玉山國家公國內位於八通關地區之金門峒大斷崖是國內規模最大的崩 塌地之一，也是河川向源侵蝕及河川襲奪之典型範例，並為斷崖、斷層、崩塌地、 分水嶺等地質研究與解說之極佳地點。本研究即將金門峒斷崖按地層、岩性等崩 塌特性之不同分為(1)C區廬山層板岩區(2)D區金門峒斷層破碎帶(3)E區佳陽層變 質砂岩夾板岩區等三區，分別於崩崖冠部埋設大樁加以量測其向源侵蝕之速率， 除定點定期野外量測與拍攝記錄外，亦針對特殊季節如颱風、豪雨、梅雨季進行 不定期之觀測，並配合不連續面之調查分析，地理資訊系統之分析與航照判釋以 探計其崩塌之發育機制，再配合前人於玉山地區之研究及文獻分析，進一步推測 其未來的崩塌演化趨勢。 金門峒斷崖之崩塌機制主要是受到金門峒斷層主構造之影響，使得岩體破碎，特 別是較部D區，另伴隨著斷層發育的高密度節理，及因解壓作用形成的張力裂 縫，板岩的板劈理，均使金門峒斷崖的岩體特性，破碎的有如膠結疏鬆的礫岩一 般，加上外在營力的作用，例如降雨沖蝕、豪雨、颱風、地震、重覆冷熱(乾濕) 作用、與凍霜作用，使金門峒斷崖以5.75m/year~8.89m/year平均速率朝南與西南急 速的向源侵蝕，其崩坍面積更於59年內擴張達2.5倍，預計3~5年之時間，可達分 水嶺開始進行河川襲奪之作用(陳有蘭溪襲奪荖濃溪):再約15年之時間會影響到 玉山至八通關之步道，其崩塌發育趨勢更有向北峰稜線迅速擴大崩塌之趨向，終 致會影響到整個八通關地區甚至玉山山脈之地形與地貌;值得興奮的是它將是我 等有生之年得見的河川襲奪地理景象。

　　　　　The Chinmentong Cliff at Patungkuan in the Yushan National Park is one ofthe largest collapsed lands in Taiwan. It is also a classic example of headward rivererosion as well as river capture. It is an excellent location for geologic research andexemplification of cliff, fault, watershed, and collapsed land. Chinmentong Cliff isdivided into the following three areas, based on the characteristics of collapse,stratigraphy and the nature of the rock, (1)Area C Lushan formation (slate) area;(2) Area D Chinmentong fault rubble belt; (3) Area E Chiayang formation(metamorphic sandstone interspersed with slate.) Wooden posts were sunk in theground at the crest of the collapsed cliff in order to measure the rate of headwarderosion. Besides doing field research and photographic records at fixed intervals,measurements were made also at unspecified time during the typhoon, heavy rain,and spring rain seasons. These were accompanied by discontinuity survey research,geographic information systems analysis, as well as aerial photography analysis toexplore the development mechanism of the cliff. Also, these were collated withprevious studies and textual evidence about the Yushan area left by others. Finally,the study estimates the future evolution of the collapsed cliff. The collapse mechanism of the Chinmentong Cliff is mainly affected by the mainstructure of Chinmentong fault, causing the rocks to be shattered, in particular, insaddle Area D, high joint density is accompanied with the development of the fault. The tensile fractures caused by the action of releasing the pressure, and thecleavage of the slate, all make, the rock composition of the Chinmentong cliffspecial. The pebbles are a conglomerate type like gum combed through the hair.With the help of the external effects, such as rain erosion, heavy rains, typhoons,earthquakes, repeated hot and cold (or dry and moist), and frost, the headwarderosion at the Chinmentong Cliff is vapid with a rate of 5.75m/year to 8.89m/year ina southerly and southwesterly direction. The surface area of the collapse hasexpanded 2.5 times in 59 years. It is estimated that in 3-5 years, it could reach thewatershed, and initiate the river capture (the Chenyoulan River capturing theLaonong River). In about fifteen years will affect the Yushan to Patungkuan trail.the trend of its collapae is to move with increasing speed toward the north peakridge line of Yushan. Ultimately this will affect the whole Patungkuan area andeven the shape and appearance of the Yushan mountain range.