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1.
Alluvial fan is among the important geomorphic positions in arid and semi-arid environments, and the history of settlement in central parts of Iran plateau has significantly been related to this landform. Soil properties are also related to the stability and the position of geomorphic surfaces. To study and compare clay mineralogy and soil physicochemical responses to geomorphic positions, 11 representative pedons on stable and unstable alluvial fan surfaces and plain geomorphic position in a transect from Sarcheshmeh Mountains toward Rafsanjan Plain were studied. Soil moisture and temperature regimes of the area were aridic and mesic, respectively. Smectite, kaolinite, chlorite, illite, and palygorskite clay minerals were found in almost all the studied soils. More salinity was investigated in stable alluvial fan surfaces than unstable positions. Evidence of a more humid paleoclimate shown by buried Bt horizons in plain surface was observed. Transformation of palygorskite to smectite in surface horizon of unstable alluvial fan position and preservation of palygorskite associated by gypsum crystals in By horizon of stable fan surfaces were among the important findings of the research. There was a significant and mutual relationship between geomorphology and physicochemical and mineralogical properties of soils under study.  相似文献   

2.
At The Rustad Quarry Site (32RI775), in southeastern North Dakota, Early Archaic artifacts, bison bone, and hearth features were found in the lowest of three Boroll-like buried soils formed in alluvial fan sediments. Alluvial fan sediments were deposited from 8000 to 4925 B.P., and were then buried by eolian sand. The alluvial fan sediments (mudflows) bury lacustrine sediments (Sherack Formation deposited from 9900 to 9500 B.P. in Glacial Lake Agassiz II), both of which overlie a Moorhead Phase fluvial terrace deposited from 10,900 to 9,900 B.P. Cultural remains were associated with five radiocarbon ages, three on archaeological charcoal (7180 B.P., 7240 B.P., and 7550 B.P.) and two on organic carbon from two welded A-horizons containing the cultural remains (7370 B.P. and 7675 B.P.). A well-developed Aquoll-like soil formed in the lacustrine sediments from 9500 to 8000 B.P. Soil morphology and chemistry of the Boroll-like fan soils, the Aquoll-like lacustrine soil, and other Boroll-like soils formed in eolian and deltaic sediments nearby suggest a subhumid, cool continental climate with riparian woodland and mixed prairie vegetation at the site and surrounding area from 11,400 B.P. to the present. This partially contradicts paleoenvironmental reconstruction from pollen sites nearby that suggest the area was covered by a closed canopy spruce forest from terminal late-Pleistocene to 10,000 B.P. Alluvial fan formation and eolian activity on the adjacent Sheyenne Delta occurred from 8000 to 5000 B.P., which indicates greater landscape instability and drier conditions during the mid-Holocene (Altithermal).  相似文献   

3.
A study area in an arid region of southern New Mexico is in basin-and-range topography and includes both a river valley and a closed basin. Holocene soils occur in valley fills and low terraces between Pleistocene fans, in and near drainageways on the fan-piedmont, on ridges, and in dunes. Holocene soils suggest the character of initial development in soils that are much older and more complex, and record the beginnings of various soil horizons. Noncalcareous brown or reddish brown B horizons have formed in low-carbonate parent materials of stable sites. Incipient development of the argillic horizon and the Haplargids occurs at stable sites in very gravelly materials that are about 1–2000 yr old. The cambic horizon and Camborthids occur in adjacent low-gravel materials of the same age. The argillic horizon occurs continuously in soils of earliest Holocene, particularly in very gravelly materials. Where soils have been truncated, as in areas affected by landscape dissection, argillic and cambic horizons are usually absent and the soils are Torripsamments, Torriorthents, or Torrifluvents depending on content of sand, gravel, and organic carbon. In high-carbonate parent materials, noncalcareous, reddish brown B horizons have not formed at any time in the Holocene. Most of these soils are Torriorthents or Torrifluvents although an incipient calcic horizon has formed in some of the oldest Holocene soils; the latter are Calciorthids. Horizons of carbonate accumulation are the best and most common pedogenic indicators of soil age. Stage I carbonate horizons are a major feature of pedogenesis in the Holocene. Because of additions of carbonate from the atmosphere, carbonate horizons are morphologically similar whether they have formed in high or low-carbonate alluvium. The carbonate accumulations are illuvial.Some Holocene deposits apparently resulted from changes in climate. Others, such as the youthful deposits of coppice dunes, apparently were caused by man's introduction of cattle and subsequent overgrazing and seed dispersal.  相似文献   

4.
Two alluvial terraces and the present flood plain were studied at two locations along the Susquehanna and Unadilla Rivers in south-central New York state. They have formed since deglaciation and incision of the stream channels into the valley train deposits. The higher terrace has noncumulative soil profiles with well-developed color B horizons predominantly of silt loam and very fine sandy loam. The terrace is weathered to a degree similar to nearby glacial outwash terraces that have caps of similarly textured sediments. Incision that produced the terrace occurred before 9705 ± 130 yr B.P. The lower terrace is characterized by relatively thick, vertical-accretion deposits of silt loam that contain sequences of thin, buried A, color B, and C horizons. They were formed between about 3240 ± 110 (14C data of soil humin) and 235 ± 80 yr B.P. Deposits above the 235 ± 80 yr B.P. stratum are unweathered. The soil stratigraphy and 14C dates of soil humin from buried A horizons are surprisingly well correlated between sites. Most sediments of the present flood plain have been deposited since 1120 ± 80 yr B.P. Incipient A horizons and oxidation of inherited organic matter in the subsoil are the only evidence of pedogenesis in the flood-plain deposits that are older than 275 ± 80 yr B.P. The most recent flood-plain fill deposited since then is unaltered. These youngest sediments of the flood plain along with the youngest veneer of vertical-accretion deposits on the lowest terrace are associated with an increased rate of deposition largely attributable to clearing of the forests by settlers, beginning in the late 1700s. Comparison of the alluvial stratigraphy with the radiocarbon-dated pollen stratigraphy of southwestern New York (Miller 1973) reveals some apparent time correlations between alluvial events and vegetation changes. This gives reason to speculate that climatic change or forest catastrophes such as disease or drought are causes of some of the alluvial events.  相似文献   

5.
A visually prominent desert soil with a horizon of clay accumulation (Typic Natrargid) has formed under an arid climate in Panamint Valley, California, in sandy, very calcareous, saline fan alluvium in less than about 3500 yr, and probably less than 2000 yr. Such soils can be used as stratigraphic markers, but could be confused with other desert soils with clay-accumulation horizons (Haplargids) which occur much more commonly on desert alluvial fans, are mostly late Pleistocene or older, and do not form in parent materials that are still calcareous. This Natrargid formed in a playa-margin environment, where clay for translocation and sodium salts that engender rapid clay movement probably were provided by dust fall.  相似文献   

6.
Analysis of a flight of alluvial terraces in the Sierra Nevada National Park near Pico Mucuñuque in the Eastern Mérida Andes has yielded information on geomorphic, pedogenic, and vegetational changes from Late Glacial time to the present. The terraces formed in large part due to stream incision/migration triggered by neotectonic uplift (>7000 yr BP) of a Late Glacial/Early Holocene glaciolacustrine lithosequence and, with the exception of the oldest/highest terrace, exhibit near-uniform lithology/parent materials. Soils developed in the terrace materials range from thin, weakly developed profiles (O/C/Cu horizons) to Entisols with O/Ah/Cox/Cu horizons and similar buried counterparts representing former short periods of floodplain stability or slow aggradation. The buried soils provide organic-rich material that yields radiocarbon ages, which provide time constraints on individual pedons and the geomorphic development of the site. Iron and aluminum extracts of soil matrix material provide information on the formation and accumulation of goethite and hematite, the relative accumulation of ferrihydrite (gain/loss), and the downward translocation of organically complexed Al as a function of soil development and age. SEM analysis of heavy mineral grains indicates varying material sources and degrees of weathering in the soil chronosequence. A qualitative study of plant functional types across the terrace sequence shows that older surfaces support greater plant diversity. The study also suggests ways in which the plant communities influence soil development at the site through varying organic matter inputs and varying soil moisture use by specific species (e.g., ferns on the oldest terrace), which may explain the absence of B horizons in the Late Pleistocene/Early Holocene soils.  相似文献   

7.
Holocene soils of a semiarid area in southern New Mexico occur on terraces and fans in and adjacent to the mountains. The illustrative soils have formed in alluvium derived from rhyolite, monzonite, and sedimentary rocks, mainly limestone.A large arid basin occurs downslope from the mountains. In the arid-semiarid transition, the effect of a gradual increase in precipitation on soil morphology is shown by Holocene soils that sensitively reflect the precipitation. Where the parent materials contain little or no carbonate, a surficial noncalcareous zone and a reddish-brown horizon of silicate clay accumulation thicken mountainward as precipitation increases. The Bt horizon is underlain by the carbonate horizon, the upper boundary of which deepens mountainward. These orographic-depth relations and soil morphology support an interpretation that some of the clay (as well as the carbonate) is of illuvial origin. In high-carbonate parent materials, a noncalcareous zone has not developed and a reddish-brown horizon of clay accumulation has not formed.In upper horizons, organic carbon increases and color darkens towards the mountains. This causes a change in soils at the categorical level of soil order; Mollisols do not occur in the arid basin downslope, where nearly all Holocene soils are either Aridisols or Entisols. In the semiarid zone, however, most Holocene soils are Mollisols. Thick, dark A horizons have formed in many of these soils. The thickness of these horizons is attributed primarily to episodes of sedimentation during soil development. Some Holocene Aridisols also occur in the semiarid zone. Generally these are on narrow ridges, where the mollic epipedon has been truncated or did not form.In some terrains the soil-geomorphic relations are complex and Holocene soils may be above or at the same elevation as adjacent, much older soils. In such situations, when the morphological range of the various soils has been determined, soil morphology may be used to distinguish the Holocene soils and surfaces from their older analogs.  相似文献   

8.
The interactions of organic matter and minerals contribute to the capacity of soils to store C. Such interactions may be controlled by the processes that determine the availability of organic matter and minerals, and their physical contacts. One of these processes is bioturbation, and earthworms are the best known organisms that physically mix soils. Earthworms are not native species to areas previously glaciated, and the introduction of earthworms to these regions has been associated with often dramatic changes in soil structure and geochemical cycles. The authors are studying C mineral interaction along an approximately 200 m long earthworm invasion transect in a hardwood forest in northern Minnesota. This transect extends from the soils where earthworms are absent to soils that have been invaded by earthworms for nearly 30–40 years. Pre-invaded soils have an approximately 5 cm thick litter layer, thin (～5 cm) A horizon, silt rich E horizon, and clay-rich Bt horizons. The A and E horizons formed from aeolian deposits, while the clay-rich Bt horizons probably developed from underlying glacial till. With the advent of earthworm invasion, the litter layer disappears and the A horizons thicken at the expense of the O and E horizons. In addition, organic C contents in the A horizons significantly increase with the arrival of earthworms. Simultaneously, measured mineral specific surface areas suggest that minerals’ capacities to complex the organic matter appear to be greater in soils with active earthworm populations. Based on the data from two end member soils along the transect, mineral specific surface areas in the A and E horizons are larger in the earthworm invaded soil than in the pre-invasion soil. Additionally, within < 5 a of earthworm invasions, A horizon materials are turned from single grain to a strong medium granular structure. While A horizon organic matter content and organic C-mineral complexation increase after earthworm invasion, they are also more vigorously mixed. This growing data set, when ultimately combined with ongoing measurements of (1) the population dynamics of earthworms along the invasion transect, (2) C-mineral association (via surface adsorption and physical collusion in mineral aggregates) and (3) dissolved organic C will show how and how much soil capacity to store C is affected by burrowing organisms, which are often the keystone species of given ecosystems.  相似文献   

9.
Outcrops of buried soils on lake-plains and glacial headlands along Lake Michigan's eastern shore suggest that periodic dune-building has occurred there after relatively long (≥100 yr) periods of low sand supply. We located, described, and radiocarbon dated 75 such buried soils that crop out in 32 coastal dune fields beside the lake. We assume that peaks in probability distributions of calibrated 14C ages obtained from wood, charcoal, and other organic matter from buried A horizons approximate the time of soil burial by dunes. Plotted against a late Holocene lake-level curve for Lake Michigan, these peaks are closely associated with many 150-yr lake highstands previously inferred from beach ridge studies. Intervening periods of lower lake levels and relative sand starvation apparently permitted forestation and soil development at the sites we studied. While late Holocene lake-level change led to development and preservation of prominent foredunes along the southern and southwestern shores of Lake Michigan, the modern dune landscape of the eastern shore is dominated by perched dunes formed during 150-yr lake highstands over the past 1500 yr.  相似文献   

10.
A soil survey around the archaeological site of Harappa, Pakistan revealed alluvial deposits of five distinct ages based on relative position in the landscape and degree of soil profile development. the youngest deposit (age 1) is in the lowest landscape position and has received flood waters as recently as 1988. Soils there are in an incipient stage of development: only organic carbon and soluble salts have accumulated at the surface of the profile. the age 2 deposit has not undergone significant pedogenic change, but is in a slightly higher landscape position than the youngest deposit. Elevated concentrations of P, and the presence of sand-sized pottery and brick fragments, indicate that this deposit was derived at least partially from archaeological material. the presence of small, soft calcite nodules (Stage II) and some soluble salt translocation are the primary pedogenic changes observed in the age 3 deposit. the age 4 deposit shows evidence of both carbonate and gypsum accumulation. Presence of large gypsum nodules in deep By horizons suggests that a high groundwater table has altered these soils. the oldest deposit, age 5, forms a late Pleistocene stream terrace of the Ravi River. the soil formed in this deposit exhibits considerable carbonate accumulation, with large, dense nodules (Stage II + ) and an argillic horizon. A 14C date from pedogenic calcite gives an age of 7080 ± 90 years B.P., indicating a minimum age of early Holocene. the soil survey suggests that the ancient city of Harappa was built on an age 5 stream terrace remnant, surrounded by Holocene floodplains and a meandering channel of the Ravi River.  相似文献   

11.
Alluvial fans are one of the most important landforms in geomorphological and paloenvironmental studies. The objective of this study was the application of clay mineral assemblages and micromorphological properties of the studied paleosols in the geomorphic surfaces of an alluvial fan in the eastern Isfahan as proxies for paleoenvironmental and paleoclimatic changes. Micromorphology, X-ray diffraction, and scanning electron microscopy approaches were used to study the representative pedons. The results indicated that the illuviation process in the calcareous soils of the arid regions of the eastern Isfahan was probably in response to Quaternary moist conditions. There was no significant difference between clay coating properties of the studied relict and buried paleosols. Clay mineralogical study suggested that kaolinite and illite were inherited from the parent materials, while smectite and palygorskite were formed in the soil environment. Paleoargillic horizon was characterized by smectite and calcic (especially the calcrete) horizons were dominated by palygorskite. Palygorskite was accumulated by both neoformation and illuviation processes. High clay content, high intensity of smectite peak, and activity of the illuviation process in paleoargillic horizon demonstrated the seasonality of climate (rainfall) even in the moist periods of Quaternary in Central Iran. Clay mineralogical assemblages suggested a trend of increasing environmental aridity in the study area. This study, therefore, highlighted the role of clay mineralogical investigations in arid lands’ geomorphological and paleoenvironmental researches.  相似文献   

12.
Gypcretes of Miocene age are preserved beneath a 9·53 ± 0·36 Ma ignimbrite along the eastern margin of the Oligo-Pleistocene Calama Basin, northern Chile. They are restricted to a single stratigraphic horizon developed within laterally extensive (>35 km) coalesced alluvial fan deposits, developed along the margin of an endorheic basin. Two types of gypcrete are recognized. Type 1 comprises almost completely gypsum-cemented sandstones containing alabastrine nodules and columns, sub-vertical and horizontal veins of fibrous gypsum and 'v-shaped' cracks infilled by clastic material, and are interpreted as surface weathered gypsic crusts. Type 2 gypcretes are composed of massive, reddened poikilitic and mesocrystalline gypsum (up to 80% of the rock) with isolated bedding-parallel, clast-rich lenses (200 × 30 cm) and sub-vertical veins of fibrous gypsum. The massive texture resembles that of well developed B horizons in Quaternary alluvial desert soils. The crystal forms suggest an origin as a subsurface gypsic crust formed by a combination of hydromorphic (poikilitic) and illuvial (mesocrystalline) processes with the fibrous gypsum veins suggestive of periodic surface exposure.
Gypcrete horizons are up to 25 m thick and composed of both gypcrete types. They represent superimposed phases of surface and subsurface gypcrete development. Quaternary gypcretes are developed in arid climatic regimes, but are not considered to develop under hyper-arid climates. An arid climate is considered to have prevailed in the study area up to 9·5 Ma after which a change to hyper-aridity favoured gypcrete preservation.  相似文献   

13.
Concentrations of CO2 in soil atmosphere and CO2 efflux were measured across a marine terrace soil chronosequence near Santa Cruz, California. Soil development, specifically the formation of an argillic horizon, has created a two-tier soil gas profile in the older terrace soils. The soil above the argillic horizon has seasonal variations in soil CO2 associated with plant respiration. The older soils with dense argillic horizons maintain a year round ～1%CO2 below the argillic horizon. The CO2efflux during the growing season is higher on the older terraces.  相似文献   

14.
Soil development in the plain landscape of the southern Argentinean Pampa is related to pulses of aeolian accretion of calcareous loess during the Holocene epoch. Such plain relief is associated with landform stability that favors pedogenesis. In some sectors of the Holocene loess mantle, detailed soil surveys show a great variability of soil morphology in short distances (<7 m), such that pedons with Bt horizon (Ap-Bt-C-2Ckm) coexist with pedons with an AC horizon (Ap-AC-C-2Ckm) in a plain landscape, within identical loess parent material over a tosca layer (2Ckm-calcrete-petrocalcic horizon), and in a similar pedoclimate. This article studies the origin of this spatial variation. Loess parent materials directly overlie the relic tosca layer, exhumed after erosion of preexisting soils of the Late Pleistocene. The contrast in soil morphology between the petrocalcic horizon and the overlying Holocene soils reflects the effect of polygenesis. The complex soil spatial distribution pattern over the tosca layer appears unrelated to its paleomicrotopography, because soils with Bt horizons are identified in positive and depressed microlandforms of the tosca. The absence of Bt horizons might be caused by formerly intense biological activity related to a stable pattern of two natural vegetation covers or a surface paleomicrotopography that supported distinct vegetation types depending on the soil moisture in each paleomicrolandform.  相似文献   

15.
14C dating and pollen analysis of the surface organic (LFH) horizons of several humo-ferric podzol profiles forming a soil catena close to the 'Little Ice Agc' outer moraine ridge of Haugabreen, southern Norway, are used to examine the timing and nature of podzol development at the low-/sub-alpine margin of the Jostedalsbreen area. Comparison with results from a palaeosol buried beneath the outer moraine shows that FH horizon development began as early as 5,265 ± 65 B.P., but that it was not synehronous across the profiles, the latest profile having a date of 3,590 ± 65 B.P. It is argued that surface organic horizons developed as a response to a deterioration of climate and possibly the recrudescence of the Myklebustbreen ice cap at c . 5,000 B.P., and that the dates for horizon initiation vary according to local topographic and soil-hydrologic conditions. It is still uncertain whether the hump-ferric podzols were preceded by brown earths or weakly podzolised sub-alpine podzolic soils, but at all sites where pollen evidence is available it appears that FH initiation took place beneath Betula woodland.  相似文献   

16.
The sorption parameters of Cu, Zn and Pb are related to the composition of the different genetic horizons of a Luvisol profile in batch sorption experiments. The affinities of metals towards the soil samples from different horizons followed the same sequence, e.g. Pb≥C>>Zn. By far the highest metal retention was found in the Ck horizon due to the alkaline conditions. It is followed by the A horizon with its high organic matter content, while the lowest sorption capacity was found in the Bt horizon. In the horizons free of carbonate, primarily Pb and Cu were immobilized. The studied soil can be characterized by high amount of organic matter, clay accumulation horizon, as well as calcareous subsoil. This kind of profile development makes soils able to immobilize a significant metal pollution.  相似文献   

17.
黄河三角洲内陆到潮滩土壤中碳、氮元素的梯度分布规律   总被引：1，自引：0，他引：1  
黄河三角洲是我国典型的通过黄河冲积泥沙填海造陆形成的近代沉积区。区域受到黄河冲积、沉积等自然过程和农业耕种熟化等人类活动的双重影响。本研究通过在黄河三角洲地区内陆到河口海湾不同距离采集典型土壤类型剖面发生层样品,探讨土壤有机碳、总氮等生源要素的空间分布规律,为阐明我国典型海岸带地区陆源碳、氮的输送及循环特征提供基础依据。研究结果表明,黄河三角洲内陆与河口地区呈现出完全不同的土壤碳、氮分布规律。表层土壤碳、氮含量在黄河沿岸及三角洲南部均表现出由陆向海逐级递减的空间分布特征,而在黄河刁口流路和清水沟流路沿行水方向有梯度升高的趋势。内陆地区土壤碳、氮与盐分呈一定的负相关关系,表明土壤碳、氮主要受到耕作熟化过程的影响;而在河口地区两者呈显著的正相关关系（p〈0.01）,表明靠近海湾地区土壤碳、氮积累可能受到细颗粒泥沙沉积和滩涂湿地厌氧等环境影响。表层土壤碳、氮比变幅在3.6~8.6之间,说明该地区土壤有机质分解较快,不利于有机碳的积累。土壤剖面中,一些特殊发生层如红色夹黏层、黑色泥炭层对土壤碳、氮的富集具有明显的作用,其中红色夹黏层的土壤碳、氮含量接近耕层土壤。总之,黄河三角洲土壤在耕作垦殖、泥沙沉积等综合作用下形成的空间分布格局以及剖面特征发生层是影响碳、氮封存、释放和增汇等循环过程的关键驱动因素。  相似文献   

18.
选取贵州喀斯特区草地生态条件下5个不同区域的石灰(岩)土为研究对象,根据相应的理化性质及发生特性,探讨了影响土壤发育的因素,确定了土壤的诊断层和诊断特性。其中,供试土壤的诊断层包括:暗沃表层、雏形层和粘化层等,诊断特性包括:热性土壤温度状况、常湿润土壤水分状况、均腐殖质特性、腐殖质特性、盐基饱和、碳酸盐岩岩性、铁质特性、石灰性等,并在此基础上对其进行了土壤系统分类,结果显示:5个供试土壤分别属于中国土壤系统分类中的普通黑色岩性均腐土、腐殖钙质常湿雏形土和腐殖钙质常湿淋溶土等亚类。   相似文献   

19.
The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ～2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ～1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (105–106 years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ18O and relatively high δ13C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ～1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 106 yr.  相似文献   

20.
Cactus Hill is located in the Virginia Coastal Plain on a terrace above the Nottoway River. The site has a record of occupation that spans the Holocene and also offers evidence of humans late in the Pleistocene before Clovis time. Soil investigations identified several deposit types, and demonstrated that multisequal eolian sands forming the site's primary core are arrayed in spatially and temporally discrete horizons. Resting atop an ancient paleosol, the earliest sand stratum (19,540 ± 70 14C yr B.P.) is marked by a conspicuous but culturally sterile buried surface horizon. Eolian sand above this surface supports another sequum in which Clovis and underlying “Blade” artifacts are associated with a fainter surface horizon and pronounced subsoil lamellae. Early Archaic and successively younger artifacts occur above the Clovis level in a more weakly developed uppermost sequum. This soil and cultural stratigraphy, together with considerations of regional topography, demonstrate that the landscape has evolved incrementally since about the last glacial maximum. © 2004 Wiley Periodicals, Inc.  相似文献   

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