Fig. 6

Clay resources map (1:530′000) by Letsch et al. (1907). The map is shown in its full extent in order to get an overview of the areas that were explored the most. Details are not readable at that map scale. See the full resolution of this map in Additional file 1. The star-, cross- and circle-like red symbols represent occurrences of different types of clays, marls or brickearth. The fat dashes mark brick factories. Note that most of the occurrences, i.e. mining sites at the beginning of twentieth century were located in the Molasse Basin and in the Jura fold-and-thrust belt