The productive layer of the Estonian oil shale deposit as seen in the Põhja-Kiviõli II opencast mine. The productive layer is made up of oil shale layers A through F1 together with the limestone layers in between. However, in some cases, layers F2, G and H (visible in the image, but not labeled) are also mined. The oil shale beds are marked on the image with capital letters, limestone layers are labeled with a combination of two capital letters, indicating at their location within the productive layer. Dashed lines indicate layer boundaries, which are harder to distinguish and are thus approximations. Stratigraphy: the productive layer of the Estonian oil shale deposit is part of the Kiviõli Member of the Viivikonna Formation. The formation belongs to Upper-Ordovician Kukruse Regional Stage (global Sandbian Stage). (Source reference: Heikki Bauert and Olle Hints "XI Baltic Stratigraphical Conference. Abstracts and Field Guide", Stop 4: Põhja-Kiviõli II open-pit mine, page 85, figure 4.4).