Shale (in German: Schiefer) is the name used for a sedimentary rock made of… mud (actually a mix of clay and some other minerals like quartz and calcite). The fine-graded rock can be broken into parallel layers and it is often used in Germany for covering the roofs of houses in mountain areas.
House covered in shale rock (source: Wikipedia)
During out trip to Harz we met plenty of houses like this but also natural formations where we could observe the rock outcrops in their natural beauty. The Okerstausee lake is for example surrounded by such formations. On several places, the rock was weathered and transformed into small fragments of sand-like size so I just took a small sample for my Sand Atlas (sample no. 2295). The yellow color is actually clay brought by the tidal action onto the lake shores.