greatest area and thickness of any of the formations in Kaihu Group (Fig. 1). South of Muriwai Kaihu Formation overlies the Miocene pyroclastics of the Manukau Breccia, but to the north it extends below sea-level and forms the greater part of South Kaipara Peninsula.

Between Bethell's Beach and Pulpit Rock (2 ½ miles north of Muriwai) sands and silts form a plateau which averages 550ft. in elevation and locally rises to as much as 690ft. where dune sands have been developed at the top of the succession. Fairly coarse grey sandstones at 250ft. above sea-level on a ridge north of Anawhata Beach, and silts and sandstones at the south end of Bethell's Beach are remnants of the southerly continuation of this plateau North of Bethell's Beach the flat surface is well preserved and is half a mile wide, widening to 1 ½ miles near Muriwai.

One of the best exposures of the basal contact of Kaihu Formation is located in the northern corner of the first bay south of Bethell's Beach where highly weathered, marine Tertiary tuffs are unconformably overlain at 130ft. above sea-level by a succession of sands, silts and muds (Fig. 2). A similar sequence is well displayed at Erangi Point, between Bethell's Beach and O'Neill's Bay, but at higher levels wind-bedded brown sands of varying thickness appear and are covered by variegated muds or lenses of pumiceous silt. Taylor (1927) recorded a small pocket of pumice silts and peaty material at 200ft. above sea-level in the upper reaches of Waiti Stream, a tributary of Waitakere River, and about two and a half miles inland from Bethell's Beach. The writer failed to locate this outcrop, but volcanic tuffs which Taylor reported as resting on top of the silt may actually be consolidated brown sands of Kaihu Formation. On Te Waharoa and Tirikohua headlands the vertical successions from the basal contact for 300ft. up to mudstones underlying the plateau surface are well exposed. Half a mile east of the coastline the plateau rises gently to 690ft. and is underlain by beds of dune sand. The plateau surface is normally underlain, however, by water-laid deposits visible in two good exposures. At one of these, a quarter of a mile east of the junction of Taiapa and Oaia Roads, eroded tuffs of the Manukau Breccia are overlain at 490ft. above sea-level by a thin band of mudstone. The mudstone merges upwards into 2ft. of coarse black sand capped by current-bedded sandstone which becomes increasingly finer in texture towards the plateau surface at 520ft. The other exposure is on the side of the most northerly obvious remnant of the plateau where a perfectly flat surface is preserved on the ridge between Paekawau Lake and Pulpit Rock, two and a half miles north of Muriwai. Here the closing sequence, above a basal contact at 280ft. above sea-level, is from current-bedded yellow sandstone to white pumice silt in a 6ft. band at 530ft.

Inland from the Muriwai-Anawhata coastline, and on the eastern side of Waitakere Hills, remnants of terraces at levels lower than 550ft. are widely distributed at heights of 350ft. and 220ft. to 240ft. Between Waimauku and Swanson these surfaces are not generally underlain by sediments of Kaihu Formation except west of Waimauku, where the 220ft. to 240ft. level overlies water-laid sands and silts 154ft. thick thinning out westwards towards a ridge of Manukau Breccia near Taiapa Road. The distribution of these Pleistocene surfaces of erosion and of deposition suggests a paleogeographical picture in which the Waitakere Hills formed an elevated resistant mass in an area to the south of a line joining Bethell's Beach and Kumeu. The existence of this highland is indicated

by the absence of Kaihu Formation on high terraces of marine erosion at 550ft, 350ft., and 220ft. to 240ft., and by the increasing irregularity of the basal contact of Kaihu Formation with the Manukau Breccia as a traverse is made south along the coastline from Muriwai to Bethell's Beach. North of the Bethell's-Kumeu line Kaihu Formation increases in thickness, underlies the same high surfaces, and for 4 miles south of Muriwai maintains a very regular basal contact.

West of Kaipara River and north-west of Pulpit Rock and Waimauku to near Kaipara South Head, Kaihu Formation passes below sea-level and in no place is seen to rest on older rocks such as Manukau Breccia. It seems most likely that a meridional fault was responsible for the termination of the Miocene volcanic rocks at Pulpit Rock. This is indicated by the abrupt cessation of tuffaceous Miocene sandstones along a line joining Pulpit Rock and Parakai Springs; east of this line these sandstones form a floor to Kaipara Valley at depths averaging 80ft. below sea-level, but to the west bore logs do not record them at depths up to 300 feet. A magnetic survey at Parakai by Jones (1939, p. 85 B) gave “no reason to deduce a basic dyke or flow close to the surface … such as gives rise to thermal springs in many cases “and thus supports the suggested location of the fault-line. There must, however, have been erosion and retreat of the fault

Fig. 2.— Stratigraphic columns of Kaihu Group: (a) at the northern corner of the first bay south of Bethell's Beach; (b) and (c) south of Shelly Beach whaif. Heights shown are measured above modern mean sea-level.

line scarp for steep bluffs of Tertiary rocks lie in a north-south line two miles to the east along the side of Kaipara Valley and continue for some miles north of Helensville. Penecontemporaneous subaerial erosion reduced the north end of the present Waitakere Hills by some 500 feet.

The absence of hard basement rocks below Kaihu Formation on South Kaipara Peninsula has tended to accelerate destruction of the narrow belt of higher terrace features that are continuous with those already described near Muriwai and Waimauku. Nevertheless, visual observations and statistical topographic analyses using the methods of Baulig (1935) have confirmed the presence of surfaces at 350ft. and 220ft. to 240ft. immediately west of the fault-line—i.e., west and south-west of Parakai. The inferred position of the 550ft. level would thus fall on the dissected but remarkably even crestline of South Kaipara Peninsula as far north as Shelly Beach. The period of fault-line development and immediately subsequent erosion therefore antedated the 550ft. sea-level and was at least pre-Pleistocene in age. The surfaces carried by Kaihu Formation may therefore be safely used for discussion of sea-level movements without complication by post-Tertiary differential warpings.

In the area between Pulpit Rock and Shelly Beach, Kaihu Formation is composed of light-coloured water-laid pumiceous sandstones, siltstones and pumiceous muds similar to those already described from south of Muriwai. The freshest outcrops are in cuttings made for roads that follow the flat-topped spurs west of Helensville and on the crest of the peninsula where wind-scour has excavated many hollows and removed part of the Recent dune sand cover.

(2) South Head Formation (mid-Pleistocene)

South Head Formation is formed by dark brown or reddish brown consolidated dune sands with large-scale wind-bedding. The sands are coarse in texture and poorly compacted, and leaching by groundwater has produced many limonite pans characteristic of this formation, especially near the shoreline. The dunes outcrop over an area of only 12 to 15 square miles at Kaipara South Head; along their southern margin they overlie Kaihu Formation, but for the main part they pass below sea-level.

In aerial photographs (Fig. 3) the outlines of Pleistocene dunes can be easily identified and show them to be large, irregular, lobate transverse dunes aligned with respect to a south-west wind and similar to modern dunes behind Muriwai beach. An interesting feature is that the transition from the subaqueous beds of Kaihu Formation to the South Head dunes is marked in the field not only by a change from red- and yellow-stained sediments to dark brown sands, but also by a change from scrub-covered wastelands to fertile farm pastures. This contrast in soil fertility is attributed to mineralogical differences between the water-laid and the wind-blown materials; that is, in the latter there is a much higher content of ferromagnesian minerals.

A most important point is that the crests of some of these fossil dunes are less than 200ft. in height, and therefore they post-date the high-level erosion-surfaces at 550ft., 350ft. and 220ft to 240ft. that cut across Kaihu Formation. However, the dunes have been cut across by a terrace at the lower level of 110ft to 130ft. This demonstrates that the ocean regressed below present sea-level after cutting the 220ft. to 240ft. level on Kaihu Formation, allowing a dune area to develop north of the distal end of a spit which joined the mainland at Pulpit