and high ranges rise steeply in each case on either side to 2,000 feet and over. Here, as in the instance of most of the large valleys of the island, the original configuration of the surface was not dependent on river-erosion. Rivers no doubt have done much to carve out the lesser and to deepen and widen the greater valleys; but, as is often remarked in this work, the main features of the surface were in existence before the emergence of the island from the sea.
The geological formation of the slopes of these two valleys is described in the accounts of the various ascents of the mountains bounding them. Since foraminiferous tuffs occur high up their sides, up to elevations of 2,000 feet and over, the valleys themselves were at one time no doubt also covered with these submarine deposits, which, however, have been in great part stripped off by the denuding agencies. They are still to be found, containing large tests of foraminifera, between Mbale-mbale and Waisali; but the basaltic andesites, originally underlying them, are more frequently exposed. One of these rocks found a little east of Waisali, which has an aphanitic appearance and a specific gravity of 2·82, is merely a basic glass in its early stage of crystallisation, being made up of very minute crystallites 1/5000 of an inch in length. On the surface in this locality there also occur basic agglomerates containing scoriaceous rocks, the products of some of the last stages of volcanic action in this part of the island.... In the case of the broad part of the Vatu-kawa valley above Na Salia blocks of basic rocks derived from the mountains around strew the bottom in great abundance. Lower down, where the valley is confined between the hills, basic agglomerates and coarse tuffs are displayed in the hill-sides.
Mention should be made here of the various hot springs existing in these valleys in the low levels near the rivers and stream-courses. In the Vatu-kawa valley they exist at Nukumbolo, and in the Mbale-mbale valley at Natoarau, Waitunutunu and other localities. These springs are described in [Chapter III].
DUNIUA LAGOON, representing an old mouth of the Ndreke-ni-wai. Behind rises the Korotini Tableland (2,000-3,000 feet). The cliff-like declivity over the head of the lagoon is the Korotini Bluff.