Trou de l'Alpe

Trou de l'Alpe is a thin fissure cracking the Urgonian limestones, opened in a meander loop that tears the surface lapiaz by a first vertical drop 60 metres high. The hole is a succession of large and wide shafts. Essentially vertical, this curious network is formed of successive shafts ending at 243 metres depth. At the end of the pit, at -243m, the constrictions of the meander block passage. The bottom, carved in a rock of lesser quality, presents no great interest. This vertiginous pit whose foundations remain defended by impassable narrowings counts among the most remarkable known on this mountainous patch, a veritable isolated balcony particularly lacerated by the faults that torment the Urgonian rocks where tenacious pines twisted by the winds cling.

Concretions sit on the walls around which it is possible to suspend the ropes. Despite the few concretions, the bottom presents no great interest.

Downstream, the physiognomy of the limestone is overturned, giving way to a tangled network of crevasses that absorb the local runoff. This curious network appears to constitute a window onto ancient underground circulations surely to be related to Trou de la Rousse, an ample cavern opened further East on the Grésivaudan and perched at the base of the cliffs of the promontory of la Rousse. However, the current bottom of the abyss remains unquestionably lower in altitude, revealing in reality the progressive migration of flows toward the South-West in the direction of the grotte du Mort-Rû, which moreover drains vast wild expanses between Roche Blanche, le Fourneau and l'Alpette des Dames. This relation was moreover proved by a tracing with uramine in 1979 by a geologist of the Institut Dolomieu de Grenoble. The cavity is traversed by a clear air current, betraying however a mysterious relation.

A small ledge of blocks at four metres depth allows one to gain a foothold in a fold of the entrance passage in order to equip this fine abyss as well as possible. Below, a strong scree ramp leads to a narrow adjacent corridor defended by a vertical constriction at the foot of which, six metres below, the surroundings of a large shaft 75 metres deep widen out. Climb over the rocky step that protects the access to the pit; then from this ledge suspended over the void, arrange the appropriate ropes on the anchors renewed in the compact rock kneaded in places with rudists. This first vertiginous section allows one to stop at nearly 150 metres underground where only a crevice at ground level gives its passage onto the top of a new vertical drop nearly forty metres high. Gaining the base of a small shaft 7 metres high, a few stepped rises lead finally to the edge of a new vertical break, gaping into a shaft about ten metres high. Then comes a zone particularly favoured by fracturing from which several lateral ways escape, diverging toward the downstream of the network.

Research carried out by speleologists on the eastern summits rising to the South-East of the massif de l'Alpe, just above the vallon de Pratcel, and bounded by the powerful cliffs of la Rousse, have demonstrated the existence of a pit that once raised hopes of promising discoveries. Trou de l'Alpe certainly holds astonishing surprises in reserve, but given the absence of galleries, it seems quite hazardous to discover new extensions that could lead toward the grotte du Mort-Rû situated 850 metres lower and three kilometres away. The constrictions of the meander prevent explorers from continuing their course downstream in the direction of the grotte du Mort-Rû: a cavern highly renowned for the gigantism of its entrance hall and for the extent of its labyrinthine network.