The Layer Landforms Of The Grand Valley In Ruyuan County, Guangdong Province

This paper briefly reviewed the studies on planation surface home and abroad , including the contribution by Davise, A.pench, w.pench, L.C.king, K.K.markofu, JuliusBudel,J.F.Gelert, B.willis, J.GAnderson, Yeliangfu, Yangzhongjian, Lijijun, Cuizhijiu, etc.Since 1889, when the concept of peneplain first appeared, more than 100 years has passed and many new terms have been put forward, including pediment, primary peneplane, etchplain, pediplain, summit surface, exhumed surface, planation sueface, etc. The meanings of these terms are the same and can be interpreted in this way: during the lengthy geological period when lithosphere is at its tectonic stabilization phase, the explanate landform close to the base level of erosion, formed by many outside force such as weathering, erosion, transportation, cut away all the exposed stratum and geological structure before it. Based on field survey and indoor analyse, this paper focused on the layer landforms of the Grand Valley in Ruyuan county, Guangdong province. By means of Arcview and relief map of 1:50000, a three-dimensional display of the layer landforms of the Grand Valley and nearby was done and the two layers explanate landforms in North and East of the Grand Valley of RuYuan were clearly displayed. Compared with geology map of 1:200000, the exposed terrane and geologic structure background in region of the Grand Valley were analyzed.The geologic structure and landform shows that Dabu and the summit surface around it , separated by faultage. Dabu bason is a downfaulted bason, and the moutains around it are rift block mountains. Combined with the background of the north of Guangdong province and weathering crust of Dabu bason, it is considered that two layers landform near the Grand Vally came into being at the same period. So the author guesses that Dabu basin is fault basin and the mountains around it are block mountains. Basing on denudation rates of the south slope of Nanling mountains, it is inferred that the two layers landform were formed at the late stage of Tertiary Period, 3 million years ago.After investigating the riverbed of Dabu River, the author drawed a sketch map of vertical section on the bed and found that there are 2-3 knick points on this riverbed. Combining with the study on terrace of Lingjiang River and karst caves, it is inferred that the place has raised un-continuously three times during Quaternary.