Zhang Tingjun, Wang Shading
This paper mainly deals with the alpine periglacial planation process, fluviatile action and the lower limit of alpine periglacial action in continental climate. The alpine periglacial planation process in continental climate can be divided into two types: 1. In relatively humid area, gelifluction develops very strongly in the middle and lower parts of mountains; while at the top and upper part of mountains, frost weathering is more important. The broken rocks move down along slopes under gravitation. Together with avalanches and some other aetions, gelifluction and frost weathering have a strong action on the slope erosion and incision, making the slope retreat parallelly or top erosion surface contract gradually. This type of process is called the periglacial planation process of slope retreating type. 2. In relatively dry areas,gelifluction is very weak, frost weathering at the top, upper and middle parts of mountains play a dominant role in periglacial processes. The broken rocks move down along slopes, bedrock at the top, upper and middle parts of mountains in enposed to the air andsuffers stoong frost weathering again. Such process repeats in cyclesin this way, resulting in the frequent denudation of the top, upper andmiddle parts of mountains by frost action, Which finally makes thetop and upper parts of mountains smooth and very gentle, covered byrock fields and rock slopes; while gelifluction in the lower parts ofmountains is limited in a few places. In this process, the retreatingvelocity of mountain slopes is less than the descending velocity of mountaiaheight, resulting in the expansion of the top surface of mountains, andthe mountain section profile has the shape of a trapezoid. This process iscalled the periglacial process of height descending type.valley development in the alpine periglacial zone is closely related to the existence of permafrost, but the main reason resulting in the flat and wide valley is the influence of small discharge of river, high load, strong aggradation and local erosion basis. The formation of asymmetric valley is mainly controlled by the water-heat difference between different facing slopes. Rivers can make permafrost thaw beneath it, forming river talik. Therefore, it is not proper to think that it is difficult for a river to cut down in vertical direction due to the existence of permafrost beneath the river bed.The lower limit of the alpine periglacial action develops on many factors. It cen not be determined by mean annual air temperature or the lower limit of the modern alpine permafrost; it should be at the lowest alrtitude of the distribution of a group periglacial phenomena formed by strong frost action. In relatively humid area, the lower limit of periglacial action is lower than that of permafrost; in relatively dry area, it is near or higher than the lower limit of permafrost.
