Martinguinda

Cavity of tectonic origin located right at the contact between the Lutetian limestone materials and the rounded and angular cobbles of sandstone origin of the Lutetian. The karst is cut by a joint running north to south, and the pit lies in the highest part of the joint, at the contact. The mouth is narrow and oval, with the western side depressed by the presence of a large limestone block.

The mouth leads to a vertical shaft of 9 metres, extremely unstable and dangerous. After a narrow and delicate passage beneath an enormous block, one reaches a 4-metre step that leads to a gallery in the form of a descending joint about 2 metres wide. This gallery leads via a ramp full of angular clasts to a new vertical step of 5 metres, in a zone narrower and more depressed than the rest of the main gallery. After this step the gallery becomes narrower and closes in an impenetrable fissure through which a strong current of cold air sometimes blows.