The hot and humid climate of the time caused the oxidation of the iron, which coloured the sediments red.
In the Mesozoic Era, the sea settled on this pelitic base rock and new sediments were deposited from a massif located in the current Mediterranean. Over tme, these new sediments produced limestones, gray marl and sandstone.
In the Cenozoic Era, the formation of the Alps resulted in this area being forced up to create Dome de Barrot whose pelites cut by the erosive action of Var and Cians are today visible for several hundreds of metres in the Gorges de Cians and de Daluis. We have reached the entrance to the Gorges and continue the journey from the point marked with a blue flag on the satellite image above. The road/tramway cut a deep cleft in the rock, and as can be seen there is a warning of tunnels ahead!The tramway is visible in this postcard image which is taken facing towards Daluis. 
The scenery is dramatic! At certain points along its length the tramway/ road clung to the rockface.
The first tunnel built for the road and used by the tramway is shown in the two images below which are taken from Googles Streetview. The south portal of the first tunnel.The north portal of the first tunnel.The first and second tunnels are marked on the map above. The first, has a combiination of green, blue and red dots. It is 117 metres long. The second tunnel is marked north of the first by two black dots. It was built for the tramway as the road bend on the valley side was too tight for trams to negotiate. It was only 31 metres long. 
The very narrow and deep gorges of the Var, north of Daluis, were always a serious blockhampering communications between the high and low valleys. A road was built in the late 19th Century to replace mule tracks and required the drilling of 10 tunnels. 
Subsequently, when secondary railways were being built, the town of Guillaumes lobbied for a railway. But the nature of the terrain would only permit the construction of a tramway between Pont de Gueydan and Guillaumes.
The ten tunnels already built became combined road/tramway tunnels, but it was also necessary to by-pass some very tight curves in the road using a dozen new
tunnels specific to the tramway. These also became road tunnels after the tramway was abandonned,
It was only north of the Gorges de Daluis, before arriving at Guillaumes, where the valley became wider, that the tramline was free to choose its own route. That section contains the last four tunnels on the line.
Because of their number, proximity and lack of specific place names, these
Tunnels do not have specific names and are often referred to as numbers in registers of structures. The numbering starts close to Daluis and heads towards Guillaumes.Tunnel 2, South Portal.The two images immediately above show the north portal of tunnel No. 2.Tunnel No. 3, south portal.Tunnel No. 3 is marked on the above map by the green, blue and ed dots. It is 233 metres long and was originally constructed for the road in the late 1800s before becoming a shared tunnel when the tramway was built. 
The north portal of Tunnel No. 3 is immediately followed by an arch bridge carrying the road  and a couple of significant retaining walls before Tunnel No. 4 is reached.Tunnel No. 4 was built for the tramway, and again avoided a sharp bend on the road. It was a straight tunnel of 241 metres in length. The picture above shows the south portal and the road curving away to the right. As can be seen on the map below the road entered a tunnel just off shot on the picture above.Tunnel No. 4 is marked with green, blue and red dots on the plan above. Immediately to the north of the tunnel, the road/tramway crossed another arch bridge. Looking back at the north portal of Tunnel No. 4 from the arch bridge above.In a very short distance we encounter Tunnel No. 5 (above), this was built for the tramway as was 69 metres long. It is marked on the plan below with a green, blue and red dot. The yellow dots represent Tunnel No. 6. Tunnel No. 5, north portal.
Tunnel No. 6 is encountered immediately, it was originally built for the road, although at the time of its construction the road was much narrower than at present. The shot immediately below shows the old arch bridge over the stream which was directly before the tunnel entrance and clearly shows the modern widening of the bridge.The south portal of Tunnel No. 6The north portal of Tunnel No. 6 demonstrates the asymmetrical shape of the tunnels after conversion for the trams. The higher side allowed for the catenary and poles which supplied electrical power to the trams.The south portal of Tunnel No. 7, which only about 15m in length. The road curves sharply to the left beyond the tunnel and the picture below looks back at the rock wall from round the corner.Tunnel No. 8 is another very short tunnel, encountered within 10s of metres of Tunnel No. 7. It also is very short, no more than 10 metres in length. The first image below shows the south elevation in the 21st Century, and the second shows the same elevation while the tram tracks and catenary were still in place. The north elevation of Tunnel No. 8 in the 21st Century.
Tunnel No. 9 is a longer tunnel (43m). It was built for the tramway, as the road heads out round another rocky promontory. Tunnel No. 9 South elevation. Tunnel No. 9 North elevation.The bridge over the Ravine immediately after Tunnel No. 9.
This next map shows the locations of Tunnels Nos. 9, 10 and 11. Tunnel No. 9 is marked with yellow dots at the bottom of the map; Tunnel No. 10 is marked with a read and a green dot; Tunnel No. 11 runs is a more southwest to north east direction close to the top of the map. Tunnel No. 10 was just 22m in length but was, like Nos. 9 and 11, built for the tramway. The tunnel roof has been removed and the road diverted along the line of the tunnel.Tunnels and bridges alternate along this route with frightening regularity! This structure is between Tunnels Nos 10 and 11.This ‘interesting’ three-way junction heralds Tunnel No. 11. It appears very narrow but was built wide enough to accommodate the 1.9 metre loading gauge of the trams on this route.The north-east portal of Tunnel No. 11. Immediately after leaving the 57 metre-long tunnel the road/tramway needed the support of a substantial retaining wall.
Tunnel No. 12 is just 16 metres long.South portal of Tunnel No. 12.North portal of Tunnel No. 12.Turning 180 degrees from the location of the last photo … the south portal of Tunnel No. 13 and the bridge over a gully immediately before the tunnel.A panoramic view from above Tunnel No. 12 of the road approaching the dramatic bridge, Le Pont de la Mariée. The 2CV is on the road adjacent to Tunnel No. 13. The north portal of Tunnel No. 13.The south portals of Tunnels Nos. 14 and 15.The south portal of Tunnel No. 15.The north portal of Tunnel No. 14.The north portal of Tunnel No. 15.The southwest portal of Tunnel No. 16, which reveals further tunnels beyond!The northeast portal of Tunnel No. 16.
The southwest portal of Tunnel No. 17, with another bridge over a ravine.The northeast portal of Tunnel No. 17.The road and the tramway diverge. Le Pont de la Mariée was built for and carried the tramway.Looking back at the tramway/road from the east bank! A drammatic photo (above) that shows the upstream part of Les Gorges de Daluis. In the foreground we can see Tunnel No. 16, then the entrance to Tunnel No. 17 and then Le Pont de la Mariée which is one of the fist reinforced concrete arched bridges in France. The route of the road is shown by a blue arrow and the route of the tramway is marked with red arrows. 
Le Pont de la Mariée was built in the clearly part of the 20th Century. It has a 62 metre span and sits 80 metres above the valley floor. It is salutary to think that this bridge only saw active use for a period of 6 years in the 1920s. So much work for so little gain! The image below was taken during its construction. The next tunnel, Tunnel No. 18, is the first of these tunnels on the east side of the River Var.  Tunnel No. 18 is marked on the plan above by the green and red dots. It is 42 metres long and was built specifically for the tramway. Access in the 21st Century is restricted to pedestrians. Its west postal is shown immediately below and its east portal on the second image below. Tunnel No. 19 is shown by the two black dots on the above map. It is just 28 metres long and also only accessible on foot.The West portal of Tunnel No. 18.The East portal of Tunnel No. 18. Tunnel No. 19: south portal (left above) and north portal (right above).The satellite image above shows the bridge and the tramway route passing through the rock in Tunnels Now 18 and 19 before heading north until quite close to the River Var. The image immediately below picks up the line of the tramway travelling further north through a heavily wooded area, and the one below that shows the tramway formation leaving the wodds and passing a campsite. The old tramway has now become an access road for the campsite.The tramway formation crosses the Vallon de Tireboeuf and continues north as shown on the map below. Two tunnels were encountered on this part of the journey – Tunnels Nos. 20 and 21. Tunnel No. 20 is marked by the green and red dots on the plan. Tunnel No. 21 is marked by the black dots.Tunnel No. 20 is also known as Tunnel de Ruine. It is no more than 20 metres long. The picture below shows it north portal. Tunnel No. 21 is in fact a 4i metre-long protective gallery over the line which the images below show clearly. The tramway ran north from the gallery towards Guillaumes on the east side of the River Var. In the adjacent satllite image the gallery can be seen close to the bottom of the picture. The point at which the road crosses the river to rejoin the tramway alignment can be seen close to the top of the picture. The modern road bridge seems to dominate the location , but the old road bridge can be picked out immediately to the north of the modern bridge.
The location is shown in the images below.
The first picture shows the modern road bridge and alongside it the original road bridge built in the mid-1800s. The picture is taken looking towards Daluis, downstream of the location.
The second image below shows the the modern bridge and, on the east bank of the river the tramway formation under high cliffs. This picture is taken looking upstream towards Guillaumes.
The third picture is taken from the south looking up the valley of the Var towards the same bridge – Pont des Roberts. 
The road/tramway followed a straight course northward to just south of Guillaumes. After a minor deviation to the left and then the right the tramway crossed the Pont de Tuébi.