## 1 Introduction  
The hydration forms of relief described in this paper are rare morphological forms appearing in the weathering zone of gypsum-anhydrite rocks [1, 2, 3, 4, 5, 6, 7, 8, 9]. They are more or less convex in shape and reach up to 2-3 m in height and over several meters in lateral extension. Their origin is related to the phenomenon of volume increase during hydration of exposed anhydrite rocks under the influence of surface and subsurface waters [10, 11, 12] (and further references in [13, 14]). During this process, which can be called expansive sypsification of anhydrite [14], the anhydrous calcium sulfate (the mineral anhydrite; CaSO\({}_{4}\)) transforms into calcium sulfate dihydrate (CaSO\({}_{4}\):2H\({}_{2}\)O; gypsum) according to the reaction: CaSO\({}_{4}\) + 2H\({}_{2}\)O \(\rightarrow\) CaSO\({}_{4}\) 2H\({}_{2}\)O. This reaction can lead to volume increase only if the system is open, i.e., when the water is added from the outside [11]. The term hydration form was first used by [PERSON] et al. [3] (p. 6) for characteristic small-scale forms of relief \"unique to karst developed on anhydrite\" when \"anhydrite hydrates and is changed into gypsum\" and \"experiences a 1.557 times increase in volume.\" They described hydration polygons, domes and crusts in Tripolitania province in Libya. Earlier, hydration domes with internal hydration caves were recognized in the Harz Mts. region in Germany [15]. The majority of the known hydration forms are created as a result of the localized hydration of the anhydrite substrate, leading to volume expansion and local uplift of the surface layer of the rock split of the substrate. These forms commonly show a narrow empty fissure or cavity inside [6; 16; 17; 18], rarely are they massive [3; 19]. Some empty cavities reach sizes large enough to be called caves. They are up to 1.5 [5; 7] or even 2.3-2.4 m high [4; 20], providing the possibility for an adult man to enter inside. The name hydration caves or swelling caves (_Quellungsholen_ in German) was proposed for them [15; 21; 22; 23; 24]. Causes of this type are very rare in the world [25; 26; 27]. The convex hydration forms of the relief were apparently created by expansion of the rock material taking place locally, very close to the surface. According to [PERSON] and [PERSON] [17], the shape of the forms is generally a product of eccentrically acting compressive stress within the localized hydration zone. The differential expansion greater at the surface than in the rock interior causes detachment of the surface layer of the rock and its uplift, leading to empty dome- or tepee-like hydration forms, including pressure ridges. The volume expansion of the rock material is led by displacive crystallization of secondary gypsum, exerting crystallization pressure on the surroundings [14]. It has been recognized that these processes and expansions leading to the growth of the forms take place not only within the uplifted layer but also in the nearest vicinity of the forms [4; 28]. The hydration forms of relief have a specified origin, shapes and sizes, and occur as discrete objects on the Earth's surface. Thus, they show the basic features defining landforms [29; 30; 31], and as such, they can be a subject of geomorphometric analysis [30; 32; 33; 34; 35]. Hydration landforms may occur in groups, creating a rare type of morphology, which can be called a hydration landscape. The discussed forms (as well as the similar weathering forms of the relief--gypsum tumuli [36; 37])--can be considered a unique type of landform characteristic of sulfate karst [38] (pp. 125-126), yet not satisfactorily recognized and known. Occurrences of hydration forms and caves have been documented in many places around the world, both in natural (e.g., Woodward Co. in Oklahoma, Ottawa Co. in Ohio, Culberson Co. in Texas, and Eddy Co. in New Mexico in the USA, Alberta in Canada, and Tripolitania in Libya) and artificial exposures of anhydrite rocks [3; 4; 15; 16; 17; 19; 22; 39; 40].