The phyllite quarry located within the ‘Góra Miłek’ reserve in Wojcieszów is one of the numerous sites in the Land of Extinct Volcanoes where rocks from the early Paleozoic era can be observed.
In a small, artificial exposure, two rock varieties can be distinguished. The first of them are the phyllites found in the north-western part. Phyllites are metamorphic rocks formed as a result of metamorphism occurring over a large area but under relatively low temperatures and pressures. They have a characteristic greenish-silvery color and clearly visible “layering,” which, in the case of metamorphic rocks, is called foliation. The primary rock (protolith) for phyllites was fine-grained sedimentary rocks such as mudstones or siltstones.
The second rock present in the quarry is greenstone. Greenstones formed as a result of the metamorphism of volcanic rocks with a basaltic character. True to their name, they are greenish in color and stand out with a homogeneous structure – meaning the absence of layering.
Both phyllites and greenstones are of similar age and have undergone a similar geological history. However, due to different protoliths, which are the original rocks, phyllites and greenstones responded differently to tectonic deformations, as evidenced by observations in the exposed rock formations. Within the phyllites, small-scale folds with a height (amplitude) of up to 20 centimetres can be observed. These are the so-called kink folds which were formed as a result of tectonic forces. Since phyllites are a ductile material, deformation resulted in the folding of layers without interrupting their continuity. On the other hand, in greenstones, which are brittle, structures called boudins can be observed. These are formed due to the stretching of the brittle rock trapped between rocks with greater plasticity, such as phyllites. As a result, the brittle element experienced fracturing, giving rise to structures shaped like cigars, which are known as boudins. Both kink folds and boudins enable the determination of the directions and orientations of tectonic forces that led to their formation. Analyzing the shapes of these structures allows us to hypothesize that they formed during the same episode of tectonic deformation.
