元谋干热河谷强侵蚀区土桥发育特征与演化过程

元谋干热河谷区的土桥地貌是当地常见的一种潜蚀地貌，是区域地貌的重要组成部分。元谋土桥是造成当地水土流失的重要原因之一，研究其形态特征及演化过程对于认识区域地貌演化及生态地质环境的治理具有重要的现实意义。2009―2013年，通过多次对土桥形态特征、分布区域的地形地貌的现场测定，对土层进行了分析。结果表明：1）元谋土桥大部分分布于冲沟中上游的沟底，少部分分布于冲沟边缘的峭壁下，有连续出现，也有单独出现；2）土桥发育的土层具有层间差异性，即桥洞所在土层较其顶部土层抗崩解性弱，易受侵蚀；3）地层构造节理及冲沟坡度的变化等因素对土桥的形成具有一定的影响作用；4）元谋土桥的形态、结构与一般天生桥或人工桥梁具有相同的桥体结构，形态规模一般都不大；5）演化过程遵循一般事物的发展规律，分为萌芽期、年轻期、成熟期、消亡期４个阶段；6）元谋土桥与陷穴、冲沟等地貌具有一定的耦合关系。

Morphological Characteristics and Evolution Process of Soil Bridge in Yuanmou Dry-Hot Valley

Soil bridge is one of the common erosional landforms in Yuanmou Dry-hot Valley. It is also an important reason causing local water and soil loss, studying the morphological characteristics and evolution process of soil bridge would have important practical significance for better understanding of the evolution of regional landforms and management of local eco-geological environment. From 2009 to 2013 the authors made field investigations on the morphological characteristics, microtopography of the soil bridge, the soil bridge position, and the shape of the longitudinal profile of the gully. And the properties of soil layers were analyzed. The results showed: 1) Soil bridge mainly developed at the bottom in upper and middle reaches of the gully, less at the bottom of cliff at the edge of gully. Some soil bridges occurred continuously and some lonely. 2) There was difference among soil layers of the bridge, the resistance of the soil layer of bridge hole against disintegration and erosion was weaker than that of the top layer of the bridge. 3) There was some influence of structure joints and gully slope on the formation of soil bridge. 4) The shape and structure of soil bridge were similar to those of the natural bridge or the artificial bridge. The scale of soil bridge was generally little. The shape of the bridge hole was mainly similar to triangle and sometimes to arc. The top of the hole mainly developed along the vertical joint and collapse was more frequently observed. The influencing factors of the soil bridge development were mainly the climate environment, the tectonic setting, the formation joint and the slope of gully, etc. 5) The evolution process of soil bridge followed the general law of thing development, which could be divided into four stages: germination stage, young stage, mature stage and withered stage. 6) There was certain coupling relationship between soil bridge and sink hole and gully in Yuanmou.