The Roman aqueduct from Fontaine d'Eure near Uzès to Nemausus (Nîmes) passes over the Pont du Gard, and many other significant bridges. The location of Nemausus was somewhat inconvenient when it came to providing a water supply. Plains lie to the city's south and east, where any sources of water would be at too low an altitude to flow to the city. Hills to the west made a water supply route too difficult from an engineering point of view. The only real alternative was to look north toward the area around Ucetia. Natural springs there offered the best chance for a steady flow.
The straight-line distance between the two cities is about 50 kilometers. The aqueduct takes a winding route measuring around 50 kilometers as well. This detour circumvents the southernmost foothills of the Massif Central. These hills are known as the Garrigues de Nîmes. They are covered in dense vegetation and garrigue. Deep valleys indent the landscape. It was impractical for Romans to tunnel through these hills. A tunnel would have required a length between 3 and 4 kilometers depending on the starting point.
A roughly V-shaped course around the eastern end of the Garrigues de Nîmes became the practical solution. The Fontaine d'Eure sits at 128 meters above sea level. It is only 19 meters higher than the repartition basin in Nîmes. This small elevation difference provided sufficient gradient to sustain a steady flow of water to the 50,000 inhabitants of the Roman city. The aqueduct's average gradient is only 1 in 3,000. It varies widely along its course but is as little as 1 in 20,000 in some sections.
Architectural Construction Methods
The Pont du Gard contains an estimated 50,400 tons of limestone with a volume of some 42,000 cubic meters. Some individual blocks weigh up to 6 tons. Most stone came from the local quarry of Estel located approximately 1 kilometer downstream on the banks of the Gardon River. Coarse-grained soft reddish shelly limestone known locally as Pierre de Vers lends itself very well to dimension stone production. Blocks were precisely cut to fit perfectly together by friction and gravity. Builders eliminated the need for mortar.
Builders left inscriptions on stonework conveying various messages and instructions. Many blocks were numbered and inscribed with required locations such as fronte dextra or fronte sinistra. These guides directed builders during assembly. A surveyor or mensor planned the route using a groma for sighting. He used the chorobates for leveling and a set of measuring poles five or ten Roman feet long. His figures and perhaps diagrams were recorded on wax tablets later written up on scrolls.
Builders made extensive use of cranes and block and tackle pulleys to lift stones into place. Much work could have been done using simple sheers operated by a windlass. For largest blocks a massive human-powered treadmill was used. Such machines remained in use in quarries of Provence until start of 20th century. A complex scaffold supported bridge while being built. Large blocks left protruding from bridge to support frames and scaffolds used during construction.