Chester K. Wentworth

Soil avalanches on Oahu, Hawaii

A large part of Oahu above 1000 feet in elevation is very rugged, with steep slopes from 35 to 60 degrees. In the rainier parts, though widely distributed, bare rock cliffs are usually not of great lateral extent and occupy but a small part of the map area. Nearly the whole area is covered with soil or rock so weathered as to support a dense cover of vegetation. By incessant creep and colluvial wash, this mantle rock assumes slopes at the limit of stability for normal conditions. In exceptionally wet weather, owing to weight of soil water and the lubrication it affords, patches of soil together with vegetation become loosened and pass downslope as soil avalanches. The angles of slope are chiefly from 40 to 50 degrees, and the thickness of material involved is rarely more than 2 or 3 feet. The avalanches play an important part in the succession of plants, but the details of this phase have not been studied. They are also thought to be important in undercutting the crests of ridges so as to develop the striking knife-edge ridges so characteristic. No close measurement of the rate of erosion by these slides is practicable, but they apparently are responsible for a relatively rapid denudation and they are probably the controlling factor in determining the general form of land sculpture in the mountainous parts of this region.

KOOLAU BASALT SERIES, OAHU, HAWAII

The Koolau volcanic dome forms the eastern and larger part of the island of Oahu. It rises to somewhat over 3000 feet above present sea level and is composed almost entirely of thin basaltic lava flows. The visible flows were erupted from vents along a fissure system more than 30 miles in length and trending parallel to the main axis of the Hawaiian archipelago. The fissure system is marked by swarms of subparallel dikes in a band 2 to 5 miles wide. The rock formation marked by the dikes has been called a dike complex. In the southern half of the dome the crest and fissure portion has been cut away by erosion from the east, but the forms and structures which remain indicate that the dome at present sea level was never more than about 20 miles wide and that its form was greatly elongate, consistent with its growth about a linear vent system. Indications that faulting took a significant part in the reduction of the southern part of the windward slope are suggestive, but no direct proof has yet been found. The remainder of the dome has been profoundly dissected to the stage of mid-maturity though enough remains to everywhere indicate its general original form. Following the period of most pronounced erosion there was a series of late eruptions both around the margins and in the interior of the southeastern part of the dome, giving rise to lava flows and pyroclastic cones of ultrabasic composition named the Honolulu series as described by Winchell (1946). Presence of these late rocks in various forms, distributed about Honolulu has attracted petrographic attention to the neglect of the main Koolau series. Following extensive field study and cutting of many hundreds of thin sections in connnection with water-supply studies, nine specimens have been analyzed with particular reference to the problem of variation. No indication of significant or systematic variation in chemical composition has been found, despite the collection of samples from widely separated points and from various depths in the dome. The Koolau basalts are slightly more silicic than most of the “normal Hawaiian basalt” averages heretofore known. Attempts have been made to derive the Koolau composition on various hypotheses, but none having independent basis of plausibility has been developed.

General Features and Glacial Geology of Mauna Kea, Hawaii

INTRODUCTION The four volcanic masses whose coalescing bases form the island of Hawaii are substantially alike in petrographie composition; all of them bear cinder cones on their flanks, and lava flows from all of them have extended far into the sea, and also inland to join their neighbors. As regards their topographic expression, they form two groups. Mauna Loa and Hualalai have remarkably flat, smooth profiles and summit craters; Mauna Kea and Kohala are rugged masses sharply incised by canyons and have no topographic feature to indicate the major source of the materials of which they are constructed. The two giant domes, Mauna Loa and Mauna Kea, reveal these differences clearly. The profile of Mauna Loa extends from the coast as an almost unbroken curve with a gradient averaging about 600 feet to the mile nearly to the rim of the active crater, Mokuaweoweo, at an altitude of 13,680 feet. 1 . . .

Multiple glaciation of Mauna Kea, Hawaii

Mauna Kea, Hawaii (13,784 feet), was glaciated four times during a period presumably correlative with the ice age elsewhere. Snow mantles Mauna Kea during winter but banks rarely survive the summer. Evidence is abundant not only for the latest glacial stage in the summit area but also for three earlier glacial advances in the zone outside and below the youngest moraine. The four indicated stages have been named, beginning with the latest, the Makanaka, Waihu, Pohakuloa, and pre-Pohakuloa stages. Distinguishing features of the several drifts are: Stratigraphic position. The three earlier ones lie under successive series of late lava flows. Moderate weathering and surface staining (to brown) of the older drifts. Climate has probably been periglacial throughout Pleistocene and recent time. Lithologic differences due to derivation from different series of surface lavas. Matrices of the older drifts are partly tuffaceous indicating contemporary volcanism. In places the oldest drifts are well-indurated tillite. The drifts are dominantly boulder beds, much water-washed. Boulders are somewhat faceted but only faintly striated. Striated pavements are known under the older drifts, but glacial erosion was generally feeble. Glaciers in the pre-Pohakuloa stage descended to about 7000 feet but only to 10,200 feet in the latest stage. Interglacial processes other than extrusive vulcanism are little known. Climate is now subarctic above the timber line; and significant soils probably were never developed.

Geology of Ulupau Head, Oahu

Ulupau Head is a secondary tuff crater forming the eastern salient of Mokapu Peninsula, northeast coast of Oahu, Hawaii. Subsequent to its building from an offshore, submarine vent, the crater was deeply eroded by wave action on the north and east sides, the rim being largely destroyed on the east, and the sea entering the bowl. Later, at a stand of the sea at least 45 feet above the present stand, a bed of oyster-bearing limestone and an overlying calcareous beach sandstone were deposited on steeply sloping rock surfaces inside the crater. Unconformable on these beds is a thick series of alluvial, tuffaceous silts, and in one sector a remarkable giant breccia, carrying 30-foot blocks of tuff and probably due to an enormous rock fall from an oversteepened marine cliff. At this time the sea stood at least as low as at present. Later reef formations and accompanying beach conglomerates indicate two more stands of the sea, the first at 25 feet and the second at about 12 feet above the present. Incorporated in the reef limestone of the northwest shore are great blocks of tuff let down by marine erosion from the rock-fall breccia and forming a remarkable reef breccia. This in turn, subject to attack by water-level weathering on the tuff blocks and solution benching on the reef limestone matrix, furnishes a striking example of the contrasted features of marine benches developed by processes peculiar to the two types of rock.

Dinwoody Glaciers, Wind River Mountains, Wyoming; with a Brief Survey of Existing Glaciers in the United States

Introduction Of the large number of active glaciers present in the higher mountains of the western United States few are several miles in length, forming parts of rather extensive glacier systems, but most of them are small—not more than 2 or 3 square miles in surface area—and but little, if any, greater in length than in width. Probably most of them are essentially cliff glaciers,[2][1] and move only very short distances down from the basin of snow accumulation. Some of the larger systems, such as those of Mount Rainier and Glacier National Park, are now well known, but others, especially the smaller ones in districts which have not been geologically mapped, are but little known except to local hunters and sportsmen. Those of the Wind River Range, including the Dinwoody glaciers here described, have been known for many years, and they are shown on the Fremont Peak topographic . . . [1]: #fn-1

Striated Cobbles in Southern States

Introduction The occurrence of faceted and striated cobbles and boulders in the drift of the northern United States and Canada has been known since the earliest geological observations in America, and their glacial origin has been recognized from the time of Louis Agassiz, nearly a century ago. These scratched boulders, along with various other lithologic characteristics of the till, have been generally regarded as convincing evidence of glaciation. In some instances identification of pre-Pleistocene epochs of glaciation has rested largely, if not wholly, on the presence of such boulders. In several parts of the United States, however, striated cobbles have been found far south of the known limit of any Pleistocene invasion, in the valleys of streams that head in glaciated territory. The abundance of these cobbles shows that during glacial time these streams carried enormous loads of coarse debris, and from the large size of some of the blocks . . .

Eustatic Bench of Islands of the North Pacific

Introduction Daly 2 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 sea-level 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 . . .