In the block of Callus, certain levels of chalk are transformed into dolomite, or more exactly into chalk dolomitique, that is a rock containing crystals of dolomite, mineral of chemical formula CaMg (CO3) 2, thus a carbonate doubles of calcium and magnesium. These dolomites are said secondary sectors, because the dolomite did not precipitate directly from the sea water but she replaced gradually the calcite or the initial aragonite. Half of the ions That2 + Is replaced by ions Mg2 +. The substitution of a low part of ions Fe2 + In the structure give a brown tint to the crystals of dolomite, it is this property which is held here for the identification.
Two levels dolomitiques were spotted:
- An indicated level by "dolomite of Etretat", approximately 1 m under the marl Southerham;
- A level in the limit Turonien / Coniacien.
The training of these levels dolomitiques is detailed here.