Deep structures of slopes connected with sandstone crags in the upland area of the Świętokrzyskie (Holy Cross) Mountains, Central Poland

•Deep-seated slope deformations in low gradient relief and siliciclastic-clayey rocks - unique or not sufficiently recognised?•Manifestation of DSGSDs in low and gentle slopes of upland area - the challenge for application different study methods.•Slope activity in the Quaternary - the problem still not appreciated in the morphogenesis of the Central European uplands.•Presence of polygenetic slope structures in the horizontally oriented geological strata.

Deep seated gravitational deformations of slopes are commonly observed and described in mountains, but faintly manifested and thus neglected in upland areas. Thanks to the geophysical methods (e.g. ERT) as well as detailed surface surveys and studies in caves, such deformations have been discovered and described in the region of the Świętokrzyskie (Holy Cross) Mountains, Central Poland, which is an upland area of usually gentle slopes of a height of several tens of metres. The deformations, which have been studied on 20-30-m-high slopes formed of sandstone series alternating with claystone, siltstone and heterolithe series, represent various movements: from slight rotational and translational (ca 1 m) movements, through horizontal and vertical shifts of several metres to significant dislocations of sandstone 'corestones' embedded within the regolith forming the slopes. The last mentioned structures are currently relics, completely masked by subsequent slope processes and their origin should be related to specific periglacial and even sub-glacial conditions of the Pleistocene. On the other hand, as confirmed by radiocarbon dating and other mapping, slight movements of relatively low slopes can even be active nowadays. These deformations that occur in regions of a relatively low relief and have so far been neglected or considered to be unique are possibly even more frequent and should be studied using modern geophysical and other methods.