Geological structure on the Pliocene paleosurface level in the nowadays 1000 m altitude were reconstructed. The lithological and tectonical circumstances in that time played a decisive role in hydrographic net origin and also on Kanomljica, Nikova, Idrijca and Belca rivers piracy. Landscape south and NW from Idrija fulfil all conditions to become classical region for initial karstification stadium examination, for that reason to this problems great attention have been paid. Particularly system of cross alpine faults and fissures played significant role on karst phenomena origin. With potholes theory the origin of Wild Lake have been explained and pointed out, that some dolinas at least have been projected already in period when Cretaceous limestones were covered by worse waterpermeable rocks. Anticline structures proved as the best indicator for karst rooms presence. On article end the results of 60 complete silicate analyses of nearly all Idrija' s rocks in table shape are presented.

The paper discusses karstification along the thrust contact between dolomite and limestone at Idrijski Log and Koševnik (west-central Slovenia). Thrust contact induced the formation of a specific contact karst assemblage. Karst morphology is less strongly expressed within the dolomite, whereas the limestones are deeply karstified, with many dolines, shafts and sinkholes. Shafts in limestone and dolines in dolomite show genetic relationship with the thrust contact, whereas preserved fragments of phreatic features originated deep beneath the thrust plane.

With detailed geological mapping at a scale 1:5 000, beside general geological data also the structural and tectonical circumstances of the surrounding of Habečkovo brezno were recognised. The complicated tension deformations developed in compressive condition and the thrust contact are the main reasons for developement of hydrological and surface karst features. With kinematical reconstruction we recognised that 400 m deep Habečkovo brezno has a special structural and hydrological location.