Such derivative maps fairly represent the properties on a landscape formed on a particular mixture of rocks over time. This expert approach to environmental interpretation of geologic maps has been tested and used successfully on the Columbia River Basin Ecosystem Management Project, USA, and for management planning in the Lake Tahoe basin, California. This poster utilizes recent geologic mapping in the Tahoe-Donner Pass area to briefly describe the ArcGeology data model and to demonstrate the evolving expert system.