Trou du Glaz
Northing: 45.317272° Easting: 5.851502° Altitude: 1,690 m
How to find it
The easiest approach to Trou du Glaz is from the Col du Coq. Simply follow the waymarked GR9 and the yellow sign posts. It takes about 45 minutes. When exiting from a pull-through, turn left along the path, and head down the zigzags at the first junction. If approaching from Perquelin, as is necessary if doing the through trip to the Guiers Mort without a second car, make your way towards the Guiers Mort, cross the stepping stones below the cave, and follow the obvious path which rises up to the cliff, and then ascends beneath it. This is mostly straightforward, but after about 45 minutes there is an apparent impasse. Here, look for an easy scramble of four or five metres up the cliff to gain access to a new traverse level which isn't immediately obvious. It takes about 1¾ hours.
The name Trou du Glaz (sometimes spelt as Trou du Glas) is thought to be derived from "glace", the French for ice, which survives all year in parts of the entrance porch area.
The earliest inscriptions found in the cave date back to 1769, and it was the subject of explorations by Édouard Martel and Robert Joly, amongst others, before it became the focus of interest for Chevalier's team between 1935 and 1947. During that time they established the connections with Grotte du Guiers Mort, Grotte Annette Bouchacourt, and P40. Since then the system has been the object of intensive exploration by successive generations of cavers, which continues today.
Trou du Glaz is at the heart of the Réseau de la Dent de Crolles, and there are many possible routes available. It is the starting point for a number of pull-through trips equipped with pull-through chains, pitch-head traverse lines, and way markings, which have been well described in route guides produced by Spéléo Secours Isère, the local rescue organisation. It is also the exit for equipped through-trips from P40, Pulpite, and Gouffre Bob Vouay, all on the plateau. All these through trips pass through the lower storey of the cave, which can be reached by two routes – the traditional route which descends the Puits de Lanterne, and an alternative which descends the Puits de l'Ogive. Hence, there are two versions for some of the through trips. The main routes staring from the Trou du Glaz are as follows:
Puits Lanterns - Puits de l'Ogive round trip
This is a fairly short round trip in the Trou du Glaz which descends the Lantern shafts, and returns through the upper storey via the permanently rigged Puits de l’Ogive. It allows a leisurely look around the entrance series, as well as a reconnaissance for pull-through trips. The route is fully described in a Spéléo Secours Isère guide (a loose translation by the author is available here – use at your own risk). The trip takes 2 to 4 hours depending on how much poking around is done, and requires 2*30 metre ropes. The route mostly follows large fossil passages.
Trou du Glaz - Grotte du Guiers Mort
Like the Berger and the PSM, the Trou du Glaz to Grotte Guiers Mort traverse is one that should be on every caver's wish list. There are now a number of links between the two systems, including the original route which was connected by Chevalier's team in 1941. This follows what starts off as a fossil meander from near P60 down a succession of shafts into the master cave. It is described in 'La Dent de Crolles et son Réseau Souterrain', pages 174-176, and also in 'À Travers le Karst', pages 115-119, albeit as a segment of the traverse from P40 to Grotte Guiers Mort.
The one most used now, and which is fully equipped with pull-through chains, traverse lines, and waymarkings is that which descends P36, and traverses into the fossil series exiting through the Réseau Sanguin into the Guiers Mort. It was first made possible in 1983. The height difference between entrances is 356 m, and the route involves abseiling about 175 m in a dozen pitches and covers over 2 km of passage. It also ascends a permanently rigged 40 m pitch. The traverse typically takes four to five hours, requires 2*40 m of rope, and is not weather dependent. It is used for commercial caving, so in season one can be pretty confident that the fixed ropes will be in order. It is fully described in a Spéléo Secours Isère guide (a loose translation by the author is available here, use it at your own risk). To find the parking area in Perquelin from Grotte du Guiers Mort, follow the path down from the entrance, and pick up the waymarked path.
Trou du Glaz - Grotte Annette Bouchacourt
This route was pioneered by Chevalier's team, with the link-up between the two caves finally made in 1946, and it follows a large fossil phreatic passage up-dip through the heart of the mountain from the western cliffs to the south-eastern cliffs. Originally the water ran down dip from Grotte Annette Bouchacourt to Trou du Glaz with four major phreatic lifts which need to be descended. The two entrances are more or less at the same altitude.
It is probably the easiest of the pull-through trips in the Réseau de Dent de la Crolles, and it provides an excellent introduction to the system. The route covers about 2,300 m, and although there is but 12 m difference in height between the entrances, it involves abseiling about 130 m in seven pitches. The pitch heads are all equipped with pull-through chains and traverse lines, and the various traverses encountered en route are also equipped with traverse lines. There is one upward pitch which has a fixed rope on it (it can be free-climbed if necessary). The route is used for commercial caving, so in normal season one can be very confident that the fixed rigging will be there. It typically takes about four hours, requires 2*40 m of rope, and is not weather dependent. Older accounts highlight the danger of a mobile boulder choke in the Annette entrance passage, but this was stabilised in 2011 with some serious iron work (see photograph below). The traverse is fully described in a Spéléo Secours Isère guide (a loose translation by the author is available here – use at your own risk). The route is also described, with a specially drawn survey and with photographs, in an informative article by Serge Caillaut in Spéléo Magazine 107 (2019) – more details available on bone fide request.
To return to Col du Coq, traverse north from the entrance for 100 metres, and find a route that picks its way down the slope for 30 metres to intercept a path which contours below the summit cliffs of the Dent de Crolles, to the Pas de l'Œille path. This joins the ascent route at the top of the zigzags.
Trou du Glaz - Grotte Chevalier
Although the enormous Grotte Chevalier was first explored in 1942 by Chevalier's team, it wasn't connected to the main system until 1984, and then from passages climbed into above P36. The first half of this traverse is the same as for the Glaz-Annette. It is a little more challenging than the Annette traverse, and features the superb 50 m Puits Maurice and a tensioned traverse above a 30 m shaft, eventually dropping into the end of Grotte Chevalier. It covers about 3 km of passage, and although there is but 27 m difference in height between the two entrances, it involves abseiling about 210 m in eleven pitches. The traverse requires 2*55 m of rope, and is not weather dependent. The traverse is fully described in a Spéléo Secours Isère guide (a loose translation by the author is available here – use at your own risk). To return to Col du Coq, traverse south from the entrance for a few metres, and find a route that picks its way down the slope for 30 metres to intercept a path which contours below the summit cliffs of the Dent de Crolles to the Pas de l'Œille path. This joins the ascent route at the top of the zigzags. Note that reports indicate that the entrance area of Grotte Chevalier has become very unstable and may well be blocked, but an alternative exit is available. See the Grotte Chevalier page for more details.