The authors propose to give details of physiography merely as required in connection with the elucidation of problems arising from certain specific topographic features, which are discussed in sequence below.
(a) Tertiary Basin north of Whisky Creek, Ramarama.
A small hollow about a mile long and half a mile wide, immediately north of the lower part of Whisky Creek, is encircled by a well-elevated rim, constituted mainly by greywacke, and unbroken but for a small deep gap on the west, which allows escape of a small stream (see Fig. 2). Fresh-water shales form the undulating floor
of the hollow, and on the west abut against a steep slope of grey-wacke which is dissected by one or two miniature hanging valleys. On the north and east the passage from Tertiary to greywacke terrain is indefinite.
Though the present cup-like form of this area is undoubtedly a consequence of selective erosion, whereby there has been removal of a noteworthy depth of soft Tertiary beds, it seems probable that the present relation of these latter to the greywacke is due to tectonic causes. The steep western greywacke margin of the basin, it is suggested, is a resequent fault-line scarp, developed with reference to a north-north-east fault with a throw to the south-east attaining not less than 150ft. in magnitude, but apparently decreasing rapidly northwards. The only evidence in favour of this suggestion is the apparent absence of coarser clastic beds, such as expectably would occur were the steep greywacke slope the wall of a hollow in which Tertiary deposition took place, whilst the Tertiary shales on the floor of the hollow now dip westward at from 20° to 25° towards the line of the hypothetical fault.
It is not clear why the stream draining this small basin did not take a southerly course along the soft Tertiary sediments which apparently provide a ready route to Whisky Creek.
Space does not permit full discussion of all explanatory hypotheses possible, so that the authors will content themselves with suggesting that the early consequent drainage at the locality concerned may have taken its course approximately south-west along the fault-angle depression caused by the fault that has been invoked, and was unobstructed by the volcanic mass now in existence, but then not extruded.
When the lavas were emitted, however, such a route would be blocked, and the ponded waters would overflow by the lowest gap—here over the western rim. An objection to this hypothesis is based on the fact that, as shown earlier, it appears as if the basaltic lavas concerned were outpoured before completion of major movement along the Papakura-Drury Fault. Thus the north-north-east minor fault postulated would have to be earlier than the major north-north-west fracturing. This conclusion does not accord with reasonable expectations, so that at first it appears by no means unlikely that the outlet stream discussed is antecedent in its nature and was superposed upon the greywacke from overlying Tertiary strata. The basalts, however, were not extruded until these latter sediments were stripped from part of the greywacke of the western rim of the basin, for they there rest upon greywacke, so that, if this explanation of the drainage be accepted, it must again be concluded that the volcanic episode was subsequent to faulting along the north-north-east line, and the same objection as before applies. Other conceivable hypotheses meet with similar difficulties, and it appears most reasonable to the authors to accept the first hypothesis and admit the possibility that the north-north-east faulting invoked preceded that which caused the Papakura-Drury fault-scarp. Such north-north-west fracturing can well explain the low elevation of Tertiary beds both at, and not far south of, the debouchure of Whisky Creek.
(b) The Main Central Tertiary Area.
As just explained, it is likely that the Tertiary strata south of Whisky Creek have been downfaulted along a N.N.E.-S.S.W. line, so that the block on which they rest has a downward tilt approximately to the south-west. The southern margin of this area of Tertiary beds follows a sinuous line where it borders the greywacke, which occurs in slopes rising fairly steeply to higher levels. A stream approximately follows the junction for some distance, though locally superposed upon the older rock. There is no topographic evidence of faulting along this east-west direction, and it is impossible to decide whether the steep contact evidenced is a result of down-flexure or fracture, or whether one views in effect the resurrected surface of the greywacke terrain on which Tertiary deposition occurred.
In the middle of the area now described, a fault, perfectly demonstrated both by stratigraphic and topographic evidence, separates the area of fresh-water Tertiary beds on the west from marine members higher in the same sequence, but at lower altitude, on the east. A resequent fault-line scarp about 150ft. in average height trends N.E.-S.W. for about half a mile, followed at its base by a small stream, whilst further east the surface rises gradually as an inclined gently-undulating plain-like area furrowed shallowly by several tiny sub-parallel consequent tributaries (see Fig. 3). This plain-like area is underlain by marine sandstones, which are freely exposed in deeper plough-furrows and represent the highest member of the Tertiary sequence in the Bombay district.
In the fault-line scarp, greywacke rises locally over 100ft. above the bed of the stream below, and is then covered by fresh-water beds exposed for a depth of 25ft. in an abandoned quarry, but thickening very materially locally where the greywacke surface beneath them declines steeply, for, a little north-east of the quarry, the fresh-water beds descend to not far above the bottom of the scarp. These beds consist of a characteristic, white, fairly compact quartz-sandstone containing leaf-impressions, and occurring in bands about 2ft 6in deep, interbedded with thinner argillaceous layers. On analogy with the best-exhibited sequence of the Tertiary strata available, it is clear that, near the quarry, the downthrow of the fault is of the order of 200ft. to the east. This throw diminishes as the fault is followed north, however, for foraminiferal shales, which are stratigraphically below the uppermost marine sandstones referred to, outcrop not far east of the fault-line in a cutting of Ramarama-Ararimu Road, where they appear to dip gently westward.
Though the relatively low general level of the marine beds of this Tertiary area as compared with that of neighbouring greywacke is in part due to the fault described above, it is difficult to decide, as in other localities, whether towards their eastern margin the various members of the Tertiary sequence overlap upon a steeply-rising greywacke basement, or whether the steep greywacke slopes, which rise above the surface of the younger beds to a height of 1046ft. in Dome Hill, represent substantially the plane of a fault. Had the dip of the younger beds shown any close approximation to the slope of the contact, flexure could have furnished a sufficient explanation of the
mutual relations, but the beds dip approximately west at an angle not exceeding 10°, whilst the contact plunges steeply in the same direction at about 25°.
In the failure of definite evidence, the authors can only suggest that it is by no means unlikely that a meridional fault with downthrow to the west determines the western margin of Dome Hill, and is partly responsible for the superior altitude of this greywacke mass as compared with that of the Tertiary beds west of it.
On the other hand, there is no possible doubt that the greywacke relief adjacent to areas of Tertiary deposition in this locality was considerable and that the land-forms were rugged, for breccia-conglomerates locally of very coarse nature occur at the base of the fresh-water series.
A convincing section of these and other beds of the Tertiary sequence can be examined near what will herein be referred to as the mid-Ingaia bridge, about 2 miles south-west of the trig. station on Dome Hill.
In a small rill alongside the road climbing the northern wall of the valley from the bridge, a skin of coarse greywacke débris about 3ft. in thickness, with its material little worn by water and including, as well as smaller fragments, blocks 2ft. and more in diameter, veneers a surface of greywacke which can be traced for about 2 chains along its steep south-westward plunge at an angle of about 25°. About 9ft. of similar material is exposed at the bridge and passes rapidly through a small thickness of true conglomerate of less coarse nature into sandstones and other fresh-water strata which do not exhibit the steeply-inclined attitude of the coarser beds, but, after an interval of a very few yards, are found dipping very gently westward.
It seems impossible to account for these coarse breccia-conglomerates as other than the deposits of vigorous youthful streams. Their relatively small depth, however, preserved along the direction of dip for not less than 5 chains, indicates that they cannot have been deposited in an alluvial cone, as must necessarily have been the case had the greywacke surface on which they were laid down had originally, as now, an inclination approaching 25°.
The conclusion, therefore, follows that this surface has been steepened by flexure or other tectonic movement, and that steep and lofty unreduced greywacke residual elevations existed on the margins of the basin concerned during the deposition of the Tertiary beds therein.
It will be demonstrated in the next section that precisely similar conclusions emerge from the evidence available in the locality there described.
(c)Tertiary Basin east of Bombay.
This basin, which may be called Bombay Basin, is a hollow displaying fairly broad valley-floors composed of modern alluvium alternating with rolling hills of various Tertiary strata. On the north-west the greywacke slopes of Dome Hill (1046ft.) rise very steeply about 500ft. above a valley-floor at their base; on the
west and south-west there is a barrier, provided by the volcanic mass of Bombay Hill, which gives place southwards to the steep greywacke slopes of Puketutu (1229ft.); eastward and south-eastward the greywacke surface beneath the Tertiary beds rises very gently to emerge in low marginal hills. Headwater tributaries of Ingaia Stream drain the basin and unite to leave it by falls cut in basalt and greywacke at its north-west corner.
Again, as in the Tertiary area north-west, there is evidence of the existence of highlands bordering the Tertiary lakes in the presence of coarse basal conglomerates of fresh-water nature in the Tertiary sequence. Similarly topographic data suggest that a powerful N.E.-S.W. fault forms the northern boundary of the basin, whilst the abutment of coal beds against greywacke at the base of the steep eastern slopes of Puketutu may be said to augment the evidence in favour of a meridional fault passing near the mid-Ingaia bridge.
This displacement near Puketutu, however, would have to be interpreted as involving downthrow to the east, whilst west of Dome Hill such downthrow, if faulting occurred, must have been to the west.
Bombay basin, therefore, is interpreted as structural in origin, developed by warp-like down-tilt of a block bounded by faults on its north-west and western margins. Whether these faults are coëval with the major Papakura-Drury fracture further west could not be determined.
The present low altitude of the floor of the basin arises from subsequent differential erosion whereby the soft Tertiary beds have largely been removed, whilst the hard greywacke has been relatively little affected.
It is suspected that the north-east fault postulated as defining the south-east border of Dome Hill has been almost contemporaneous with the meridional one similarly invoked, and has probably been continued to the south-west by a sag which may have caused the rapid decline in the elevation of the surface of the greywacke from Puketutu, where is is at about 1000ft., northward until, in a distance of about 3 ½ miles, Tertiary beds appear beneath basalt at an elevation of about 600ft., the greywacke basement being stratigraphically about 200ft. lower. Not far from where this happens, the same lavas have also buried greywacke spurs descending westward from Dome Hill, and have so deviated Ingaia Stream, which drains Bombay Basin, that it has turned north and carved for over a mile a deep gorge in greywacke spurs upon which it was superposed.
Prior to the outpouring of the lavas, the most northerly of the three present head-water tributaries of Ingaia Stream was reinforced by the drainage of the large intermontane depression at Ararimu which now flows north to Mangawheau Stream. A narrow sinuous through-valley, with a very low saddle at the present air-gap (1 ½ ml. north-east of Dome Hill), now connects this latter depression with the Bombay one. It may be suggested that this early reinforced stream flowed south-west direct to the lowlands near Bombay township, its course continuing the line of the N.E.-S.W. fault that
is believed to delimit the contact between the greywacke of the southern slopes of Dome Hill and the Tertiary beds south of it, and following the axis of the sag that has been postulated as forming an extension to this fault. Whether or not this course is an inherited one consequent upon early tectonic movements is immaterial, for in effect it is now subsequent in the portion that borders the southern slopes of Dome Hill, and undoubtedly long had been so prior to the eruption of the basalts, for, when this latter event took place, the stream had excavated its bed through the Tertiary cover until the greywacke basement was exposed in the north-west corner of Bombay Basin. Later this greywacke was covered by lavas, extruded from Bombay Hill as centre, which effectually masked the westward continuation of the valley in addition to diverting Ingaia Stream and causing it to encircle the eastern and northern margins of the volcanic mass erected. The regular unbroken descent west of the slopes of Bombay Hill as a sheet of lavas masking any pre-existing scarp along the line of Papakura-Drury fault, shows that volcanic extrusion culminated after faulting had ceased. It is thus by no means improbable that the reversal of drainage indicated by the air-gap north-east of Dome Hill was a consequence of warping or tilting connected with faulting, and was in no way a result of the blockage of westward-flowing drainage by lava barriers, but on this point there appears to be no conclusive evidence.
Similar reversals have, however, occurred in an air-gap valley about 4 miles east of Paparata and in Happy Valley, but in both these cases the cause appears largely to have been temporary obstruction of southward-flowing streams by floods of lava.
(d) Happy Valley and the Early History of Wairoa River.
Interesting problems arise in connection with the extraordinary alluviated intermontane hollow called Happy Valley and its modern drainage, whilst intimately associated with them is the early history of Wairoa River. The full history of this river will not be elaborated, but only such parts as are necessary for discussion of the origin of Happy Valley.
This latter valley is a broad hollow carved in greywacke; in its northern portion it trends south-south-east for three miles or more and then bends to the south-south-west for a further 2½ miles (see Fig. 5). Its floor has been aggraded to a considerable depth by gravels which are relatively coarse 20 or 30 ft. below the surface, where encountered in wells, though nearer the surface finer detritus is dominant. As a consequence of the alluviation, a broad plain over a mile in width has resulted. In the southern and mid portions its surface is almost horizontal at a level varying from about 400 to 420 ft. above sea-level, but in the north-west it is considerably terraced, the uppermost terraces, however, being at substantially the same height as the undissected plain elsewhere, though rising slightly towards Wairoa River.
The greywacke slopes on the western side of the valley are relatively gentle, but on the east wall of its northern arm there is an abrupt, lofty slope, bordered by occasional alluvial fans, which is
interpreted as a scarp developed in connection with the great N.N.W.-S.S.E. Wairoa Fault. There is much to suggest that it is a resequent fault-line scarp; as, however, its actual nature does not greatly affect the main problems of this section, this point will not be discussed.
Three streams have their source in the depression, leaving it by diverse routes. Two drain the southern and central portions, and each, in its escape, cleaves the hills forming the south-eastern rim of the basin. The more northerly of these two streams follows the line of Wairoa Fault, here apparently greatly reduced in throw, and after traversing lofty falls eventually reaches Mangatawhiri River just after it has debouched from a deep gash it has carved in the mountainous highlands further east. The other follows the southern arm of Happy Valley and flows west until it joins up with other southward-flowing tributaries to form Paraureroa Stream, which passes south through a mountain barrier over 1000ft. in height by means of a remarkable narrow gorge and finally unites with Mangatawhiri River on the low-lying plains of Pokeno Valley (see Figs. 6 and 7).
In the scarp north of the tennis courts in the middle portion of Happy Valley, the third diverging stream has its birth, and pursues its way north-west across the plain, entrenched to an increasing degree below this latter, until it joins Wairoa River where this stream makes its remarkable boat-hook bend, and turns north-west to follow a narrow, but not deep, valley excavated along the line of Wairoa Fault, and eventually flow into the Firth of Thames. Thus some of the drainage of Happy Valley escapes to the east coast of North Island, whilst the rest goes by way of Mangatawhiri River into Waikato River and thus to the west coast.
Before its alluviation, Happy Valley was strikingly more widely open than its narrow outlet valleys, and several causes appear to be responsible for this fact. So far as the north-west branch of the valley is concerned, it is probable that its gently-sloping wall represents a pre-Tertiary surface of greywacke which has been tilted towards the east by downthrow along Wairoa Fault, resurrected by erosion, and subsequently modified by such erosion (see Fig. 8). A close comparison is afforded by the fault-angle depression of Papakura Valley (see Laws, 1931). It is likely that the amount of downthrow varied at different points along the line of Wairoa Fault. For example, it almost certainly is greatly reduced near the boat-hook bend of Wairoa River, where two sub-parallel members of the fault-system define a perfect fault-splinter (Fig. 4), whilst topography suggests that the displacement also was reduced as the fault approached the present course of Mangatawhiri River.
Another factor that may have operated is a north-west warping or tilting of the major earth-block concerned towards the north-west as an accompaniment of movement along the N.E.-S.W. Pokeno Fault. This would compel any undefeated southerly-flowing stream to carve a deep water-gap in the uprising block, and it is believed that the acute V-shaped pre-basaltic gorge of Paraureroa Stream is an example of such compulsion. On this hypothesis Pokeno Fault
must be of later date than Wairoa Fault. It is true that this interpretation of Paraureroa Stream as antecedent is open to challenge, but no more satisfying explanation of the facts appears to be available.
The writers are unable to offer any satisfactory explanation of the wide south-west arm of Happy Valley and can only make the tentative suggestion that it occupies a local downsag.
Upon early incomplete examination of the area, the writers formed the tentative conclusion that, prior to taking its present north-west route from the boat-hook bend referred to above, Wairoa River flowed south-east along the northern arm of Happy Valley and escaped from it by the route along the south-east continuation of Wairoa Fault. The substantial uniformity of level of undissected portions of Happy Valley plain and the coarseness and high degree of rounding of some of its materials show clearly that the aggradation evidenced is the work of a far larger stream than now crosses the plain. Such a stream can only have been Wairoa River.
With further observation, the writers have come to agree with Mead (1930) that the main outlet of this early Wairoa River was not as stated above, but along the south-west arm of Happy Valley and through the gorge of Paraureroa Stream.
After leaving the alluvium, the present waters from Happy Valley following this route cross a narrow barrier of basalt for about 150yds. and then tumble over falls about 100ft in height caused by the basalt, but not exposing its base. This shows that the pre-basaltic valley at this place was excavated in excess of 100ft. below the level of Happy Valley. In contrast, the greywacke barrier in which the more northerly outlet stream has cut its bed even to-day is not more than about 40ft. below the surface of the plain, whilst in the past it must have been at a substantially higher level. It is obvious, therefore, that this outlet cannot have been in existence at the time when the deep pre-basaltic gorge of Paraureroa Stream was in effective operation and that a change of conditions was necessary before this northern spillway was initiated.
This change was brought about by the outbreak of volcanic activity from the centre mapped about a mile west of Trig. 619 and possibly also from the hill on which this trig. station is placed, for this bears a cap of basalt. The flood of lava followed down the valley of the stream here named Air-gap Stream into the Paraureroa Valley, continuing well into the outlet gorge, though apparently it did not reach the flats of Pokeno Valley. This is not the only local example of occupancy of stream-valleys by lavas; another occurs about 4 ½ miles further west, where basalts have followed a valley carved deep in the greywacke hills and, emerging on the Pokeno flats, have there spread out in fan-like fashion.
The surface of the flow that occupied the Paraureroa valley attained a height of 100ft. or more above the present level of Happy Valley and must have ponded effectively the waters of Wairoa River and those from the air-gap valley. Soon, however, overspilling of the ponded waters took place more or less contemporaneously over the basalt in Paraureroa valley and, further north, along a gap now occupied by the stream following the line of Wairoa Fault to Mangatawhiri River.
The period of maximum ponding must have been relatively short, for it is not represented by any shore-line benches or terraces, so that at first there must have been fairly rapid lowering of the levels of the barriers until they became stabilised upon fresh unweathered greywacke in the north and upon fairly resistant massively-jointed basalt in the Paraureroa, at a height represented by the present level of Happy Valley (400ft.-420ft.). Following this stabilisation came the infilling of the depression to its present level. Whilst in all probability there may have been early spilling of the ponded waters north-west down the course of the modern Wairoa River, such as would have assisted in the capture that later became complete, such capture was not effective until the building of Happy Valley plain was practically finished; this is indicated by the discovery in post-holes just upstream from the basalt barrier at the south-west end of Happy Valley of gravels of far coarser and more rounded nature than could be ascribed to the modern stream there existent. When, however, capture was perfected, the north-west reversal of drainage reflected in the modern streams was speedily effected. There is, however, nothing to indicate whether the considerable recession of falls, whereby the greater part of the basaltic fill of Paraureroa valley has been removed, has been the work of the larger stream that followed this route before the reversal of drainage or that of the small modern stream. This recession has continued until a strip of basalt not more than 150yds. in width intervenes between the falls and the alluvium of Happy Valley.
An interesting further example of the influence of the lavas upon the modern drainage pattern is afforded by a small stream west of Air-gap Stream; this has followed for over two miles the contact between basalt on the east and greywacke on the west until it joins Paraureroa Stream.
Nothing so far has been mentioned as to the date of the reversal of part of the drainage of the air-gap valley. The aggradational up-building of this valley reached approximately the level of Happy Valley, so that its history may be held to parallel that of this latter area, and to have entered on a new phase subsequently to the blockage of the southern end of the valley by basaltic flows. The stream now leaving the northern side of the air-gap is a reversed one in its head-water portion, and enters Wairoa River near the elbow of capture of this latter. The pirating stream can only have gained sufficient virility to be able to deepen its valley floor and effect rapid headward extension after the captured Wairoa waters had substantially deepened their new valley.
It will be noted that, in this section of the present paper, the authors have assumed that no movements of tilting or warping have occurred since the excavation of the original Paraureroa outlet-gorge. The fact that this was carved in pre-basalt times approximately to the level of the Pokeno Valley plains demonstrates that substantially this is true; it is, nevertheless, possible that a slight degree of back-tilting is indicated at the head of Air-gap Stream, for, in spite of the fact that, at an early date, this stream carved a terrace not less than 20ft. in height from the broad alluvial flat occupying the air-gap, to-day there is a broad swamp bordering the terrace (see Fig. 9).
Dr. W. N. Benson very kindly drew the attention of the authors to the similarity of certain of the features of their district to those of the Puy-de-Dôme region of the Auvergne area in Central France, and lent a paper by Glangeaud (1908) and the Clermont sheet (No. 166) of the Service Géologiques des Mines, issued in 1910, which illustrate the geology of this region. On an incomparably grander scale than in the New Zealand area described, Pleistocene flows from volcanoes of the Chaîne des Puys and other centres have travelled many miles along valleys excavated in an elevated plateau of ancient crystalline rocks until they have covered part of Tertiary lowlands further east, which are separated from the ancient rocks by a great N.N.W.-S.S.E. fault. In addition, there is in the Auvergne, as in the Bombay area, remarkably close relationship between the distribution of the eruptive vents and the fracture systems.