In the tunnels you can observe an approximately 25 cm thick layer of flint in the layered limestone. This is the so called layered flint that you can see at 1 metre intervals. The layers of flint are sloping slightly towards West because the limestone was more powerfully pushed up on the eastern side. At some places you can see plates of flint crossing the layers. This part of the flint is filling up the cracks in the limestone.
The layered flint was formed at the same time as the limestone. It is probably the result of e very complicated process on the former sea floor. When the limestone deposited, sponges with a silicon skeleton were living on the sea floor. The silicon is unlike limestone stable under acidic and soluble under alkaline conditions. The depositing of limestone algae was an ongoing process and the silicon skeletons of the dead sponges dissolved because of the alkaline conditions. The silicon was moved up by the water until the solution reached the acidic upper part of the mud layer. Here the silicon crystalized into microscopic quartz crystals which today can be found as layers and lumps of flint.