Eijstatic Bench of Islands of the North Pacific 1 by Chester K

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

EIJSTATIC BENCH OF ISLANDS OF THE NORTH PACIFIC 1 BY CHESTER K. WENTWORTH AND HAROLD S. PALMER (Read before the Society December 29, 192k) CONTENTS t Page I ntroduction...... 522 Description of the eustatic bench...... 522 General features...... 522 Relation to structure...... 526 Relation to coastal exposure...... 527 Other evidences of recent strandline shift...... 528 Causes of the strandline shift...... '.... 530 Age of the shift...... 530 Detailed accounts of islandbenches...... 531 Tabulation of the islands...... 531 Gardner Island...... 531 La Perouse Rock, French Frigates Shoal...... 532 Johnston Island...... 532 Necker Island...... 532 Nihoa Island...... 533 Kaula Islet...... ‘...... 533 Niihau...... 534 Lehua...... 534 Kauai...... 534 Oahu...... 534 Molokai...... 539 Maui...... 539 Lanai...... 539 Kahoolawe...... 539 Molokini...... 542 Hawaii...... 542 Palmyra...... ,...... 543 Washington...... 543 Fanning...... 543 Christmas...... 543 Jarvis...... 544 Howland and Baker...... 544

1 Manuscript received by the Secretary of the Society March 30, 1925. Published with the permission of the director of the Bishop Museum. (521)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 5 22 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC

I ntroduction Daly2 has compiled descriptions of raised wave-cut platforms from about thirty places scattered about the world. The present authors have been 'struck by the fact that a bench from 4 to 12 feet above mean sealevel exists on all of the volcanic islands of the Hawaiian chain, without exception Every island.in the chain shows it. One or the other of us has observed the bench on La Perouse Rock at French Frigates Shoal, Necker Island, Nihoa Island, Kaula Island, Kauai, Oahu, Molokai, Lanai, Maui, Kahoolawe, and Hawaii. Study of photographs in the files of the Honolulu office of the Hawaiian District of the United States Lighthouse Service indicates that the bench exists on Lehua and Molokini. A photograph, blurred by the motion of the surfboat from which it was taken, suggests that Gardner has the bench. Reliable oral testimony indicates that the bench is to be found on Niihau, and we have also observed the bench on the small islets off the shores of Oahu. Additional evidence of a recent higher stand of the sea is fur nished by the emerged masses of reef or of detrital limestone observed on Fanning, Christmas, Jarvis, Washington, Howland, and Baker islands of the Line group. Shortly before the present paper was submitted in final form for publi cation, Dr. Wetmore’s paper on the bird life of some of the islands in question appeared in the National Geographic Magazine, and the bench on Necker Island is shown in one of the illustrations accompanying this paper.3 There has also appeared recently a report on the reefs of Samoa, in which a wave-cut bench, now some ten feet above sealevel, has been described.4 D e s c r ip t io n of t h e e u st a t ic B e n c h GENERAL FEATURES . The bench is best developed on tuff cones, such as those in the Koko Head region at the east end of Oahu. The tuff cones are much younger than the flow lavas that form the main mass of the island, for in places the tuff extends into valleys cut ia the lavas. The bench is in general from 10 to 30 feet wide. In places it narrows or tapers out entirely.

2 R. A. Daly : A recent worldwide sinking of ocean level. Geol. Mag., vol. lvii, no. 072, June, 1920, pp. 246-261 ; also, A general sinking of senlevel in recent time. Trans. Nat. Acad. Sci., 1920, pp. 246-250. 3 Alexander Wetmore : Bird life among lava rock and coral sand. National Geo graphic Magazine, vol. xlviii, no. 1, July, 1925, p. 80. * A. G. Mayor: Structure and ecology of Samoan reefs. Pub. 340, Carnegie Institu tion of Washington, 1924, pp. 1-2.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTION OF THE EUSTATIC DENCH 523 Elsewhere it may be as much as 120 feet wide, as in the flow lavas on the west side of Nihoa Island, or even 150 feet wide, as in the tuff on Manana Island, off the east end of Oahu. The bench ranges in elevation from 4 to 12 feet above mean sealevel. We believe that the bench' is in general a little higher on the most exposed .1 8 0 ' / 7 0 " /¿O' ISO ' .

Gmrttner

A/ec/rer /y,-fioa Frenc/t Frig a t* SA

O Oa.hu FTo /eka.f /fau/a . i * * . / % > rta.hoo/aiv& O—

• \fohn-ston

Fa /mt/ra.

yVas/rington .

0 Fanning

^ Chris/'ma.s Hotr/and • Ba.Atsr 1 8 0 * ~ 7 7 0 *• 7SO' F ig u re 1.—Map of Portion of North Pacific Ocean Showing locations of islands mentioned in the text and including all the high islands within its limits. parts of promontories and lower along the shores of bays, but no statis tical data can be given at this time. At all events, the bench varies considerably in elevation, even in short distances. The variations are so irregular that they can not be ascribed to movements in the lithosphere,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 5 2 4 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC but must be due to original differences in texture or jointing, or both. The emergence of the bench appears to indicate a negative shift of sealevel of 12 to 15 feet. Ignorance of the depth of effective cutting during the forming of the bench makes closer estimates impossible at present. It does not appear, however, that there lias been a shift in the North Pacific region as great as the 20 feet deduced by Daly.5 Minor variation of elevation, which is another way of saying “rough ness of the bench surfaces/’ is due in part to lithologic variation, in part to varied inclination of tuff beds or lava-flows, and in part to jointing. The smoothest bench surface is developed on tuff, a rock more free of joints than most. Absence of joints and the granular texture favor smoothness. Benches cut on basalt flows are commonly rougher, but vary greatly in this respect. Dense flows that have few joints give smoother

Figure 2.—-Sketch illustrating Structure Relations on normal reversed dip Coasts benches than do flows of the “slag heap” type, formed by excessive shat tering of the rigid lava-crust by the drag of the still fluid interior of the flow. The normal type of shore structure for volcanic islands is such that the flows strike parallel to the shore and dip seaward, and may be called “normal-dip coasts” (see figure 2). “Reversed-dip coasts,” where the strike is parallel to the shore, as before, but the dip is toward the land, are rare, as are also “cross-strike coasts,” in which the dip is parallel to the shore. Cross-strike coasts (figure 3) seem to have rougher bench surfaces than either of the other types. There are also many unexplained irregulari ties. For example, on the Koko Crater coast, Oahu, there is one stretch of compound bench 400 feet long and 50 to 100 feet wide. Four levels, separated by gently sloping scarps, are clearly recognizable. The scarps are from four inches to a foot and a half high. 9 Op. cit.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTION OF THE EUSTATIC BENCH 525 In places where the bench is cut in tuff: it may be so level for a con siderable area that no.point of the tuff surface departs more than four or five inches from a mean level, as indicated by quiet pools of wave-thrown

F i g u r e 3.— Northwest Cape, NecTcer Island View of bench looking southwest along- northwest side. spray. Several such places are to be found on the Koko Crater coast, but the most striking is on Manana (Eabbit) Island, off the east end of Oahu (figure 9).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 526 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC In some instances such level parts of the bench are bordered on one or more sides by slightly higher masses of tuff and the water pools are maintained at a nearly constant level. No experiments nor critical studies have been made to determine the condition of the rock. We sug gest that these “water-level” portions of the bench are due to intensified weathering in the zone extending a few inches above and below the water level of the pool. The combined work of water and air in the zone which is exposed alternately to each seems analogous to the more rapid rotting of poles, piles, and other wood structures at soil or water lines. If this weathering process be operative, any portions of the wave-cut bench now exposed above sealevel, but in reach of dashing spray and suffi ciently inclosed to contain incipient water pools, would become the sites of larger and larger pools, as abrasion followed the “water-level” weather ing. This feature has not been observed on basalt benches and the process is believed to have appreciable effects only on the tuff. Since basalt is almost universally cut by joints, it is rare for pools to be held on the basalt benches: the water drains away readily through the joints. Since there is no local water-table in the basalt, there is no such zone of weathering, and consequently no localization of abrasion by wind and spray. RELATION TO STRUCTURE That the bench is not of structural origin is shown by the fact that it cuts obliquely across inclined flows of lava and beds of tuff. The most striking evidence of this is at the crater of Puu Pehe, at the south end of the Island of Lanai, where the bench cuts across the axis of the crater rim, with its indipping and outdipping layers. Below the 600-foot high cliffs of the cross-strike west coast of Nihoa are a number of fragments of bench. Their level surfaces truncate the lava-flows, which dip about 10 degrees and also cut across several dikes. The dikes strike northeast and continue vertically to the top of the cliff behind the bench. Elschner6 speaks of several levels of terrace, one above the other, on Necker Island. It is true that there are subplane surfaces at higher ele vations than the eustatic bench, but they are structural in origin, being determined by the difference in resistance to weathering of the various lava flows. These surfaces dip with the 10 degrees dip of the flows, whereas the level eustatic bench truncates the flows. It is obvious that most coasts of volcanic islands will be of the normal, or seaward-dipping, type. Eeversed-dip coasts are most facilely ex 6 Carl Elschner: The Leeward Islands of the Hawaiian group, 68 pages, reprinted from Sunday Advertise*', Honolulu, 1915.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTION OF THE EUSTATIC BENCH 5 27 plained as due to faulting, but not necessarily most validly. If faulting is infrequent, reversed-dip coasts should be scarce.. The greatest amount of bench should be found on coasts of normal dip, because that kind of coast is most common. It is to be noted that when a tuff cone is so re duced that it becomes a crescent with the erstwhile crater forming the concave side, both coasts will be of the normal dipping type, because the layers of the inner part of the crater rinj dips toward the center. In the Hawaiian chain of islands only three' reversed-dip coasts are known: parts of the north coast of East Molokai, the north coast of Nihoa, and the south coast of Xecker. It should be remembered that Nihoa and Necker are comparable to one another in size, lithology, and exposure. The bench is much more poorly developed on the reversed-dip coasts of each of these islands than on the normal-dip coasts. The bench is also poorly developed on the north coast of East Molokai—a coast of reversed dip. Consideration of the manner in which joint blocks bounded by flow bedding are held together on the two types of coast indicates that erosion of reversed-dip coasts proceeds more slowly than erosion of normal-dip coasts, at least so far as plucking by wave suction is con cerned. Figure 2a represents a normal-dip coast. Suppose that the waves have removed block number 1; then block 2 becomes unsupported on the seaward side and especially liable to be removed in turn, thus ex posing block 3, and so on. Figure 2b represents a reversed-dip coast. Suppose the waves have plucked out block 1. Block 2 and those above it will fall immediately, but the other blocks farther inland will be wedged together and will be able to resist plucking rather effectively. A' similar conclusion has been reached by Lake.7 RELATION TO COASTAL EXPOSURE Since northeasterly trade winds prevail in the Hawaiian region, it might seem likely that northeast exposures would control the distribution of the bench. Necker and Nihoa Islands are alike in being isolated and built of flow lavas, and they are of comparable size. On Necker the bench is far better, developed on the windward than on the leeward side; on Nihoa the reverse is true. Kaula, Lehua, and Molokini are of closely ‘comparable size. Kaula is fully exposed to the wind; Lehua is somewhat sheltered. Molokini is decidedly sheltered, as it is only three or four miles down the wind from the big mass of East Maui (Haleakala). All three are fragments of tuff cones. On Kaula and Molokini there is almost continuous bench on the 7 Philip Lake : Physical geography, p. 212. Cambridge, 1919.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 528 EUSTATIC BENCH 03? ISLANDS OF NORTH PACIFIC windward side and none on the leeward side. Similarly, Lehua has bench on much of the windward side, but none on the leeward side. In the case of the three tuff islands, there seems to be a correlation, but with the two lava-flow islands there is no correlation of bench de velopment with exposure to prevailing winds. However, there are not enough examples available to make a statistical study possible. It is evident that configuration of the adjacent submarine surface, with its effect on waves and' currents and accumulation of debris, and the details of prevailing wind movements must be closely studied before con clusions are reached as to relative rigor of abrasive processes. Another consideration is that in those situations where the bench was extensively developed during' the pre-shift stand of the sea a similar strong wave action is likely to have destroyed the bench before the present time. Such preponderant tendency to destruction is believed to be favored by the following factors : 1. Strong wave action develops a bench at greater depths than weak. Such a bench might even be low enough so as not to have emerged at the time of shift and hence have been far more quickly removed than a higher bench. 2. The linear rate of marine transgression is progressively reduced as the bench becomes wider. The capacity of strong wave action to cut away an existing bench and reach the same stage of slow attack on a high cliff is thought to be greater than that of weaker wave action, each con sidered with respect to a bench cut by its own attack. This factor would favor the more general preservation of benches in the sheltered situations.

OTHER EVIDENCES OF RECENT STRANDLINE SHIFT Emergence of the land, either by rise of land or lowering of water level, is shown by three classes of evidence. The first class comprises the ex posure, at elevations now well above sealevel, of features cut by marine action, notably by wave erosion, such as the bench under discussion, sea cliffs, caves, and stacks. The second class includes similar exposure of features due to marine deposition, such as bars, offshore beaches, and coralline reefs. The third class comprises drainage changes due to changes of baselevel. V o drainage changes which could be correlated with the shift under discussion have been noted in the Hawaiian region, perhaps because of the shortness and steepness of the stream courses. Numerous valley mouths have been drowned, which indicates a shift in the other direction, evidently before the emergence of the eustatic bench. There is invariably a pronounced sea-cliff behind the eustatic bench of

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTION OF THE EUSTATIC BENCH 5 2 9 the Hawaiian region, except where tiny islets have been completely trun cated by the bench. Associated with the eustatic bench are sea-caves and arches in situations now a little too high to be effectively attacked. In a number of places are stacks which are separated from the mainland by a considerable expanse of more or less continuous bench (see figures 13 and 14). Many of the valleys of Oahu have deep alluvial fills, so steep that they may better be called constricted alluvial fans than floodplains. Many of these fans terminate in scarps 5 to 15 feet high and as steep as the angle of repose of the coarse, unconsolidated material of which they are made. The drowning of valley mouths indicates that formerly Oahu stood several hundred feet higher than it does now, as did others of the islands. Presumably the alluvial fill was deposited during the time of high position. Subsequently the sea rose higher than its present level and stood there long enough for the scarps to be cut. Though the mate rial of the alluvial fill is now weak and rotten in large part because of deep weathering, it is probable that it was then fresher and withstood the attack of the relatively weak waves of the narrow estuaries. The time involved seems comparable to the time involved in cutting the eustatic bench in tuff or lava. On the south coast of Oahu, from Makapuu to Barber’s Point, there is a great deal of raised reef. The maximum elevation of undoubted reef is about 35 feet, and a considerable part of the reef formations of Oahir is too high to be associated definitely with the eustatic shift under discus sion. Material which has been described by laymen as reef at higher elevations than 35 feet has always on examination turned out to be either lower than was thought or to be composed of either wind-blown calcareous sand or of excessive accumulations of calcium carbonate by percolating water. Oahu has a better development of both raised reef and living reef than any of the other islands of the Hawaiian group. In at least one place on the Koko Head coast of Oahu is a mass of coral and other calcareous debris which appears to have been deposited during a higher sealevel and in association with the cutting of the bench. The four southern Line Islands have raised formations of reef or detrital limestone which reach a maximum elevation of about 12 feet above sealevel. Since at the present time similar reefs are being built in situations where they are awash at low tide, it appears that a shift in sealevel of about 12 feet is indicated, and these probably give the most accurate single measure available.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 5 30 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC

CAUSES OF THE STRANDLINE SHIFT Changes in the relative elevation of land and sea of the sort in ques tion may be due to uplift of the land, to withdrawal of the sea, or both. For any restricted locality it is usually impossible to determine which has taken place. The emergence indicated by the features described above is believed to be due to a lowering of the level of the sea. Uniform uplift without appreciable tilting of an area of three million square miles, such as that represented by the Leeward, Hawaiian, and Line Islands, seems highly improbable. Furthermore, the interpretation of eustatic shift falls in line with a growing mass of evidence for similar lowering of sealevel along all the coasts of the world, as pointed out by Daly and others. ' No evidence is afforded by the field relations in Hawaii bearing on the cause of such a lowering of sealevel; and the larger problem of such world-wide oscillations of sealevel lies beyond the scope of the present descriptive paper.

AGE OF THE SHIFT No means is at hand for fixing the date of the shift with great accuracy. The emergence postdates the latest secondary eruptions on Oahu, for the youngest tuff craters in the Koko region carry the exposed wave cut bench around their margins. These craters are enormously younger than the Salt Lake craters, which are associated with terraces and other physiographic features believed to have been formed in Pleistocene time. The later Koko region craters are estimated to be not over twenty thou sand years old and may be no more than five, or possibly even one, thou sand years old, and an upper limit is thus set for the age of the eustatic shift. Certain coasts on Hawaii formed of prehistoric flows from Kilauea and Mauna Loa do not show the bench and the shift thus appears to be definitely prehistoric.

t a b u l a t i o n o f t h e i s l a n d s The following table indicates the islands visited by each of the authors: Visited by Visited by Island. Wentworth. Palmer. Gardner ...... French Frigates Shoal...... Johnston ...... Necker ...... a Nihoa ...... * ICaula ...... Nlihau ...... Lehua ...... Kauai ...... * Oahu ...... * * Molokai ...... * * Maui ...... * * Lanai ...... * $ Kahoolawe ...... •...... * Molokini ...... Hawaii ...... * * Palmyra ...... * Washington ...... * Fanning ...... * Christmas ...... * Jarvis ...... * Howland ...... Baker ......

GARDNER ISLAND Gardner (Gardiner) Island is a small rocky islet in latitude 25° 01' north, longitude 167° 59' west, with an elevation of 170 feet. A smaller rock is close to its southwesterly shore. A reef extends about one-half mile from the smaller rock. A bank, with 17 to 20 fathoms, extends about 5 miles northwest, northeast, and southeast and 10 to 12 miles southwest.8 Rock specimens collected on Gardner by the members of Trip B of the 1923 Tanager Expedition are all basalt. Landing was said to be difficult. The fact that landing was at all possible suggests the existence of some of the bench on this island. A photograph in the Bishop Museum col lection (number 10094) is blurred by movement of the small boat from which it was taken, but suggests the presence of a bench. A bench was s Coast Pilot notes on Hawaiian Islands. IT. S. C. & G. S;. 2 ed.. 1030, p. XXXV—Bull. Geol. Soo. Am., Vol. 36, 1924

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 5 32 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC noted by Dr. A. Wetmore, of the United States Biological Survey, who was a member of the same party. He reports that it was best developed on the northeast side of the island.

LA PEROU SE ROOK, FRENCH FRIGATES SHOAL French Frigates Shoal is a crescent-shaped atoll, formed by two arcs of reef of different curvature. In the area included by the two reefs are several small, low sand islands. La Perouse Eock lies midway between the ends of the crescent and to leeward of the leeward and less sharply curving line of reef. It is 350 feet long from northwest to southeast, 40 to 80 feet wide, and 120 feet high. It is composed solely of basalt flows which dip gently to the northwest. 'Landing was made on a small fragment of bench on the southwest side of the rock by the" members of Trip C of the 1923 Tanager Expedition. The maximum width is 10 or 12 feet and it extends some 60 feet. It is 5 to 8 feet above sealevel. Mr. A. E. Arledge, formerly Superintendent of the Hawaiian Lighthouse District, said that he landed at the northwest end of the rock on the bench and traversed the bench for half the length of the northeast side, to a point where he was able to ascend to the summit. It is possible that he landed on a structural bench developed on a particularly resistant flow and followed thé surface of this flow. Two hundred feet northwest of this islet is a still smaller one, with a flat top 30 by 70 feet and 8 feet high. This looks to be a bit of high bench, but it may be of structural origin. The bench on the southwest side of the main islet seems not to be structural, as a flow surface may be followed upward from the bench.

JOHNSTON ISLAND Indurated calcareous conglomerate and sandstone lie 4 feet above mean tide on Johnston. No undoubted emerged reef rock was seen during a four hours’ visit ashore.

NECKER ISLAND Necker Island has a maximum height of 276 feet and an area of 41 acres. It is about 4,000 feet long east and west and averages 400 or 500 feet in width. From the west end of the island a peninsula, “Northwest Cape,” fextends north-northeast, giving the island the shape of an “L.” A bank surrounds the island, giving depths of 14 fathoms and upward for 5 to 30 miles distance. The rocks arc all basaltic. Tie flows dip 11 degrees northeast and strike north 85 degrees west. A number of vertical dikes bear about northeast-southwest. Some of the flows and

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTIONS OF THE ISLANDS AND THEIR BENCHES 5 38 dikes contain megascopic olivine and in a few augite can be seen. There are no pyroclastic rocks. In the bay between Northwest Cape and the main branch of the island there is considerable actively growing reef, which partly breaks the force of the waves on this side. A wave-cut bench extends along most of botli the east and west shores (figure 3) of Northwest Cape and adjacent shores of the main island and was noted in several patches on other parts of the shore. Along the shores of the more protected bay at the north the bench is about 4 feet above sealevel, whereas it is about .8 feet high on the exposed west end of the main island.

NIHOA ISLAND Nihoa Island (also called Modu Manu, Moku Manu, and Bird Island) has an area of 127 acres and a maximum elevation (Millers Peak) of 895 feet. The west, north, east, and southeast sides are nearly vertical, large parts being 600 feet or more high. These shores are also nearly straight. The water parting is everywhere near these precipices, so that 90 per cent of the surface drains southward by seven valleys, with slopes that average 25 degrees. The south shore is formed by Adams Bay, which is of rather irregular outline, due to the alternation of valleys and spurs. The extreme length of the island is 4,800 feet and the average width about 1,400 feet. The rocks are all basaltic. Numerous dikes bearing north 60 degrees east cut the flows, which strike north 55 degrees west and dip 10 degrees southeast. No pyroclastic rocks were seen, though a lookout was kept for them. Nihoa is on a bank with 20 to 40 fathoms of water. In general the shores of Nihoa either have a distinct, undoubted bench four to eight feet above sealevel or the bench is entirely lacking. The bench extends along most of the shores oh the south, or Adams Bay, side, but is found only as short stretches on the other sides of the island.

KAVLA ISLET Kaula Islet, or Kaula Rock, is a fragment of the rim of an extinct tuff crater. It is about 2,000 feet -long and its height is probably about 550 feet. As seen from the bridge of the Tanager during a circumnaviga tion, the islet appears to consist solely of tuff, which is in all probability of basaltic composition. A photograph by the Naval Air Force shows the bench extending along more than half of the inner curve of the crescent. Another photograph of the outer curve shows no bench. Deep, reefless waters surround Kaula.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 5 34 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC Mr. E. E. Tinkham, Superintendent of the Hawaiian Lighthouse Dis trict, stated that the bench extends the full length of the inner curve and supported his statement with excellent photographs. NII11AU Until recently, little information has been available concerning Niihau because of the policy of the owners of the island. Mr. Charles Gay, who lived for many years on Niihau, states that much of the southeast coast is precipitous, but that on the gentler northwest coast and on the south west end of the island there are numerous fragments of the bench cut in flow lavas. Similar testimony comes from Mr. is. E. A. Hinds,9 who recently studied the geology of Kauai and Mihau. He finds the bench clearly developed on Xiihau, but the evidence of emergence is less abun dant than on Kauai. LEIIVA Lehua is a remnant of a small, parasitic crater at the north end of Xiihau. According to the Lighthouse Service, half of the crater rim has been destroyed and the north half of the windward side, which is also the concave side, has a well-developed bench, as is usual on tuff shores. The photographs at the Lighthouse Service office show the bench un- equivocably. It cuts across ash beds that dip seaward about 25 degrees and appears to be 25 to 100 feet wide. At the back of the bench, is the appropriate wave-cut cliff. KAUAI On a brief visit to Kauai in 1924, the bench was observed at a number of points on the north coast west of Hanalei Bay and at Kawai Point, south of Nawiliwili Bay, on the east coast. The same feature is found rather broadly developed at the “Spouting Horn.” east of Lawai Bay, on the south coast. Hinds10 found abundant evidence of emergence on Kauai, which he plans to describe in detail'in his forthcoming report. The amount of emergence indicated by the bench and other features agrees with that found by the writers on other islands. OAHU On the island of Oahu the bench is best developed in the southeast por tion, extending along the coast from Pearl Harbor eastward to Makapuu Head and thence northwestward to Mokapu Peninsula. In the vicinity

9 X. E. A. Hinds : Private communication. 10 X. E. A. Hinds : Private communication.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTIONS OF TIIE ISLANDS AND TllEIR BENCHES 535 of Pearl Harbor and the Salt Lake tufî craters are a number of localities where low sea-cliffs have been cut in positions slightly higher than are now accessible to the waves. Certain coral formations in this vicinity now somewhat above sealevel probably date from the same time as the cliffs, though other much higher and older emerged reefs are also found in the region. Fragments of bench partially mantled by beach sediments are found along the seaward margin of Diamond Head and the same are imperfectly shown on Black Point. Round the entire margin of the Koko Head Peninsula it is exceptionally well developed in the tuff of which the peninsula is exclusively composed (figure 4). The best single

F ig u re 4.— View of eustatic Bench on Coast of Koko Head, Oahu expanse is found round the shore of Hanauma Bay, where the bench is broken in but two or three places in over a mile, is from 5 to 10 feet above sealevel, and ranges from 10 to 50 feet wide (see figures 5 to 7). At least half of the perimeter of Manana Island just north of Makapuu Head is marked by an emerged wave-cut bench ranging from 5 to 15 feet above sealevel. The east shore of this island is for the most part very precipitous, with deep water at the foot of the cliff. At the site of a former crater throat the concentrically dipping beds have been trun cated by a superb bench 150 feet wide and 400 feet long. The structure has apparently favored deeper cutting inland and the inclosure of a spray-fed pool which has now extended itself by water-level weathering

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 536 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC

F igure 6.—Near View of Bench on east Shore of Hanauma Bay, Oahu The bench is here cut across the edges of tuff beds, dipping 20 to 25 degrees.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTIONS OF THE ISLANDS AND THEIR BENCHES 537

F igure 7.—Small Cove in Bench on west Side of Hanauma Bay, Oahu Tliis level floor has been produced in part by “water-level” weathering, as described in the text.

F igure 8.— View looking eastward along south Coast of Manana Island, Oahu The bench here shows far more vigorous erosion and less etching due to weathering than those in the Hanauma Bay region.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 5 3 8 BTJSTATIC BENCH OF ISLANDS OF NORTH PACIFIC

F ig u re 9.— View of “water-level” Pool on Bench on the cast Coast of Manana Island This patch of bench is developed across the concentric indipping tuff beds of one of the volcanic vents responsible for the building of the island and is a remarkable example of wave-work in a locally favorable situation.

F ig u re 10.—View of icest Coast of Lanai The view is taken looking north from a point south of Knumalapau Harbor. Eustatic bench is shown broadly developed at the foot of the cliffs in the middle distance.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTIONS OP THE ISLANDS AND TIIEIR BENCHES 5 39 over the whole area of the bench. Probably less than 2 per cent of the area differs from the mean level by over a foot (figures 8 and 9). The bench is clearly developed on the more protected parts of ITIupau crater, whereas the exposed parts have steep cliffs flanked by deeper water. A fine bench may be seen surrounding Moku Mann Islands one mile north of Ulupau, but these have not been visited. The bench has been identified on the coast northeast of Waialua, near Makaha, and southeast of Nanakuli. Truncated alluvial deposits along many parts of the wind ward coast and west of Waialua also indicate wave-work at a slightly higher level than is possible at present. MOLOKAI The bench is fairly well developed on all parts of the Molokai shore which were visited. These include the westernmost ten miles of the south coast, short stretches of the west and north coasts as far around as Waikolu and the vicinity of Kaunakakai, Kamalo, Pukoo, and Halawa. The north coast from Halawa to Waikolu was skirted in a small boat. Only small fragments of bench are found at places flanking the high cliff of this coast. Much of the south coast of Molokai now consists of a nar row alluvial plain, less than 10 feet above sealevel, which extends inland along the gulch channels and merges with the stream grades. Between the streams the sloping spurs are commonly truncated by old sea-cliffs 10 to 50 feet in height, and the eustatie bench which is believed to be continuous is veneered by the alluvium of the plain. Toward the west end the alluvial plain becomes narrower, and well defined wave-cut benches were observed at a number of places at elevations of 6 to 10 feet above sealevel. Considerable masses of calcareous sandstone were noted in situations too high to have been formed under existing conditions. MALI The bench has been observed at a number of places on the shores of West Maui and on headlands west of Maliko Bay on East Maui. Bench is developed at a number of places on the south shore of West Maui, from MacGregor’s Landing to Lahaina, and in a few places it truncates lava- flows that dip about 15 degrees seaward (figures 11 and 12). L A X AT The Island of Lanai shows on its west and southwest coasts one of the best examples of bench cut in lava rock seen by the writers. It is prob able that this impression is due in part to the somewhat detailed examina-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 540 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC

F igure 11.— View of Bench looking west past Mouth of Maliko Gulch, East Maui In the distance are the slopes of West Maul. Bench is cut In basalt.

F igdbe 12.— Sea-cliffs and Bench between Waihee and KahaTeuloa View is on northeast side of West Maui. The rock is basalt.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTIONS OB’ THE ISLANDS AND THEIR BENCHES 541 tiun made of this island, and that similar conditions will be found else where by students making more extended studies on other islands of the group. From the northwest point of the island to Kaumalapau Harbor (figure 10) and along much of the south coast east and west of Manele is a succession of alternating bays and headlands. The wave-cut bench is present on the majority of the headlands at 8 to 10 feet abov<3 sealevel and has associated with it a fine series of sea-caves and stacks produced at the time of higher sealevel (see figures 13 and 14). The bays are commonly fringed by coarse gravel beaches merging inland to poorly assorted alluvium from the gulches. Commonly the bench is not over 10 or 15 feet wide, but in places the gently dipping lava flows have been truncated by the abrasive action of the waves over areas 100 or more feet in width and twice as long.

F igure 13.—Eustatic Bench surrounding icave-cnt Stacks The location is on the west coast of Lanai, south of Honopu Bay. The stacks are known locally as the “Five Sisters.” South of Kaumalapau Harbor is a straight section of coast consisting of an almost vertical cliff ranging from 200 to 1,000 feet in height. This cliff is believed to be due to faulting. No good bench is known along it, though a boulder beach 10 or 20 feet wide lies at its foot and is ac cordant with present sealevel. The -north, northeast, and east coasts of Lanai are low and formed largely of recent alluvial and marine sediments. Detrital limestone formations of marine, origin extend to 10 feet above sealevel at a few points, and some of the spurs are truncated by old wave-cut cliffs some what back from the present shore. KAHOOLAWE A. well defined wave-cut bench about 10 feet above sealevel was seen at the boat landing near the east end of the north coast of Kahoolawe.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC Similar benches were seen on adjacent parts of the coast from the boat. Fragments of bench here and there around the margin of the deep in dentation on the south coast were seen from the highland to the north. MOLOKINI Molokini lies between East Maui and Kahoolawe and is a fragment of a tuff crater rim. The concave side faces west-southwest, according to the Lighthouse Service, though the Coast Pilot says it faces northwest. Statements by the Lighthouse Service officials, supported by excellent

This shows a wave-cut stack surrounded by eustatic bench, near Manele, on the south coast of Lanai. photographs, indicate that bench is well developed along the whole of the convex, or windward, side of the island. HAWAII No detailed study of the Hawaii coast has been made by either of the authors. When passing the east coast of Hawaii north of Hilo the bench was seen at various points, but the precise localities were not recorded. The peninsula pierced by Onomea Arch has at its, outer end a distinct bench which extends inland with progressively lesser widths. Messrs. Dranga and Thaanum11 report that there is no bench on the coasts formed by Mauna Loa or Kilauea lavas, except possibly at Ka Lae, the southern 11 Oral communication.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 DESCRIPTIONS OF THE ISLANDS AND THEIR BENCHES 5 43 point of the island. According to them, the bench is common along the Mauna Kea coasts, but they are not informed concerning the Kohala coast. There is uncertainty concerning the Hualalai coast. It seems probable that appreciable volcanic activity from Mauna Ivea and Kohala ceased before the time the bench was cut, but that Mauna Loa and Kilauea certainly, and Ilualalai possibly, have covered the bench, suppos ing it to have existed, with more recent lavas. PALMYRA !No evidence of emergence was found on this island, and it is believed that it has been developed wholly since the shift of sealevel took place, and would therefore not be expected to carry a bench or emerged reefs or detrital formations. WASJIINOTOY Emerged masses of reef rock were seen at a few places on the shores of Washington Island, but none was observed at elevations clearly indicating emergence as great in amount as that shown on some of the other islands. It is believed, however, that such would be revealed by sufficiently detailed study, sincc the general elevation of the island and the present complete inclosure and freshened condition of the lagoon indicate very decisive shift of the strandline. The fresh-water lake and peat bog now occupy ing the site of the former lagoon have been described elsewhere.12

F A Y \ 7 V<7 .Vo emerged masses of undoubted reef rock in place were seen during two one-day visits to Fanning Island. At a number of places detrital limestone formations were observed at 3 or 4 feet above sealevel, which appeared, to indicate emergence, and it seems probable that Fanning has experienced a history similar to that of adjacent islands. CHRISTMAS, Great masses of deeply weathered coral reef rock and detrital lime stone are found at a number of places in the interior of Christmas Island at an elevation of about 10 or 11 feet above sealevel. The ancient lagoon has been converted by withdrawal of the sea into a large number of in tensely salt and very shallow lakes. The irregular sporadic distribution of these water bodies can not be satisfactorily explained otherwise than as the residual pools of the once continuous and deeper central lagoon.

12 Chester K. Wentworth: Geology of the Line Islands. (In press.) Bishop Museum, x92o.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021 544 EUSTATIC BENCH OF ISLANDS OF NORTH PACIFIC The margins of the old lagoon are rather clearly indicated in the line between the lake-studded flat and the slightly higher rimland of the island. JARVIS Jarvis Island is the only one of the Line Group which has a “dry lagoon” and furnishes clear topographic evidence of emergence. Addi tional indication of such a shift of the strandline is contained in the masses of emerged reef rock and detrital limestone which are found at a number of places on the island. No opportunity to measure the exact amount of emergence was found on Jarvis Island.

HOWLAND AND BAKER We are indebted to Dr. T. A. Jaggar,13 of the Hawaiian Volcano Ob servatory, for the following information concerning these islands: The west, or leeward, sides are about 20 feet high and the east side of each about half as high. Old cemented coral conglomerate is found near the top of each. So far as can be judged from Dr. Jaggar’s description and without personal observation, there is a greater similarity between these two islands and Jarvis than with any others of the Line Group and, as pointed out by Jaggar, clear evidence of emergence amounting to somewhat less than 20 feet.

18 T. A. Jaggar: Personal communication.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/36/3/521/3429964/BUL36_3-0521.pdf by guest on 30 September 2021