The Roselière (reed beds) of Maincourt

At the heart of the Chevreuse Valley Natural Park, a hike that allows you to wander through a protected natural site with a trail laid out by the Park. In the Bois de Maincourt, one can see ancient sandstone quarries. The course of the river Yvette, which the trail follows on the way back, finally allows a patrimonial approach with old mills and old washhouses.

Technical sheet No. 4098728

A Lévis-Saint-Nom walk posted on 19/03/15 by LB_Neil. Last update : 06/10/19
Calculated time Calculated time: 2h50[?]
Distance Distance : 9.08 km
Vertical gain Vertical gain : 76 m
Vertical drop Vertical drop : 77 m
Highest point Highest point : 173 m
Lowest point Lowest point : 105 m
Moderate Difficulty : Moderate
Back to starting point Back to starting point : Yes
Walking Walking
Area Area : Vallée de Chevreuse
Location Location : Lévis-Saint-Nom (78320)
Starting point Starting point : N 48.719994° / E 1.95902°
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The walkway on the discovery trail


Park in the car park of the trail, located in a turn of the D58 at the Lavagot mill between Maincourt-sur-Yvette and the Saint-Nom Church. Access via the N10 and Le Mesnil-Saint-Denis or the N118 and the D906 via Chevreuse.

PNR posts
In the first part of the hike, find your way on the ground by small marker posts laid out by the Regional Natural Park (Parc Naturel Régional, PNR) of the Chevreuse Valley, on top of which a metal plate White wears a black arrow indicating the direction to follow.

(S) Take the path that starts at the bottom of the car park and head east. At a three-way fork, ignore the red and white markings that invite you to go diagonally left and go straight towards a large information board. Referring to the map on this panel, follow the green line and the orange route.

(1) Shortly after the sign, head to the right on the path laid out with a pontoon over the marsh (several information boards along the way). Head around a reed bed on the right. Cross the Yvette at a footbridge. Soon after, at the Pinguenet pond (signposted) there is an observation post above the water on the left. Immediately after, at a fork on the road, turn right.
NB Here, the IGN map is not updated: the purple trail that continues along the Yvette and crosses just before point (4) no longer exists.
Make a large loop on the left with a very short passage where the pontoon gives way to a dirt road. Reach a T-junction which marks the end of the road built in woods.

(2) Leave the path laid out in the "small loop" from left (back to the car park). Turn right on the dirt road from the "big loop", which is between two wetlands and crosses some streams with wooden walkways. Shortly after the information panel at the mill, veer left at a property and follow the path lined with two fences. Pass a forest fence and continue to Maincourt in a small street. Turn left (green gate, Red and White markings) and take the old mill dam, with reed beds on your left.

(3) At the crossroads located at the end of the dam, a PNR post indicates that you should make a round trip to the washhouse in Maincourt, taking the marked path in Red and White, which is off to the right. If planning to walk only the “discovery” part of this trail (returning to the car park after point (5), see practical information), then it worth going to see the washhouse (the trip there and back will take about 10 minutes). If you are planning on doing the whole thing you will pass the washhouse on the way back.
Continue along the dam and beyond, crossing the Yvette, and immediately follow a bend to the left in front a green gate and pass a forest fence. At a large information panel (the same as the beginning), ignore two paths leading off to the right. Immediately following this, pass by an NRP marker post and a few metres further on, reach another PNR post.

(4) A Red and White marking on a tree in the shape of a perch indicates that you should go straight on. Ignore this marking and follow the PNR signpost: head right onto a trail that climbs amid blocks of sandstone outcrops. The slope is steep but the climb is short. At the summit of the hill (a big oak tree), turn left. Walk along the edge of the shelf, passing by a beautiful view on the left and the first old sandstone quarry on the right. Further on, make a short detour to the left as far as the information panel which faces a sandstone bench, retrace your steps and continue to the west extending the main path. Reach a second old sandstone quarry (information board).

Red and White + Blue and White markings (sparse, one as much as the other)

(5) At a fork just past the old quarry, leave the trail and ignore the PNR post and the Blue and White markings that indicate that you should turn left. Take the right lane, to the north (you will not find markings for a few hundred meters but, after the intersection in a turn, you can see arrows for hikers coming in the opposite direction). At the next fork (tree with plot No. 28.22 and a marker), take the left lane, following the path of where we come from.

’’Mountain bike signs for "Chevreuse" + Orange markings''

(6) Ignore the Red and White arrow that indicates a left turn at an acute angle, follow "Pommeret" (sign) and immediately reach the Six Croix crossroads. Ignore the Blue and White arrow that indicates that you should cross a forest fence and leave the wood: take the first road to the right headed east after the fence (follow the mountain bike sign for "Chevreuse"). Stay on the edge of woods on the well maintained path. At an intersection with a large oak bearing with plot no. 28.26, turn left and pass a forest fence. Ignore a path starting on the right and walk along the meadows with prominent a barn. At a T-junction, turn left towards the hangar. Reach a street, cross the hamlet of Grand Ambésis passing a castle on your right.

’’Mountain bike signs for "Chevreuse"''

(7) At the exit of the hamlet (electric pole), turn right at an acute angle ("Firefighters Access” barrier). Be alert, there is no sign indicating the exit of the hamlet, locate the path that starts at the corner of a garden, heading south-east. Follow the side of the field on your left and on your right a hedge then some woods. At a T-junction (tank on the left), turn right at a right angle. A fork at the entrance of the wood, take the left lane. Journeying along the south edge of woods. At the next intersection, turn left at a right angle out of the woods and follow the edge of a field on your left. In a further brief section through the woods, ignore the path on the right. Continue to the south-east, along a meadow on your right. Reach houses and a crossroads of streets at a place called Bel-Air.

’’Orange markings''

(8) Ignore the mountain bike sign for "Chevreuse" that tells you to go straight and instead turn right at an acute angle along Allée Butte Ronde (a do not enter sign on the right side of the street). The street soon becomes a path that goes down steeply into the woods. In a wide turn to the left, ignore a path that comes from the right and, at the fork immediately following it, take the left lane. Continue down the path becoming increasingly boxed in.

”Red and White markings”

(9) At the bottom of the descent, reach a crossroads with a fence that is at an angle: continue opposite towards an ONF panel visible on the reverse. After fifty metres, arrive at the sign for “Forêt Domaniale de Rambouillet", turn right at a right angle on the GR11 towards the Coignières SNCF station (small sign high on a tree). Until Maincourt (3), the GR winds a lot while keeping a general west-north-west direction. Head through a beech forest with many outstanding specimens and note the sandstone block outcrops on the right of the slope. Head on to the Source aux Fées washhouse (information panel). Continue on the path and quickly reach the edge of a property (an old mill) on the left, with the Yvette running alongside. At a fork, take the left lane and then the bridge over the Yvette to reach the Maincourt washhouse (information board). Continue west and arrive at the crossroads already taken on the outbound leg located at the end of the sea wall of the mill.

(3) At the intersection, turn right at a right angle cross again Yvette, immediately follow a bend to the left opposite a green gate and pass a forest fence. Pass by the trail’s large information board and reach the PNR marker post followed on the outbound leg.

(4) Continue to the west on the GR11 (marking on a tree in the shape of a perch) and follow the course of Yvette.

(1) Find the start of the path on the left with a man-made wooden surface and continue straight ahead. Pass by the very first information panel and, at the next junction, continue straight. This quickly leads back to the car park.

Waypoints :
D/A : km 0 - alt. 116 m
1 : km 0.16 - alt. 119 m - Start of landscaped path
2 : km 0.44 - alt. 108 m
3 : km 0.71 - alt. 109 m - End of landscaped path
4 : km 1.2 - alt. 108 m - Digue du Moulin Maincourt
5 : km 1.35 - alt. 116 m - Fork, PNR post.
6 : km 1.99 - alt. 160 m - Fork right after the second old quarry
7 : km 2.43 - alt. 169 m - The route branches off just before the Six Croix c
8 : km 3.79 - alt. 163 m - Exit of the Grand Ambésis
9 : km 6.17 - alt. 163 m - Bel-Air
10 : km 6.91 - alt. 108 m
D/A : km 9.08 - alt. 116 m

Useful Information

The estimated time for the walk is 3 hours with few stops. The trail lends itself well to enjoying the sights and views, and it is worth taking slowly to see everything...

‘’Warning'': In wet weather, the road built on a pontoon between (1) and (2) is slippery.

‘’Variation'' - You can shorten this to only hiking the initial “discovery” section, which takes you over the swamp and a view over the old quarries. In this case, follow the PNR posts from beginning to end:
- Follow this route from the start to point (3).
- Make a round trip to the Maincourt washhouse
- Continue the route via points (4) and (5).
- At the crossroads (5), leave this route, turn left (Blue and White doubled with Red and White markings).
- Head down a steep descent for a few metres to the west of point (1), reach the path that follows the course of Yvette.
- Take this road to the right and reach the starting point.
Expect a 1 hour walk without the stops for the whole route.

Most roads are well stabilised and the ground is generally dry. Wet sections are found between points (9) and (1) and the undeveloped parts of the marsh trail. All the developed sections are accessible to people with reduced mobility.

On the “discovery” section of the trail from (1) to (5), dogs must kept on leads and bicycles are prohibited. The Maincourt Swamp is a protected site: follow the other guidelines listed on the large information panels.

A 1/25000 map is required.

Visorando and this author cannot be held responsible in the case of accidents or problems occuring on this walk.

During the walk or to do/see around

The Maincourt Swamp and trail laid by the PNR of the Chevreuse Valley, between points (1) and (4) and then (4) and (1):
- Reed beds and other vegetation adapted to the environment.
- Many birds (do not hesitate to come early to have more chances to observe and enjoy them in peace).
- Numerous panels all along provide information.
A place that is likely to interest a wide audience!

The upper part of the early trail, between (4 and)5:: has two old sandstone quarries.

The course of Yvette between (1) and (3) then between (9) and (1):
- Source aux Fées and Maincourt washhouses
- Old mills
- Beech forest with several outstanding examples

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The GPS track and description are the property of the author.