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Parking is available in the car park for the nature trail, situated on a bend in the D58 road, near the Lavagot mill, between Maincourt-sur-Yvette and the church at Saint-Nom. Access is via the N10 and Le Mesnil-Saint-Denis, or via the N118 then the D906 via Chevreuse.
PNR signposts
Throughout the first part of the walk, the route is signposted by small posts from the Haute Vallée de Chevreuse Regional Nature Park (PNR), topped with a white metal plate bearing a black arrow indicating the direction to follow.
(S/E) Take the path that starts at the far end of the car park and heads east. At a fork in the road, ignore the red and white markings which suggest turning left diagonally and continue straight on towards a large information board. Referring to the map on this board, follow the green route and then the orange route.
(1) Shortly after the sign, turn right onto a wooden walkway over the marsh (there are several information boards along the way). Go round the reed bed on the right. Cross the Yvette via a footbridge. At the Pinguenet pond (sign), there is access on the left to a viewing platform above the water.
(2) Immediately afterwards, at a junction on the boardwalk, turn right. Further on, the boardwalk gives way to a footpath. At a fork in the road, turn left and step onto a boardwalk again. You’ll come to a T-junction (boardwalk on the left, dirt track on the right).
(3) Turn right onto the ‘circular’ path, which runs between two wetlands and crosses a few streams via wooden footbridges. Shortly after the information board about the mill, turn left level with a property and follow the path bordered by two fences. Pass through a forest gate and emerge onto a small street in Maincourt. Turn left (metal gate, red and white marker) and follow the old mill embankment, with the reed bed on your left.
(4) At the junction at the end of the embankment (PNR signpost), continue along the embankment, cross the Yvette, take an immediate left-hand bend in front of a green gate and pass through a forest gate. At a large information board (identical to the one at the start), ignore two paths branching off to the right. Immediately afterwards, pass a PNR signpost and, after a few more metres, reach another PNR signpost (a red and white marker on a tree, the size of a pole).
(5) Turn right onto a path that climbs amongst exposed sandstone boulders (follow the sign on the PNR post and ignore theGR®® markings). The slope is steep but the climb is short. At the top of the hill (large oak tree), turn left. Walk along the edge of the plateau, passing a beautiful viewpoint on the left and a first old sandstone quarry on the right. Further on, take a short detour to the left to the information panel facing a sandstone outcrop. Retrace your steps and continue westwards along the main path. You will reach a second old sandstone quarry (information panel).
Red and white + blue and white markings (sparse in both cases)
(6) At a fork in the road just past the old quarry, leave the nature trail and ignore the PNR signpost and the blue and white marker, which direct you to turn left. Take the path on the right, heading north (you won’t find any markings until a few hundred metres further on, but if you look back after the junction, you can see arrows for walkers coming from the opposite direction). At the next fork in the road (plot no. 28.22), take the path on the left, continuing along the path you’ve just come from.
“Chevreuse” mountain bike signs + orange markings
(7) Follow the signs for “Pommeret” (ignore the red and white arrow indicating a left turn) and you will immediately reach the Carrefour des Six Croix. Heading east, take the first path on the right from the gate (follow the “Chevreuse” MTB sign). Stay along the edge of the woodland on this well-maintained path. At a crossroads (plot no. 28.26), turn left and pass through a forest gate. Ignore a path branching off to the right and ride alongside the meadows, keeping a farm shed clearly in sight. At the T-junction, turn left towards the shed. You’ll come out onto a road; ride through the hamlet of Grand Ambésis, passing a castle on your right.
Mountain bike signs “Chevreuse”
(8) On leaving the hamlet, turn sharply right (at the ‘Accès pompiers’ barrier). Take care: there is no sign indicating the exit from the hamlet, so look carefully for the path leading off at the corner of a garden. Ride alongside fields on your left and, on your right, a hedge followed by woodland. At a T-junction (with a reservoir on the left), turn right. At a fork in the road at the entrance to the woodland, take the left-hand path. Continue southwards along the edge of the woodland. At the next junction, turn left, leave the woodland and stay on the edge with a field on your left. During a brief stretch through the undergrowth, ignore the path on the right. Continue south-east, alongside a meadow on your right. You’ll reach some houses and then a crossroads in the hamlet of Le Bel-Air.
Orange markings
(9) Turn sharply right onto Allée de la Butte Ronde (no entry). The road soon gives way to a path that descends steeply into the woods. In a wide bend to the left, ignore a path coming from the right and, at the Fork in the road that appears immediately, take the left-hand path. Continue downhill, as the path becomes increasingly narrow and steep. At the bottom of the descent, you’ll reach a junction with a wire fence at an angle: carry on straight ahead towards an ONF sign, which you’ll see from behind.
Red and white + blue and white markings
(10) At the “Forêt Domaniale de Rambouillet” sign, follow theGR®®11to the right towards Coignières SNCF (sign high up on a tree). The path winds its way whilst maintaining a general west-north-westerly direction. Pass through a beech forest with several remarkable trees and note the sandstone boulders on the slope to your right. Pass the Source aux Fées wash house (information sign). Continue along the path and you will soon reach the edge of a property on your left (an old mill), with the River Yvette running alongside it. At a fork in the road, take the left-hand path and cross the bridge over the Yvette: this will take you to the Maincourt wash house (information board). Continue westwards and you will come to a crossroads you passed on the way out.
(4) At the crossroads, turn right, cross the Yvette again, immediately take a sharp left-hand bend in front of a green gate and pass through a forest barrier. Walk past the large information board for the nature trail and reach the PNR signpost you followed on the way out.
(5) Continue westwards on theGR®®11(marker on a tree in the shape of a pole) and follow the course of the Yvette from a distance.
(1) Look out for the start of the wooden walkway on your left, then continue straight ahead. Pass the very first information board you come across and, at the next junction, continue straight ahead. You will very quickly reach the car park (start/end).
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Then, the top of the route is very pleasant with clear views. On the way back down, be careful, it can be slippery, and the path is quite narrow, but it's still very pleasant overall. Worth doing again!
Visorando Member
Difficult 
Easy 
