From the Kilian Crater to the Temple of Mercury, the trachyte route

In Laschamps, park on a gravel area (carpooling area) on the west side of the village, at the crossroads between the D52 and D767 on the edge of the Col de la Moréno road. This car park is generally less busy than the "hikers" car park located a hundred metres away on the D52, on the other side of the crossroads.

No signposts

(S/E) With your back to the crossroads, immediately take the small road, Chemin Combeta (no vehicles over 6 tonnes), between the car park and the Laschamps exit sign, which branches off to the right (north-north-west). Ignore the street on the right and, further on, continue along the path that follows the tarmac road. When you reach the D942, cross with caution (dangerous road).

Pay close attention to the directions until you reach Col de Ceyrat, as this is an unmarked forest trail with several paths that are not shown on the map, while others are shown but no longer exist (GPS or Visorando app useful).

(1) On the other side, follow the path opposite, which is closed to motor vehicles and marked with a red arrow. Barely a hundred metres further on, just after a bend, be careful at a fork in the path: follow the one on the right, marked by a red arrow that is not very visible (the path on the left does not appear on the IGN map). Join a wider path and follow it to the right (ignore the red arrow that invites you to turn left).

Shortly afterwards, ignore a red-marked path on the left, another unmarked path further on, and then another path on the left at a sign for "Saule Meursault". You will come to an intersection with a path going up to the left, marked with a red cross, and a red-marked path going down to the right.

(2) Continue straight ahead on the path, ignoring these paths. About 100 m further on, ignore a path on the right, then arrive at a fork in the path. Continue to the right and then ignore a path on the right just after. The path descends steeply. Arrive at a fork in the path (the one on the right is not shown on the map) and continue to the left.

(3) About 200 m further on, look out for a path branching off to the right between forest plots 5 and 6, marked with red numbers painted on the trees (if you miss this path, you will soon come to a path marked^GR®®). Follow it and, at the next fork (number 6 painted on a tree), keep to the right. Notice the deep ravine on the left, this is Kilian Crater, which the path skirts around its eastern edge, and which is unfortunately too wooded to be spectacular.
Ignore two more or less distinct paths on the left (I don't know if they lead to the bottom of the crater) and arrive at a road (D68).

Red and white^GR®® markings

Follow the road to the right (wide shoulders but remain cautious) to reach the Col de Ceysat. Turn left just behind the Auberge des Gros Manaux, climb a few steps, pass the memorial to the Ardents on the right, and follow the yellow directional signs (Col de Ceyssat marker).

(4) Follow the signs for Sommet du Puy de Dôme, which is the Chemin des Muletiers. Higher up, cross the old road and the Panoramique des Dôme railway line, continue on the path opposite which winds its way up to the summit and ends with a few steps. Go straight ahead, leaving the station on your right, and join the small road and the bottom of the steps leading up to the temple and the antenna, following the yellow directional signs.

(5) Climb the steps, pass to the right of the Temple of Mercury, continue uphill and then go around the station and the antenna, either by taking the shortest path, as shown on the trail, or by taking a short detour to visit the viewpoints via the Chemin des Curieux. Return to the small road at the bottom of the steps.

(5) Follow the small road and head towards Sentier des Chèvres. Further on, leave the road for Sentier des Chèvres, which branches off to the right and starts with steps. Further down, you will reach the Traversin crossroads, with its fountain displaying yellow directional signs.

(6) Turn left towards^GR®®4, Laschamps.

No markings

Turn onto the first path on the left after leaving the^GR®®. Continue through this tunnel of greenery and further on, you will reach a path with^GR®® markings and directional signs.

Red and white^GR®® markings

(7) Follow it to the left towards Col de Ceyssat to go around the Puy de Dôme to the west. Ignore a hairpin path on the right (not shown on the map), then a path on the right (at the directional signs, follow Col de Ceyssat), then finally return to the Col de Ceyssat signpost and the intersection you passed on the way there, at the foot of the Chemin des Muletiers.

(4) Join the D68 and follow it to the left with caution. After about 100 metres, at the bend in the road, at the directional signs, turn right and follow the red and white^GR®® trail towards Laschamps. At each intersection, follow the clearly visible markings, but remain vigilant as there are many trails, some of which are not shown on the IGN map, and a few turns that are easy to miss. You will come to the D942.

(8) Cross the road, taking care, and follow the path opposite, which starts between two crosses. You will come to the D767a. Follow it to the right, taking care.

No markings

(9) Turn onto the first street on the right, Chemin de la Vialle, at the corner of the cemetery, then turn left at the next intersection to return to the car park (S/E).

Medium difficulty, rather difficult for non-mountaineers, due to the steep climb up the Chemin des Muletiers (but no technical difficulty). Standard hiking equipment required.
The fountains at Col de Ceyssat and Traversin are turned off in winter.
Exploration of the Kilian Crater is at your own risk; I have not yet tried it myself.
The Puy de Dôme is always very crowded, of course, so hiking is more enjoyable outside the summer season.

Discover more hikes in this area with a description or the Visorando app here and more generally in the Puy-de-Dôme here

Photo album here

The mystery of the trachyte of the Temple of Mercury

The Temple of Mercury was built in two successive stages, first in arkose (50 AD), then, as it became very popular, enlarged around 150 AD with another rock, trachyte (from the Greek trakus = rough). It was long assumed that the source of this stone was the Puy de Cliersou, whose quarries can still be seen, particularly from the summit of the Puy de Dôme (cave entrances).

However, during the storm of 1999, the forests covering the puys were partially devastated, particularly at the Col de Ceyssat, between the Puy de Dôme and the Puy des Grosmanaux, which we bypass on this hike. Here, we discover the remains of a large Roman city that was built with the same stones as the Temple of Mercury: the quantity of material used no longer corresponded to the volume of the Cliersou caves, which were far too small!

In 2008, Didier Miallier (a researcher at the UCA's Particle Physics Laboratory) was curious enough during a hike to descend into the Kilian Crater (a researcher who knows how to research must be curious!).

The Kilian Crater is one of the most recent in the Chaîne des Puys and one of the least known because it is hardly visible, buried in the forest and not even on a summit! This crater was formed by a very violent explosion on an ancient cinder cone (now the Puy des Grosmanaux) 9,300 years ago. At that time, the Puy de Dôme had already been formed for about 2,000 years.

So, Didier Miallier, our researcher, discovered a cave dug into trachyte similar to that of the Temple of Mercury at the bottom of the Kilian Crater (a researcher who knows how to search should be able to find something without luck... but it helps!).

Studies later confirmed that Kilian Crater had indeed been used by the Romans for their temple and their city, and later by builders in the Middle Ages. This explains the unofficial Latin-Greek term via trachyte in the title of this hike, as the route goes around Kilian Crater before reaching the Temple of Mercury, 400 m higher up.

Kilian Crater is heavily wooded, but you can tell it is very deep, and you can still make out the old quarry terraces that intrigued Didier Miallier at the time. There is no obvious path down to the bottom, but if any hikers discover a discreet but passable trail, I would be interested in the GPS track.

And what about the Puy-de-Dôme? It seems obvious that its name comes from the word "dome" (a dome-shaped mountain)... but no! It seems that it comes from the Gallo-Roman god Mercury-Dumias (one of the avatars of Lug/Mercury). Puy being a deformation of "podium", the Puy de Dôme would in fact be the podium of Mercure-Dumias, abbreviated to Podium de Dumias, or at least his temple. That said, the local name Dumias for the god Mercury probably comes from the fact that the "dome" (dome-shaped mountain) was the god's second home in the Arverni region!

More info here