Deciphering the origin of allochthonous sandstone boulder trains within a mudstone escarpment, Stołowe Mountains, SW Poland
Parzóch, Krzysztof; Migoń, Piotr
published: Jan 1, 2015
ArtNo. ESP023105901013, Price: 29.00 €
Two trains of allochthonous sandstone boulders occur within deep ravines incised into a steep mudstone escarpment in the Stołowe Mountains tableland, SW Poland. They comprise 99 and 47 boulders longer than 1 m respectively and extend for 300–800 m. In the northern, longer train, boulders are distinctively clustered, while clustering is much less evident in the southern train. No consistent differences in rock hardness have been recorded within the northern train, indicating similar weathering history. No sandstones cap the escarpment but a further group of residual sandstone boulders dots the plateau above it which is considered to be the source of boulders. The origin of the trains is complex and unusual, and does not fit any of the three models present in the literature, emphasizing in situ mechanical breakdown and transport under periglacial conditions, selective erosion of blocky regolith, and rock fall from caprock, respectively. Boulders on the plateau are probably remnants of a long eroded sandstone mesa. Long-term retreat of the mudstone hill slope undercuts the plateau, including the remnant boulders, which begin to creep or slide down the slope, towards the bottoms of the ravines. Here they accumulate and remain stable, until they are undermined by ongoing fluvial erosion or break down due to associated tensile stresses. The subsequent movement of boulders leads to their wider dispersal along the valley floor, to form the trains. This scenario of boulder stream origin has few if any parallels in the literature.