The bridge has three tiers of arches, standing 48.8 m (160 ft) high. The whole aqueduct descends in height by only 17 m (56 ft) over its entire length, while the bridge descends by a mere 2.5 cm (0.98 in) – a gradient of only 1 in 3,000 – which is indicative of the great precision that Roman engineers were able to achieve, using only simple technology. The Pont du Gard was constructed largely without the use of mortar or clamps. It contains an estimated 50,400 tons of limestone with a volume of some 21,000 m3 (740,000 cu ft); some of the individual blocks weigh up to 6 tons. Most of the stone was extracted from the local quarry of Estel located approximately 700 metres (2,300 ft) downstream, on the banks of the Gardon River.] The coarse-grained soft reddish shelly limestone, known locally as "Pierre de Vers", lends itself very well to dimension stone production. The blocks were precisely cut to fit perfectly together by friction alone, eliminating the need for mortar. The builders also left inscriptions on the stonework conveying various messages and instructions. Many blocks were numbered and inscribed with the required locations, such as fronte dextra or fronte sinistra (front right or front left), to guide the builders.