Archaeological stone of Easter Island

With the scarcity of other resources, stone assumed great importance in the culture of Easter Island. The volcanic rocks display a compositional continuum paralleled by changes in physical characteristics. The most abundant rock type, porphyritic hawaiite, was the least useful as it is poorly jointed and difficult to work. Mugearites and benmoreites usually have a flaggy structure and were used as dressed stone in the early ahu, in the houses of Orongo and in stone implements. Rhyolitic obsidian was used for scrapers, knives, and a variety of weapons, especially the tanged mataa. Statue carving reached its zenith on Easter Island largely because of the availability of a suitable rock type, the Rano Raraku tuff. The tuff was not erupted from the present Rano Raraku crater but from another vent southeast of the surviving portion of the cone. Red scoria from Puna Pau was quarried for the topknots. © 1993 John Wiley & Sons, Inc.     

Petrology and geochemistry of Easter Island

Easter Island has developed around three volcanoes—Poike, an older (3 m.y.) strato-volcano, Rano Kau, a caldera, and the fissure complex of Terevaka and its associated cones. The lavas show a wide compositional spread from tholeiites and olivine tholeiites to hawaiites, mugearites, benmoreites, trachytes and rhyolites (comendites). Hawaiite is by far the most abundant rock type and trachytes and rhyolites are relatively rare. Intermediate and acid rocks are concentrated in the southwestern part of the island on or around Rano Kau.The basaltic rocks, which are plagioclase-phyric or aphyric, are transitional hypersthenenormative types characterized by high contents of Fe, Ti and Zr but low K and Mg. The Poike basalts are marginally lower in Zr, Nb, Y and Zn compared with those of the younger volcanoes, but the trachytes from this centre show anomalously high concentrations of Rb, Zr and Nb.The island's youngest flow, the Roiho basalt, is an olivine tholeiite with distinctly more alkaline affinities: it is olivine-microphyric with relatively high contents of Mg, Ni and K.The study was initiated whilst this author was at Department of Geology and Mineralogy, University of Oxford.     

Drought, vegetation change, and human history on Rapa Nui (Isla de Pascua, Easter Island)

Stratigraphic records from lake sediment cores and slope deposits on Rapa Nui document prehistoric human impacts and natural environmental changes. A hiatus in sedimentation in Rano Raraku suggests that this lake basin dried out sometime after 4090-4410 cal yr BP and refilled only decades to centuries before AD 1180-1290. Widespread ecosystem changes caused by forest clearance by Polynesian farmers began shortly after the end of this drought. Terrestrial sections show a chronology of burning and soil erosion similar to the lake cores. Although changing sediment types and shifts in the pollen rain suggest that droughts occurred earlier in the Holocene, as yet there is no evidence for droughts occurring after AD 1180-1290. The timing of the agricultural colonization of Rapa Nui now seems well established at ca. AD 1200 and it was accompanied by rapid deforestation that was probably exacerbated by the island's small size, its droughty climate, and the rarity of primeval fires. Detailed records of a large interval of Rapa Nui's ecological history remain elusive due to the drought hiatus in the Rano Raraku sediment record. We find no evidence for a “rat outbreak impact” on Rapa Nui's vegetation preceding anthropogenic forest clearance.     

Plate tectonic significance of Late Oligocene/Early Miocene deep sea sedimentation at Maewo, Vanuatu (New Hebrides)

Eight lithofacies representing a westward trending, deep sea fan, dominantly deposited from mass flow mechanisms, are recognised in geologic sections in the lower part of the Sarava Formation, of Late Oligocene/Early Miocene age, on Maewo Island, Vanuatu, New Hebrides. Also present representing the floor on which the deep sea fan prograded are non-calcareous, red siltstone and minor green siltstone which indicate deposition beyond the calcareous compensation depth, i.e. a depth greater than 4.25 km, and rare thin airfall ash.Previous workers proposed that rifting occurred in the area now occupied by Maewo during the Mid Miocene. However, the great depth at which the Late Oligocene/Early Miocene strata were deposited suggests that rifting occurred prior to the Late Oligocene. Rifting may have occurred even earlier because Pentecost Island, which lies south of Maewo, has a dismembered ophiolite suite which ranges in age from 35-28 Ma (Oligocene). The ophiolite suite may have formed in an interarc environment.The writer's reconstruction of the Oligocene arc system of the New Hebrides is an analogue of the present day Mariana Arc System. Interarc rifting ceased by the Early Miocene and during the Mid-Late Miocene the subduction of zone may have migrated westwards to lie along the Maewo-Pentecost axis.     

Late Miocene fluvial sediment transport from the southern Appalachian Mountains to southern Florida: An example of an old mountain belt sediment production surge

Past geomorphological models assume that erosion of sediments from old mountain belts occurred at a relatively constant rate, based on comparatively uniform isostatic adjustment caused by unloading. Late Miocene strata of the south‐eastern United States provide an example of pulsed tectonism resulting in a surge in siliciclastic sediment production and transport. Regional tectonism (uplift of the southern Appalachian Mountains) and climatic conditions during the Late Miocene resulted in the long‐distance (up to 1000 km) fluvial transport of coarse siliciclastic sediments onto a stable carbonate platform in southern Florida. The sediments are unusual in that they are significantly coarser than marine‐transported sands in southern Florida, with discoidal quartz and quartzite clasts up to 40 mm in diameter locally present, and have relatively high potassium feldspar contents (up to 16% in some sample fractions), whereas feldspar is rare in modern Florida beach sands. It is suggested that previously documented rejuvenation of the southern Appalachian Mountains during the Middle to Late Miocene time, coupled with the Messenian sea‐level low, generated the increased rate of sediment production and necessary hydraulic gradient to allow rapid transport of coarse sediments. Tectonic influence on the river pathway in Florida, as well as in the southern Appalachian Mountains, may have maintained the river on the narrow carbonate platform. The Florida Platform during the Late Miocene must also have had a sufficiently wet climate to cause episodic transport of the coarse sediments. Siliciclastic sediment transport on the Florida Platform during the Late Miocene greatly differed from Pleistocene to modern conditions, which are dominated by the transport of fine‐grained sands by longshore marine processes.     

A Late Cretaceous pole for the Pacific plate: implications for apparent and true polar wander and the drift of hotspots

We present a Late Cretaceous (81 Ma) pole position for the Pacific plate derived from paleomagnetic analyses of basalt samples from Detroit Seamount (of the Hawaiian–Emperor seamounts) that were oriented using Brunhes-age overprints. This pole is at much higher latitudes than the previously published Late Cretaceous pole positions based on the modeling of magnetic anomalies observed during marine surveys over seamounts. Our new pole suggests that the Pacific plate would have moved rapidly between 95 and 81 Ma at speeds as high as 19.8 (−10.8/+11.2) cm/year. The Pacific plate at this time was smaller than the present-day plate and had a substantial subducting boundary. The high-velocity estimates are comparable with those of other paleoplates having similar characteristics. Therefore, plate tectonic driving forces can explain the motion and there is no need to invoke true polar wander. Decreases in mantle drag associated with vigorous Late Cretaceous volcanism in the Pacific, however, may have contributed to the rapid plate speed. The new pole position, together with other reliable paleomagnetic indicators of Pacific apparent polar wander, further supports the notion of drift of the Hawaiian hotspot during the Late Cretaceous.     

多期活动古隆起复合叠加过程解析——以塔里木盆地轮南古隆起为例

中国含油气盆地以叠合盆地为主,经历多期次构造运动,造成盆地内部多期古隆起的复合叠加;目前对于多期活动古隆起复合叠加过程的解析研究较为薄弱。本文以塔里木盆地轮南古隆起为例,利用最新的三维地震数据和钻井资料,应用构造解析和古构造复原的方法,明确轮南古隆起经历寒武纪-早中奥陶世前古隆起、晚奥陶世古隆起形成、晚泥盆-早石炭世叠加改造、晚二叠世-三叠纪古隆起定型、侏罗纪-古近纪古隆起埋藏和新近纪以来构造掀斜6个演化阶段;揭示现今以寒武-奥陶系碳酸盐岩为主体的前中生界隆起是由位于研究区西北侧北东向展布的晚奥陶世古隆起、研究区中部北北东向展布的晚泥盆-早石炭世古隆起和研究区北侧近东西向展布的晚二叠世-三叠纪古隆起三者复合叠加而成;不同时期古隆起成因机制有差异,但都与盆地周缘洋盆闭合、造山作用关系密切。     

鄂尔多斯盆地西部晚三叠世构造属性探讨

有关鄂尔多斯盆地西部晚三叠世的构造属性问题,前人的观点大多认为该时期为(类)前陆盆地,笔者对此提出置疑,主要有3个方面的依据:首先,对西缘汝箕沟、石沟驿和崆峒山地区晚三叠世延长组的3套粗碎屑沉积进行了重新认识。汝箕沟延长组确实为边缘相沉积,但其附近层位板内玄武岩的出现,表明该区晚三叠世为拉张环境下的沉积。最新研究表明,石沟驿地区延长组沉积厚度并不大,不超过1500m。崆峒山砾岩沉积可能受西南部的秦祁造山带影响所致,不能作为盆地西部沉积的代表。其次,通过编制晚三叠世延长组地层等厚图及一系列东西向地层剖面对比,发现该时期以往认为的盆地西部从南至北的巨厚沉降带并不存在。最后,通过地震剖面、平衡剖面和裂变径迹测试数据分析,指出西部现今存在的横山堡后冲构造带和马家滩大型逆冲推覆构造带并未形成于晚三叠世,其最早形成于晚侏罗世。故鄂尔多斯盆地西部晚三叠世构造属性并非为前陆盆地,而是残延克拉通内叠合盆地的组成部分。该认识对盆地西部的油气勘探具有重要意义。     

克拉美丽山南缘西段盆山耦合机制

克拉美丽山位于准噶尔盆地东部,晚古生代克拉美丽洋盆向北俯冲消亡,西伯利亚板块与准噶尔地块在该地区发生碰撞造山。目前,就石炭纪之后克拉美丽山的构造活动存在持续挤压、拉分、伸展、挤压-伸展转换多种观点,构造样式也各不相同。本文应用断层相关褶皱理论,从盆山过渡带现今构造样式入手来探讨克拉美丽山南缘西段盆山耦合机制。研究结果表明,克拉美丽山西段在石炭纪之后经历了中二叠世早期、早三叠世早期、晚三叠世末期、晚侏罗世-早白垩世、晚白垩世早期和古新世末期6 次构造隆升。前4 期相对稳定沉积,晚白垩世早期,晚古生代地层沿着下二叠统底部的泥岩层滑脱面以叠瓦状构造楔样式向南楔入,构造缩短量大于15 km,现今盆山构造样式初步形成。始新世构造楔遭受后期突破断层改造。始新世后,克拉美丽山大规模的构造活动基本停止,地层遭受剥蚀最终形成现今地质结构。     

Cirque glaciers as morphological evidence for a thin Younger Dryas ice sheet in east-central southern Norway

Three localities with marginal moraines deposited by former cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude cirques of central Rondane were still covered by ice when the low-altitude cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the 'first' nunataks appeared. The occurrence of the cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian.     

藏北羌塘盆地上三叠统那底岗日组与下伏地层沉积间断的确立及意义

藏北羌塘盆地上三叠统那底岗日组呈近东西带状展布，下部主要为凝灰岩、凝灰质砂岩夹流纹岩、玄武岩，上部主要为含砾砂岩、泥质粉砂岩和生物碎屑泥灰岩。笔者等野外调查中首次在下伏的上三叠统肖茶卡组顶部发现古风化壳，结合在那底岗日组底部识别出来的不同底界类型和底砾岩特征，证实那底岗日组与下伏地层之间存在沉积间断。早期磁性地层研究中也曾发现了该沉积间断的存在，间断时间被认为约2Ma。运用已有的生物地层资料确定肖茶卡组地层的沉积时间（晚三叠世诺利期一瑞替期），从而推测那底岗日组火山岩形成时代略晚于这个时间，很可能早于前人关于该套地层的时代认识。那底岗日组火山岩可能对研究东特提斯地区的晚三叠世生物绝灭、气候变化以及海退事件有重要意义。     

Implications of Late Pleistocene Glaciation of the Tibetan Plateau for Present-Day Uplift Rates and Gravity Anomalies

Minimal and maximal models of Late Pleistocene Glaciation on the Tibetan Plateau are considered. The large ice sheet models indicate that disintegration of the ice sheet could have contributed up to 7 mm/yr of present vertical uplift and 2 mm/yr of horizontal extension. The former value can account for more than 50% of the observed uplift in central Tibet. The peak free-air gravity anomaly arising from the deglaciation would be around −5.4 mGal. In contrast, the smaller ice sheet models do not contribute significantly to the signals of present uplift and gravity anomalies. Modern geodetic measurements therefore have the potential to constrain the Late Pleistocene glaciation of the Tibetan Plateau. Assuming a large ice sheet over the Tibetan Plateau, the disintegration can contribute up to 6 m of eustatic sea-level rise.     

Zircon U–Pb ages and tectonic implications of Paleozoic plutons in northern West Junggar,North Xinjiang,China

North Xinjiang, Northwest China, is made up of several Paleozoic orogens. From north to south these are the Chinese Altai, Junggar, and Tian Shan. It is characterized by widespread development of Late Carboniferous–Permian granitoids, which are commonly accepted as the products of post-collisional magmatism. Except for the Chinese Altai, East Junggar, and Tian Shan, little is known about the Devonian and older granitoids in the West Junggar, leading to an incomplete understanding of its Paleozoic tectonic history. New SHRIMP and LA-ICP-MS zircon U–Pb ages were determined for seventeen plutons in northern West Junggar and these ages confirm the presence of Late Silurian–Early Devonian plutons in the West Junggar. New age data, combined with those available from the literature, help us distinguish three groups of plutons in northern West Junggar. The first is represented by Late Silurian–Early Devonian (ca. 422 to 405 Ma) plutons in the EW-striking Xiemisitai and Saier Mountains, including A-type granite with aegirine–augite and arfvedsonite, and associated diorite, K-feldspar granite, and subvolcanic rocks. The second is composed of the Early Carboniferous (ca. 346 to 321 Ma) granodiorite, diorite, and monzonitic and K-feldspar granites, which mainly occur in the EW-extending Tarbgatay and Saur (also spelled as Sawuer in Chinese) Mountains. The third is mainly characterized by the latest Late Carboniferous–Middle Permian (ca. 304 to 263 Ma) granitoids in the Wuerkashier, Tarbgatay, and Saur Mountains.As a whole, the three epochs of plutons in northern West Junggar have different implications for tectonic evolution. The volcano-sedimentary strata in the Xiemisitai and Saier Mountains may not be Middle and Late Devonian as suggested previously because they are crosscut by the Late Silurian–Early Devonian plutons. Therefore, they are probably the eastern extension of the Early Paleozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China. It is uncertain at present if these plutons might have been generated in either a subduction or post-collisional setting. The early Carboniferous plutons in the Tarbgatay and Saur Mountains may be part of the Late Paleozoic Zharma–Saur volcanic arc of the Kazakhstan block. They occur along the active margin of the Kazakhstan block, and their generation may be related to southward subduction of the Irtysh–Zaysan Ocean between Kazakhstan in the south and Altai in the north. The latest Late Carboniferous–Middle Permian plutons occur in the Zharma–Saur volcanic arc, Hebukesaier Depression, and the West Junggar accretionary complexes and significantly postdate the closure of the Irtysh–Zaysan Ocean in the Late Carboniferous because they are concurrent with the stitching plutons crosscutting the Irtysh–Zaysan suture zone. Hence the latest Late Carboniferous–Middle Permian plutons were generated in a post-collisional setting. The oldest stitching plutons in the Irtysh–Zaysan suture zone are coeval with those in northern West Junggar, together they place an upper age bound for the final amalgamation of the Altai and Kazakhstan blocks to be earlier than 307 Ma (before the Kaslmovian stage, Late Carboniferous). This is nearly coincident with widespread post-collisional granitoid plutons in North Xinjiang.     

Stratigraphy and thermoluminescence dating of Late Glacial cover sand in Denmark

In western Jutland deposits with a texture and structure similar to Dutch cover sands have been found. Investigations have been carried out on the lithology and the lithostratigraphic sequences. Pollen analysis is used for biostratigraphic correlation, and ages have been obtained by the radiocarbon method as well as by thermoluminescence techniques using both quartz and eldspar. The five sites are presented separately and the data are integrated into a general stratigraphic outline of the Late Glacial cover-sand sequence in Jutland. During the latest part of the Upper Pleniglacial and the earliest Late Glacial, older cover-sand type was deposited. Locally this sediment was also deposited during the later parts of the Late Glacial. Younger cover-sand type was deposited primarily during the Early Dryas and Late Dryas, but some sand of this type was also laid down during the Allerød and the earliest Holocene. The palaeobotanical records are discontinuous, but plant growth was probably present throughout the Late Glacial; only during the Altered did the deposition of sand decrease long enough to allow a relatively stable vegetational development. The investigation shows that the sequence of events in Jutland is similar to the Dutch record.     

上海市第四系的工程地质研究

通过２０余项工程代表性的钻孔岩芯分析,对比了上海市１２０ｍ以浅晚第四纪各沉积层的成因类型,形成时代,岩性及其物理力学性质,并按地基工程要求。编制了“地基土层次名称表”。表中将沉积层划分为１２个工程地质层及众多亚层。在此基础上,参照已有２０００多项工程勘察资料,编制成多时段１：５万古地理图,从而摸清了本区各土层的空间分布规律及其结构变化特征。根据地层结构差异,编制了 桩基工程地质分区图”,阐述了天然地基和三个不同桩基区岩土工程地质条件及应用中需注意的事项,为上海市工程勘察、设计和麓工提供了可靠的科学依据。     

Late Pleistocene Mérida Glaciation, Venezuelan Andes

The name Mérida Glaciation is proposed to designate the alpine glaciation which affected the central Veneruelan Andes; during the Late Pleistrocenc. Two main morainie levels have been recognired: one between 2600 and 2700 m, and another between 3000 and 3500 m elevation. The snow line during the last glacial advance was lowered approximately 1200 m below the present snow line (3700 m). Rodiocarbon dating indicates that the moraines are older than 10,000 years B.P., and probahly older than 13,000 years B.P. The lower morainie level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin).     

Late Paleozoic volcanic record of the Eastern Junggar terrane, Xinjiang, Northwestern China: Major and trace element characteristics, Sr–Nd isotopic systematics and implications for tectonic evolution

The Eastern Junggar terrane of the Central Asian Orogenic Belt includes a Late Paleozoic assemblage of volcanic rocks of mixed oceanic and arc affinity, located in a structurally complex belt between the Siberian plate, the Kazakhstan block, and the Tianshan Range. The early history of these rocks is not well constrained, but the Junggar terrane was part of a Cordilleran-style accreted arc assemblage by the Late Carboniferous. Late Paleozoic volcanic rocks of the northern part of the east Junggar terrane are divided, from base to top, into the Early Devonian Tuoranggekuduke Formation (Fm.), Middle Devonian Beitashan Fm., Middle Devonian Yundukala Fm., Late Devonian Jiangzierkuduke Fm., Early Carboniferous Nanmingshui Fm. and Late Carboniferous Batamayineishan Fm. We present major element, trace element and Sr–Nd isotopic analyses of 64 (ultra)mafic to intermediate volcanic rock samples of these formations. All Devonian volcanic rocks exhibit remarkably negative Nb, Ta and Ti anomalies on the primitive mantle-normalized trace element diagrams, and are enriched in more highly incompatible elements relative to moderately incompatible ones. Furthermore, they have subchondritic Nb/Ta ratios, and their Zr/Nb and Sm/Nd ratios resemble those of MORBs, characteristics of arc-related volcanic rocks. The Early Devonian Tuoranggekuduke Fm., Middle Devonian Beitashan Fm., and Middle Devonian Yundukala Fm. are characterized by tholeiitic and calc-alkaline affinities. In contrast, the Late Devonian Jiangzierkuduke Fm. contains a large amount of tuff and sandstone, and its volcanic rocks have dominantly calc-alkaline affinities. We therefore propose that the Jiangzierkuduke Fm. formed in a mature island arc setting, and other Devonian Fms. formed in an immature island arc setting. The basalts from the Nanmingshui Fm. have geochemical signatures between N-MORB and island arcs, indicating that they formed in a back-arc setting. In contrast, the volcanic rocks from the Batamayineishan Fm. display geochemical characteristics of continental intraplate volcanic rocks formed in an extensional setting after collision. Thus, we propose a model that involves a volcanic arc formed by northward subduction of the ancient Junggar ocean and amalgamation of different terranes during the Late Paleozoic to interpret the formation of the Late Paleozoic volcanic rocks in the Eastern Junggar terrane, and the Altai and Junggar terranes fully amalgamated into a Cordilleran-type orogen during the end of Early Carboniferous to the Middle–Late Carboniferous.     

Weichselian Late Pleniglacial surface winds over northwest and central Europe: a model–data comparison

Reconstructions of the Weichselian Late Pleniglacial wind direction in northwest and central Europe are reviewed and compared with palaeoclimate simulations performed with an atmospheric general circulation model. These reconstructions are based on proxy data containing information on former wind directions, such as relic dune forms, sediments and wind‐polished rock surfaces. The objective is to investigate whether: (1) the proxy information is internally consistent; and (2) in agreement with the model simulations. We find a general consensus in the proxy‐based reconstructions, indicating a dominant westerly to northwesterly wind in winter during the Late Pleniglacial. The model results indicate over the study area an atmospheric circulation in winter that is dominated by southwesterly to west‐northwesterly winds, which are stronger than the southwesterly winds in the present‐day climate. The main driving factors behind the anomalous atmospheric circulation in the Late Pleniglacial are the Laurentide Ice Sheet and a colder North Atlantic Ocean with a relatively extensive sea‐ice cover, leading to an eastward relocation of the Icelandic Low and an enhanced pressure gradient over northwest Europe. The minor difference in Late Pleniglacial wind direction between the reconstructions and model can be explained by a combination of uncertainties in the proxy data and the relatively low spatial resolution of the applied climate model. Copyright © 2006 John Wiley & Sons, Ltd.     

Discontinuity surfaces and event stratigraphy of Okha Shell Limestone Member: Implications for Holocene sea level changes,western India

The Okha Shell Limestone Member of Chaya Formation is the coarse grained, shell rich deposit commonly recognized as the beach rocks. It has been age bracketed between Late Pleistocene and Holocene. Late Quaternary sea level changes have been studied with beach rocks along the Saurashtra coastal region. The present study has been carried out in the Okhamandal area of the Saurashtra peninsula especially on the Okha Shell Limestone Member as exposed at various locations along the coast from north to south. Temporal and spatial correlations of the observations have revealed three events in the Okha Shell Limestone Member of Chaya Formation that are correlated laterally. The events show depositional breaks represented by discontinuity surfaces, the taphofacies varieties and ichnological variations. The present study in the context of available geochrnological data of the region suggests a prominent depositional break representing low sea level stand (regression) during an Early Holocene during the deposition of Okha Shell Limestone Member.     

Holocene changes in tropical wind intensity and rainfall: Evidence from southeast Ghana

Remnants of a fixed aeolian dune ridge occur along the southeast coast of Ghana, just behind the present shoreline. Aeolian sands also cover extensive areas of the Accra Plains. No dunes are present here, the sands mainly occurring as sheets which blanket an early Holocene landscape. The sediments are of mid-Holocene age and were deposited during the interval 4500 B.P.–3800 yr B.P., when the southwesterly winds were stronger than they are at present and much of tropical Africa seems to have been subject to marked aridity. The onset of drier, windier conditions around 4500 yr B.P. brought to an end the more equable climates than had characterized much of West Africa during the earlier Holocene. Aridity, intensified winds, and desert expansion between 4500 and 3800 yr B.P. parallel environmental conditions in tropical continental areas at the height of the Late Pleistocene glaciation.