Canche aux Merciers car park — Croix Saint-Jérôme 2.7km
From the end of Canche aux Merciers car park, near the information boards, the Canche aux Merciers footpath, a wide, grey sand path, leads due west towards the site of the same name. Follow signs for "Départ promenade de la Vallée d’Arbonne" until you reach the Carrefour de la Charme crossing. Located 300 metres from the car park, the site of Canche aux Merciers is very popular because, in addition to a large "sandy beach", there are many rocks where children and adults alike can practice rock climbing.
At the crossing (1), turn left onto the Chemin de la Charme, which runs alongside the Canche and and has yellow waymarks for the Vallée d’Arbonne walk. Follow the path until it reaches the Maison du Poteau crossing. At the Maison du Poteau crossing, leave the yellow waymarks and carry straight on, still on the Chemin de la Charme between forest plots 116 and 115. After 400 metres, at the start of the descent, the path crosses the Denecourt-Colinet n°16 footpath, which is waymarked in blue. The white sand gradually becomes a more and more striking feature of the landscape around you especially to the left of the path. Another 450 metres further on (2), the path crosses the "25 Bosses" (25 Bumps) footpath (red waymarks): stay on the Chemin de la Charme which runs between forest plots 116 and 117.
Before long, you will see the red waymarks of the 25 Bosses footpath (3). From here you will follow the red waymarks until the path descends from Rocher du Guetteur. In the meantime, you will first go past the houses of the neighbourhood of Croix Saint-Jérôme.
Croix Saint-Jérôme – the Sables du Cul du Chien: 3km
This is the hardest part of the walk as you will now climb 4 “bumps” or hills which are each about 50 metres high and have steep slopes covered in rocks that the path sometimes winds between and at other times takes you straight over.
The first hill is called Roche au Four, and it has beautiful views of the region and above all of the next series of hills, the 3 Pignons, to which the Massif des Trois Pignons owes its name, and which are actually 4 in number.
The path goes quickly down and crosses the wide track of the Vallée Close (4). Then there is a climb up to Rocher de la Souris, immediately followed by a rugged descent, and then another climb up to Rocher de Jean des Vignes. The following hill, the Rocher du Guetteur, is a little further away and the path makes a long loop to get there. The climb is steep here too. When you reach the top, there are two points of interest: a kind of chalice surmounted by a cross carved into the sandstone and, just beside it, a strange rock resembling a seat, hence the name of the Rocher du Guetteur (the Watchman’s rock) given to the hill. There are beautiful views from here over the forest and the Sables du Cul du Chien. Once again, the descent among all the rocks is tricky.
When you reach the bottom (5), leave the 25 Bosses path and instead follow the unmarked path to the left. This path winds gently down through the pine trees until it reaches the crossing with Chemin du Mont Pivot. Take the left-hand fork and follow the path between forest plots 136 and 137. The path will take you around the bottom of Rocher du Guetteur before arriving at the Chemin des Sables du Cul du Chien. At the crossing, turn right onto the Chemin des Sables du Cul du Chien. Leave the hills you have climbed behind you, cross the wide track of the Chemin de la Plaine de Jean des Vignes (6) and go straight until you reach the famous "sea" of white sand. This really is one of the most beautiful places in the Trois Pignons forest!
The Sables du Cul du Chien - The Diplodocus: 1.4 km
As the stretch of sand is vast, it is not easy to find the path again to carry on as there are no waymarks (hence the importance of having an IGN map and a compass with you).
So, when you get to Les Sables, the easiest way is to walk along the sandy beach on the right to see one of the strangest rocks in the forest, the Bilboquet. The land rises slightly as it reaches some rocks and birch trees which mark a kind of separation from a second large stretch of sand. At the entrance to the second beach on the left, with a wooden fence around it, is the Bilboquet. It resembles the shape of a dog’s head. Erosion, and above all, rock climbing (now prohibited) have worn it away somewhat. There are other beautiful rocks in the nearby area, including one that looks like an elephant's head, but it is hard to find: keep walking for another 60 metres to the southwest, with the Bilboquet behind you, and then turn left onto a kind of wide sandy path that leads between two birch trees to find the elephant, which is about fifty metres further on.
To leave the Sables du Cul du Chien, walk along the edge of the sandy beach to the southwest with the Bilboquet behind you until you find, 160 metres further on, the trace of a path (7). Turn left. The Chemin de la Roche aux Sabots, marks the boundary between forest plots 143 and 144 and runs around the rocks of the Sables du Cul du Chien from the south. When you reach a kind of T-junction, take the path to the right (the path to the left takes you to a large sandy slope) until you reach the Ancien Chemin de Melun (8). Take the path opposite you with white waymarks, which marks the boundary between forest plots 144 and 145. The path leads to the Rocher du Général, where it joins the Chemin de la Plaine de la Mée. Turn left onto it and carry on until you reach the crossing with the Chemin du Rocher Fin. Turn right and follow the path for about 200 metres. The Diplodocus is not far away (on the right of the path). Again, this is a wonderful rock climbing site with impressive and remarkable sandstone blocks, especially the huge standing stone known as Diplodocus.
The Diplodocus – The Tortue: 1.3km
Walk out from the rocks due East, following a branch (or variant) of the 25 Bosses path, which serves as a shortcut (discrete red and white waymarks): cross the Chemin du Rocher Fin, and then the Chemin de la Mée and, 70 metres further on, you will reach the Chemin de Melun au Vaudoué which runs between forest plots 148 and 149 (9). Take the left-hand fork and follow this path for 500 metres until you reach the crossing with the 25 Bosses footpath. Turn left onto the 25 Bosses footpath (red waymarks) and after 300 metres you will reach a rock resembling a turtle: the Rocher de la Tortue (10).
The Rocher de la Tortue – Canche aux Merciers car park: 3.6km
Retrace your steps until you get back to the crossing with an unnamed path on the border between forest plots 128 and 148, then take this path to the right. The path eventually joins the Chemin de la Mée, which you then take north. A little further on, the GR1 joins in from the left. Follow the path with its very characteristic red and white waymarks for 1.6 km until you reach the Sapeurs du Génie crossing. In the meantime, you will leave Chemin de la Mée to climb up onto a beautiful plateau covered with pine trees and heather after crossing the 25 Bosses footpath once again. At the Sapeurs du Génie footpath (12), the GR1 goes off to the right. Take the opposite direction and carry straight on for about 250-300m before taking a sharp right onto a barely visible path with worn white waymarks, which separates forest plots 111 and 112. You will soon reach the edge of the plateau. The path goes down into a kind of ravine to reach the valley floor and the Chemin de la Vallée d’Arbonne, which you go straight over. You will reach the end of the Vallée d’Arbonne walk at the yellow waymark (1), which you began at a few hours earlier, and you can make your way back to Canche aux Merciers car park.
