The Hulun Buir High Plain( HBHP) (47°40'～49°40'N, 114°30' ～ 121°20'E;550 ～ 750m a. S. 1. ) is in the piedmont and denuded areas in a transition zone from semi-arid to semi-humid climate zones in the northwest region of Northeast China, where regional environments are sensitive to climate changes. Since the Last Glaciation Maximum, a series of environmental changes, such as lake level fluctuations and peat development, alternate deposition of aeolian sands and paleosols, as well as the concurrent shifts in the southern and lower limits of permafrost. In order to better understand the changes of the southern limit of permafrost, field investigations were organized for the past permafrost and periglacial remains in the Xing'anling Mountains in Northeast China in 2007 and 2008.From June to August in 2007, large numbers of sand and soil wedges and involutions were identified on the first terraces of the Wu'erxun and Hui'he rivers in the Xinba' er' hu West Banner,and in Xinba' er' hu East Banner and Ewenke Banner on the South-Central HBHP. The shapes, scales, sedimentary structures of these wedge structures and involutions and their host strata, and nearby topography and surface conditions were carefully observed and recorded. In the profiles,generally 2m to 4m in the exposed height,soils were sampled at intervals of 0. 2～0. 4m in vertical strata and analyzed for data on grain size distribution, pollens, and 14C dating. Their ages were estimated on the basis of their relative positions in strata and their C dating.Analysis and cross-examinations indicate that these sand wedges were formed during the Early Holocene( ca. 10. 0 ～ 7. 5kaB. P. )when the mean annual air temperatures were about -6. 5～-7. 0℃ in the study region,whereas these soil wedges were formed during the Late Holocene ( ca. 2. 3 ～ 2. OkaB. P. ) when mean annual airtemperatures were about -5. 0～-5. 5℃. It was colder and drier during the Early Holocene, when aeolian actionand deposition prevailed and permafrost occurred more extensively on the HBHP. In the Late Holocene, the air temperatures were roughly the same with those in the present-day northern Da Xing'anling Mountains,I, e.,about-5. 0～-5. 4℃. Thus,the permafrost conditions then,such as ground temperatures and the thickness of permafrost,resembled those of the present-day regions north of Mangui in the northern part of Northeastern China. The air temperatures during the formation and growth periods of these sand wedges and veins in the Early Holocene were 1. 5 ～ 2. 0t lower than those in the present-day northern Da Xing'anling Mountains. Therefore, mean annual groundtemperatures on the HBHP was colder than -1.5～-2. 0℃ ,and permafrost was greater than 80 ～ 100m in thicknessduring the Early Holocene. The southern limit of permafrost was about 4° ～ 5℃ south of its present-day position, I. E., at 43° ～44°N.However, although the HBHP is rich in paleoclimatic evidence, further systematic investigations and in-depth analysis deem necessary for a better understanding of permafrost evolution in the study region, Northeast and North China,and Eastern Eurasia since the Last Claciations.%砂、土楔等楔状构造是多年冻土曾经存在的可靠证据,其与多年冻土及年均地、气温度的关系也是重建古冰缘环境的依据.在呼伦贝尔高平原中南部的乌尔逊河、辉河、新巴尔虎东旗和鄂温克旗发现大量砂、土楔及伴生的冻融褶皱.通过对其形态、规模及围岩(土)结构分析,并根据楔内充填物及围土的14C定年结果判断,砂楔形成于早全新世(约10.0 ～ 7.5kaB.P.),当时研究区年平均气温为-6.5 ～ -7.0℃；土楔形成十晚全新世(约2.3～2.0kaB.P.),其时年平均气温为-5.0 ～ -5.5℃.这表明,早全新世更为于寒,风沙堆积活跃,多年冻土分布广泛；进入晚全新世后,气候转暖,呼伦贝尔气温与现今大兴安岭北部(即满归以北)的年平均气温(-5.0～-5.4℃)大体相当.据此推测,当时呼伦贝尔高平原的冻土发育程度与满归以北地区现今大片多年冻土状况相似.早全新世砂楔(脉)形成时期气温比现今大兴安岭北部低1.5～2.0℃.依此可见,早全新世该区多年冻土地温低于现今大兴安岭北部的地温(-1.5～-2.0℃),冻土厚度超过80 ～100m.
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