Przegląd Geograficzny (2022) vol. 94, iss. 1

Rock mushrooms in Poland and the world – terminology, distribution, views on evolution

Filip Duszyński, Piotr Migoń

Przegląd Geograficzny (2022) vol. 94, iss. 1, pp. 5-30 | Full text

Various shapes are taken on by the bedrock outcrops emerging from regolith cover, typically described using the generic term ‘crag’ or ‘tor’ (the latter term most frequently in relation to granite outcrops). Consequently, specific terms have been proposed to account for this variety. Among these outcrops are those consisting of a narrow lower part (stem) and a wider upper part (cap), resembling a mushroom. In English, they are named pedestal rocks if built of hard, well-lithified rock; or hoodoos if the rock is softer, but there is no established boundary line between the two. Not uncommonly, however, and perhaps less formally, they are referred to as ‘rock mushrooms’ (or mushroom rocks), to emphasise the unusual shape. In Polish, the term equivalent to ‘rock mushroom’ has been used at least since the 1930s, and appears to be a legitimate part of geomorphological vocabulary. In this paper, we present the occurrences of rock mushrooms in different lithologies, identify geological controls and review various hypotheses regarding their origin and evolution.

Rock mushrooms are known from various lithological settings, although some bedrock types clearly favour their origin more than the others. First of all, these are sandstones and conglomerates. Rock mushrooms are up to 10 m high, whereas height proportions between the stem and the cap vary, resulting in a great variety of specific shapes, from large monoliths on low (<1 m) pedestals to tiny caps on top of slender stems >5 m high. In Poland, the north-eastern part of the Stołowe Mountains abounds in rock mushrooms developed in Upper Cretaceous sandstones; but they also occur in other parts of the Sudetes, where Cretaceous sandstones crop out; and in the flysch Carpathians. Specific variants of rock mushrooms in clastic rocks include those related to non-uniform silification of sandy sediments (e.g. Fontainebleau Forest, France) or secondary ferruginisation of sandstone beds (e.g. Kokořinsko, Czechia). Rock mushrooms are also known from limestone and dolomite terrain, with the massive forms in Ciudad Encantada, Spain, being probably the tallest known from literature. The latter are up to 15 m high and have developed within a dolomite succession. Heights above 10 m are also attained by rock mushrooms in the volcanic succession of Cappadocia, Turkey, locally described as ‘fairy chimneys’. A great variety of shapes are documented from this region, with conical caps being very common. Granite rock mushrooms are comparatively rare, and in this case a clear distinction between the stem and the cap usually proves difficult. More commonly, the outcrops assume a shape resembling the letter ‘S’ or ‘Ω’, with basal undercutting grading smoothly into a wider upper part. A specific term ‘flared slope’ has been proposed to account for this basal concavity. Finally, cap-on-stem situations typify eroded glacigenic deposits, best known from the Alps, where boulders embedded in till or outwash sediments provide a protective cap to the underlying mass. As the overall shape is often conical, the term ‘earth pyramid’ is used in some languages (e.g. Polish and German).

Three main types of geological control may be identified for rock mushrooms. Relation to rock layering is most evident, with a more-resistant bed supporting the cap. Higher resistance may be due to lithological characteristics (e.g. sandstone over shale, ignimbrite over lacustrine silt) or structural differentiation (e.g. various density of bedding, changes in primary porosity, homogeneous over thinly bedded sandstone, different degree of welding in ignimbrites, non-uniform silification or ferruginisation). Another group arises in situations of more-resistant elements being distributed without any order, as within glacigenic deposits. Consequently, whereas in the former cases it is possible to observe caps of adjacent rock mushrooms at the same level, no comparable patterns exist in the latter. The third group includes rock mushrooms not showing evident rock control, and of origins relating primarily to greater efficacy of rock disintegration in the lower part of the outcrop.

Rock mushrooms have more than one origin, and many can in fact be polygenetic. In each case, however, rock disintegration is clearly more efficient in the basal part. The reasons for enhanced efficacy at this point vary, and include: (a) aeolian undercutting – this view prevails in primary and secondary geographical education, even as wind-abraded rock mushrooms are by no means the most common examples; (b) differential weathering related to lithological or structural heterogeneity of rock, even as the exact mechanisms of weathering may vary; (c) subsurface (subsoil) weathering (etching) leading to the (e) overland flow and gully erosion – these processes are fundamental for rock-mushroom evolution in poorly-lithified deposits; (f) negative feedback between stress and erosion on exposed bedrock outcrops.

Rock mushrooms are thus good examples of geomorphic equifinality, with the consequence that any a priori generalisations regarding their origin may prove misleading. They are also good illustrations how both substrate (rock or sediment) and process shape landforms. So it is that these not only have scenic value (as “natural curiosities”), as has been recognised for many years now, and indeed emphasised in the context of tourist activity; but also considerable educational value to be taken advantage of in both geoeducation and geotourism.

Keywords: skalne grzyby, ewolucja rzeźby, wietrzenie, geomorfologia strukturalna

Filip Duszyński [], Uniwersytet Wrocławski, Instytut Geografii i Rozwoju Regionalnego
Piotr Migoń [], Uniwersytet Wrocławski, Instytut Geografii i Rozwoju Regionalnego