In mountainous regions comprising weathered soils and rocks, it is important to delineate the spatial distributions of geomaterials with different weathering and decomposition grades in depth. This paper presents a methodology to cost-effectively and accurately identify the subsurface zones of volcanic weathering. The methodology consists of in situ digital monitoring and a number of associated data analysis methods. The in situ digital technique is the drilling process monitor. It can automatically, objectively and continuously measure and record parameters associated with the full drilling process of a rotary-percussion drilling machine when it is being used to drill a production hole (such as probe holes and soil nail holes) in the ground. The data analysis methods are used to identify relevant factual data from the full drilling process data for zoning. The calculations are simple and straightforward tasks. Factual data presented in the paper illustrate that the proposed methodology can be a simple and cost-effective tool to record and utilize by-products of current normal drilling practice for ground investigation, geotechnical design and verifications, drilling construction management, as well as drilling quality control. The proposed methodology offers great potential in furthering the geotechnical knowledge and engineering practice in Hong Kong and many other mountainous regions comprising weathered soils and rocks. There is also significant potential for other applications in civil and mining engineering through estimation of the mechanical properties of rock from the monitored drilling data.