Alum Shale

The Alum Shale is a black, organic rich shale formation exposed across parts of Sweden, spanning the Cambrian through Ordovician periods and locally more than five hundred million years old at its base, historically mined for alum and, more recently, for uranium and oil shale, while also preserving trilobites and other early marine fossils, including the well known Orsten-type deposits within it that preserve microscopic animals in exceptional three dimensional detail. The formation is one of the most extensively studied Paleozoic rock sequences in Scandinavia.