From OpenStreetMap Wiki
This is used to describe the geological makeup of an area. See the page Geological for an introduction on its usage.
|geological||moraine||A moraine is any ice formed accumulation of unconsolidated glacial debris (soil and rock) which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by glacial ice.|
|geological||outcrop||An outcrop site is a place where the bedrock or superficial deposits have become locally exposed and are directly accessible to analysis.|
|geological||palaeontological_site||A palaeontological site is a place (or group of physical sites) in which remains of ancient living forms is preserved. Area under the study of the forms of life existing in prehistoric times, as represented by the fossils of plants, animals, and other organisms investigated using the discipline of palaeontology.|
|geological||user defined||All commonly used values according to Taginfo|