Geographically, Thrace is a region located in southeastern Europe within the territories of Greece, Bulgaria and Turkey. In Turkish Thrace, karst occurs extensively in Eocene limestones, although some limited karst occurs in marble of the metamorphic series of Palaeozoic age. The karstification base is shallow to very shallow and most of the dolines and poljes have been captured by surface streams. Subsurface drainage has been changed to surface drainage in most parts of the region. Caves and cave relicts are concentrated mainly at three different altitudes, and almost all caves are horizontal or sub-horizontal. With these characteristics, Turkish Thrace hosts a distinct type of karst compared to that of other regions of Turkey, and particularly to the well-developed active Taurus karst.