Following the Orge from Arpajon to Sainte-Geneviève-des-Bois.

Departure from Arpajon station (RER line C, on the section between Paris and Dourdan-la-Forêt). Take the Place de la Gare exit, on the side of track 2 (Dourdan-Paris direction; if arriving in the other direction, take the underground passage).

Red and white markings (sparingly used)

(S) With your back to the station, turn right and walk down the avenue. Take the first left, Rue Roger Lhuillier, then the first right. At the end, cross a wide street and follow Rue du Jeu de Paume for a few metres. Then take Passage du Jeu de Paume on the right. You will come out onto a street; follow it to the left.

No signposts

At the end of a left-hand bend in the street, leave the^GR® for a moment and turn right onto Rue Raspail, which leads to the monumental market halls. Then turn left twice, leaving the market halls on your right.

Red and white markings

At the end of the street, turn right and rejoin the GR^®. Cross both arms of the Orge river (town hall on the left) and continue straight on along Grande Rue until you reach the church.

No markings

(1) At the corner of the church, turn right onto Rue de la Résistante (let the^GR® continue straight ahead. N.B. The purple line on the IGN map has not been updated). At the first stop sign, continue straight ahead and enter Saint-Germain-Lès-Arpajon. At the second stop sign (turret opposite), continue straight ahead along Rue du Docteur Louis Babin.

Red and white markings

(2) At the corner of No. 36, turn right onto Rue Ernest Labbé and rejoin the GR^®. Then cross Place de l'Église. At the corner of the church, turn right, go through a gate leading to a public space and turn left towards the town hall. In front of the town hall, take a slightly downhill tree-lined path on the right. Cross a stream and then the Orge river on the Passerelle Louis Maillard footbridge.

No markings
From this point until point (14), almost at the end, the hike follows the Promenade de l'Orge, marked with signposts and information panels. The purple line on the 1:25,000 map is supposed to represent the^GR®655, but it is only at point (7) that you will find it

(3). Turn left immediately and follow the Orge on your left. Cross a small road and continue on the stabilised path along the river. Then cross the Orge on a footbridge and follow it on your right. The river divides into two (small waterfall), continue along the secondary branch (the Grande Boële) on your right. At a crossroads, just before a bridge over the Boële, continue straight ahead and cross a stream via a footbridge without a railing (the 1:25,000 map is inaccurate at this point, but the 1:6,250 map is accurate).

(4) At the next crossroads (bridge on the right), continue north towards the Bassin du Petit Paris (signpost). On a wide, shaded path, pass a housing estate on your left. At a fork approaching a high-voltage power line, turn right. After a left-hand bend, pass under the high-voltage power lines and you will find the Grande Boële on your right. Just before a bridge, take a first turn left then right with the path, cross a small road and continue on the path opposite after the forest barrier.

(5) Ignore a path on the left, turn right and take the footbridge over the Grande Boële. At the T-junction that follows, turn left. Walk along the Orge on your right and ignore the first footbridge that crosses it. At the second footbridge, turn left and cross the Grande Boële again. Walk along the river on your right, then, on leaving Leuville-sur-Orge, turn left and stay on the path running parallel to the Route de Brétigny. At the information sign, turn right, cross the road and continue straight ahead on the paved path along the edge of the field on your left. At the fork, turn right and cross the Grande Boële for the third time.

(6) At the fork at the end of the footbridge, turn left and follow the Bassin du Petit Paris on your right. At the end, turn left and cross the Grande Boële for the fourth and last time.

Red and white markings
There are also yellow markings, but they are discontinuous: stick to the red and white markings.

(7) Turn right immediately and cross a tributary of the Orge, the Salmouille. Pass a playground on the right-hand side, ignore a path on the left and cross the Orge. Then turn left immediately onto a wide, stable path, with the river on your left and the Bassin de Carouge a short distance away on your right.

(8) At the end, at a crossroads, pass under the two bridges of the N104, still along the Orge, and take a cycle path for about a hundred metres. At the end of a right-hand bend, leave the path and turn left onto a path that enters a wood. At the end of the wood, continue between two fences. At the T-junction, follow the small road on the left that climbs towards the centre of Longpont-sur-Orge and ignore the turnings on the left.

(9) At the corner of No. 13, turn right down Chemin du Moulin de Basset (cobbled street). Opposite the gate of the mill, turn left and continue along a path with the Orge river on your right. After passing under a power line, at the three-way junction, turn right. Then cross a road at a pedestrian crossing and continue opposite into a wood (chicane). At the next junction, turn right.

(10) After about 100 metres, ignore a footbridge over the Orge on your right and turn left onto a stabilised path that passes between two ponds (note the view of the Château de Lormoy on your left). At a fork not shown on the map, follow either path, as they rejoin further on. Just after this junction, turn left (leaving the main path to continue along the pond on the right) and take a footbridge. At the next fork, turn left and climb slightly along a meadow on your left. After about 250 metres, turn right onto a discreet path through the undergrowth (if you miss it, you have a second "chance" a few metres further on). After a few metres, turn right onto a wider path and you will come to a forest barrier overlooking the chevet of the Basilica of Longpont.

(11) Turn left onto the^GR®111and go around the basilica on the left to find the entrance through a side door on the north side. Retrace your steps back to the forest barrier.

(11) With your back to the basilica, turn left (east) onto the^GR®655, pass two concrete posts on your left and follow a path that descends between meadows. At the bottom, at a crossroads, turn left and walk along the Orge river on your right (majestic plane trees). At the end of the path, cross a road and, immediately afterwards, take the footbridge on the right over the Orge. Turn immediately left and continue along the river for about 200 metres.

No markings
Up to the point (13), the map shows a few paths that do not exist (see satellite view): follow the description carefully

(12) At the first fork, turn right (let the^GR® continue straight ahead). Immediately afterwards, at a three-way junction (signpost), turn left. At the fork about 50 metres further on (bench), follow the left-hand path north-north-east (the right-hand path leads to a house). At a crossroads (signpost), continue straight ahead. After crossing a copse, at a fork, take the left-hand path and walk along a meadow on your left.

(13) After a sign indicating a view of the summit of the Tour de Montlhéry (to the south-west), at a new fork (bench), take the path on the right. You will soon come to a wooden pontoon; follow it to the right to cross a reed bed. At the end, turn left onto a stabilised path and walk along a river on your right. At a three-way junction, turn right. Ignore a footbridge on the right and continue along the river until you reach a road.

(14) Turn right and cross the river. Immediately take the first right, Rue du Vieux Perray. At the stop sign, turn left and go up Rue du Colonel Yves Massie. At the crossroads, continue straight ahead and you will soon reach Sainte-Geneviève-des-Bois station, with access to the platforms, either immediately for a return to Paris, or after taking the underground passage for a return to Dourdan or Étampes (E).

Train timetables: consult the Transilien website.

For motorists: the Arpajon and Sainte-Geneviève-des-Bois stations both have large car parks, so it is possible to leave a car at one of them or a car at each. Both car parks are located on the side of track 2 (platform towards Paris).

Low-cut hiking boots or good trainers are suitable for this hike, which mainly follows well-maintained paths and tarmac roads. In all likelihood, this hike is easily accessible in winter.

There are no water sources along the route. There are numerous bars, restaurants and shops in Arpajon, between (S) and (1).

Dogs must be kept on a leash along the entire length of the Promenade de l'Orge, between (3) and (14). The area is shared with other users, who may be numerous at weekends: other walkers, runners and cyclists. A minimum of attention and courtesy is required.

A detailed map is essential (at least the one accompanying this description).

Hike completed by the author on 14 October 2017.

- Pleasant walks along the Orge. The Promenade de l'Orge is managed and maintained by the Syndicat mixte Orge Aval.
- Arpajon market halls (15th century), between (S) and (1).
- Arpajon Church (11th-16th century), open daily from 8:00 or 9:00 a.m. to 7:00 p.m. (1).
- Saint-Germain Church in Auxerre (11th-12th century), between (2) and (3).
- Longpont Basilica (11th-13th centuries), open daily from 9 a.m. to 7 p.m. (11).