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Leaving from Égly station (RER line C, on the section between Paris and Dourdan).
Yellow markings
(S/E) Leave the station on the Ollainville side and head north on the Route d'Ollainville.
(1) Immediately after crossing the River Orge (signposted wash-house on the right), turn left onto a level track and follow the River Orge on your left. After passing a sort of sentry box, reach the eastern end of the Bassin de Trêvoix (information panel).
Unmarked
(2) Turn right onto a wide gravel path and walk alongside the pond on your left. Cross the weir. After passing houses behind fences, turn right at another information panel to take the bridge over the Rémarde.
Yellow signs
(3) Turn right into the small road. Before the entrance gate to the Moulin de Trêvoix, turn left onto a track (EDF substation) that slopes gently upwards between fields. Ignore a track coming from the right, veer left and stay below the D116d road.
(4) Turn right into the small tunnel that runs under the D116d (the signs appear above the tags). Pass a group of small sports fields and allotments (Terrasses de Trêvoix) on the left.
(5) Just after a metal forest fence, turn left onto a path that starts at the foot of the "Rue de Trêvoix" sign. At the end of the path, turn right at an angle into the Chemin des Vignes (pass to the left of the "one-way traffic" sign). At the end, zigzag left-right into Rue des Vignes.
Then take the first path on the left, the Sentier de la Fontaine Bouillant (signposted on the ground). Head towards the bell tower, ignoring another path on the left. Cross the Rue du Pré d'Arny and continue straight ahead along the path. At the end, turn left into Rue de Fontaine Bouillant. Note a wash-house on the right and reach the church of Saint-Didier.
(6) At the head of the church, turn right into a sort of square and walk past the war memorial. Turn into the Rue de la Libération and zigzag right-left onto the Sentier de la Fontaine Beurlin. Find the Fontaine Beurlin wash-house on the right and continue straight ahead at the next crossroads. At the end of the path, turn right into a street and then the first left, the Sentier du Saut de Loup. Turn left into Rue des Acacias and then right into Chemin de Soucy.
(7) Go through the forest gate, enter the forest and head north, with the wall of a private property on your right.
No signposting
(8) After approximately 350m (tagged wall), leave the waymarked path and continue in the same direction, turning left (north-west) onto a sandy track. Climb slightly and reach a crossroads (ruined hut slightly higher up on the left).
Red and white markings
(9) Turn left (forest fence) and follow the GR®111from west to south-west. Go past a caravan site and bungalows. At the next crossroads (a sort of small roundabout), take the path in the middle that goes slightly left towards a small house (ignore the path that goes down to the right after a forest barrier). After passing this house (signposted "Fautrat"), let the GR®111go left and turn sharp right to pass a forest fence.
Yellow markings (rare)
(10) Immediately after the barrier, at the corner of signpost no. 31 "L'Epine", turn left (west-south-west) into the undergrowth. At the edge of the wood, at the intersection with a yellow 2-sided sign, continue straight ahead between the fields (ignore the left-hand path, which is better marked). Cross a small road and continue straight ahead with a horse pasture on your right. Enter a small wood and cross a small river, La Charmoise. Cross the D3 road(take care) and continue straight ahead along the path between the fields.
Option for those not afraid of tarmac - Turn left and follow the D3 road, ignoring the small road to the right and, at the next roundabout, turn left into Rue de la Galloterie. Follow the route as described.
(11) At signpost no. 42 "Courson", turn left onto a grassy track towards Fontenay-les-Briis. Turn left onto a small road (signpost no. 41). After about 150m, turn right into Rue de la Galloterie (signpost no. 40). At the roundabout, continue straight ahead on the same street. Ignore the lane on the left (which is just an extension of Rue de la Galloterie), then turn left into Rue de Cocagne (Impasse) and head downhill.
(12) Just before the small bridge, turn right onto a narrow path at the corner of Borne n°38 "La Charmoise". The path winds its way between the river on the left and the garden fences on the right. Next, leave the river bank and walk along the edge of a field on your right, on a path that is always narrow and sometimes faintly marked. Pass a water treatment plant and continue along the edge of the field. At the end, turn right along a fence on your left. Turn left into a very narrow path. At the end of this short lane, between the houses at nos. 16 and 18, turn left into a street (yellow beacon on the right-hand side).
(13) At the crossroads, turn left into Rue de la Butte Bouillon, where you will momentarily see the red and white GR®111markings. Cross the Charmoise and go past the first forestry barrier. At the second barrier (marker no. 37), leave the GR®111to the left and continue uphill. After walking along the edge of the forest, enter the undergrowth and cross a forest barrier.
(14) At the crossroads, head for the faint path that bends quickly to the left (this can also be considered the 2nd path on the left). At the next fork in the road, turn left onto an even less well-marked track that runs alongside fallow land on your left. In the wood that follows, the path is better marked. Continue between fields with greenhouses clearly visible on the left and reach houses and meadows.
(15) At the crossroads, turn right (Yellow beacon, "No Trespassing" sign) and ignore the yellow arrow indicating that you should turn left. After approximately 200m, leave the main path which bends to the left (house in the woods), and continue straight ahead (east) on the path which leaves behind a forest fence. Ignore the footpath junctions on either side, one on the right and one on the left, and you will come to a road (Chemin du Bois Pernot). Continue to a T-junction and turn left into Rue des Groseilliers.
(16) Immediately after a post with a yellow cross on the left-hand pavement, turn right into the narrow Sentier de Verville. This path bends left into the Rue de la Libération. Take this street on the left (D116, towards Arpajon). Turn right towards the church (sign) and leave the yellow markings for a moment. At the corner of the church porch, turn left.
(6) Continue east on Rue de Fontaine Bouillant. Reverse the route taken on the outward journey: pass the Fontaine Bouillant washhouse then turn right onto the path of the same name; cross a road, continue straight ahead along the path, turn right into Rue des Vignes then left-right into Chemin des Vignes; turn left into a path.
(5) At the end of the path, take the track on the right (forest fence). Pass the Terrasses de Trêvoix again and take the small tunnel under the DD116d.
No signposting
(4) Continue straight downhill (leaving the path you took on the outward journey on your left). At the bottom of the descent, turn right onto the small road. At the end of this road, turn left onto a larger road, keeping to the left-hand pavement. Cross the Rémarde and, a few metres after road no. 7, turn left onto the track leading to the Bassin de Trêvoix (information sign).
Yellow signs
(17) Turn right onto the wide gravel path and follow the pond on your left. After a succession of left-right-left bends, you can go left to see the Pierre de Beaumirault (menhir; a good place to stop). Continue eastwards along the entire south bank of the basin.
(2) Go straight on at the information sign. Very soon afterwards, go round a sort of sentry box on the left and continue east along a path that runs alongside the Orge, on your right (beyond it, you can see some small houses around the "La Grande Ile" lake).
(1) When you reach a road, turn right and cross the bridge. Follow the Route d'Ollainville back to Égly station (S/E).
Visorando member
Easy 
