GEOLOGICAL CHARACTERISTICS
This tertiary “coastal” conglomerate is different from that of Grasbergde Rorschwihr or Stegrebende Rorschwihr. Indeed, it contains pebbles of sandstone limestone from the lower Dogger (upper Aalenian) which outcrops upstream; this sandstone, ferruginous, brick-red, often oolitic limestone is always rich in iron oxides which are found to be very resistant to erosion. This typical facies, relatively fossiliferous in ammonites (Ludwigia murchisonae and Ludwigella concava) and lamellibranchs is often reworked into large blocks or slabs in the conglomerate. On the north-eastern flank of the Kugelbergde Rorschwihr, the conglomerates contain limestone pebbles from the lower Lias (Sinemurian). This shows that erosion had already reached this level after having cleared the Bajocian, then the Aalenian and the Pliensbachian. These conglomerates based on lower Lias are therefore younger than those built solely on the basis of Oolitic Dogger (principle of reverse sedimentation). These conglomerates with altered liasic fauna: grypheas, rostrums of Belemnites, ammonites (Amaltheus, Pleuroceras) and nodules or ferruginous debris of ovoid marls, are located below the land of Lias, at the northern end of Kugelbergde Rorschwihr. In the whole of the Kugelbergof Rorschwihr, it is nevertheless the limestone pebbles of the Grande Oolithe du Dogger (Bajocien) that predominate. The source rock outcrops at the top of the Kugelbergwhere the fissured karstic limestone beds, altered on the surface, support veneers of the siderolithic Eocene (lower and middle Eocene). These remains of associated continental ferruginous deposits, or freshwater limestone media, are very rare. They are the result of Cretaceous-Tertiary tropical climate change.
This geological overview is taken from a private study carried out by the INSTITUT DE GEOLOGIE DE L’UNIVERSITE LOUIS PASTEUR DE STRASBOURG (Claude SITTLER and Robin THIRION) at a scale of 1/5000 on behalf of DOMAINE ROLLY-GASSMANN.