In the southwestern part of Taiwan, most foothills are covered by mudstones. The mudstone, formed during Miocene to Pliocene and Pleistocene, is a young and weak sedimentary rock. When subjected to water, slaking of the mudstone can easily take place and in dry seasons, severe shrinkage can occur. Thus, erosion of mudstone slopes is easily induced by rainfalls. Heavily eroded natural mudstone slopes in southwestern Taiwan frequently become "bald" (i.e. not covered by any plants or bushes) and are very susceptible to geologic damages such as mudflow and slope failure. One interesting observation of the mudstone slopes in southwestern Taiwan is that most of the bald slopes are south-facing. The causes of this directional variation are thoroughly examined in this paper. It is concluded that solar radiation, rather than the direction of typhoons, is the main reason for forming bald slopes in the south-facing direction.