Even though no first-century plans or other written records about the project exist, much is known about the tools and methods of work used.
One of the most amazing aspects of this monumental project was the ability of the engineers and surveyors to lay out its placement and slope with the rudimentary layout tools of the day. The best equipment available at the time is considered to be 30 times less accurate than the simplest layout tools we use today.
The groma was an elegantly simple apparatus with five plumb bobs suspended from a swiveling head used to establish straight lines and 90-degree corners at a distance. When the central plumb bob was placed over a reference mark, the tool was driven into the ground and the head would be rotated to align the principal axis by sighting along two of the strings. The perpendicular axis also could be determined by sighting along the other two strings because the arms that held the plumb bobs were perpendicular to each other.
The chorobate was used to find level or make slope calculations over a distance. After the device was lined up and shimmed level according to its water trough and plumb bobs, the operator would sight across the top through a peep sight at a surveyor's rod held a distance away. Leveling of objects such as stones was done with an A-shaped level by lining up its attached plumb bob string with a central vertical marking. Squares, compasses, rules, and, yes, even plumb bobs were used by the Romans much as they are today.
The Romans quarried the limestone used for building the Pont du Gard (as well as for subsequent repairs) about 600 yards downstream. It is interesting to note that the tool used to excise the stone seems to also have established the uniform thickness found in most of the blocks used throughout the structure. Stonecutters would incise a groove in the stone with a long-bladed, two-sided pick called an escoude. When the outline was as deep as the tool would cut (16 to 18 inches), a shallower groove was cut along the bottom of the stone to establish a plane of weakness at that depth, then wooden wedges were pounded in to fracture the stone along the desired lines. When the stone was pried free, a small crane with a manually operated windlass lifted it onto a cart pulled by oxen.
Since the Pont du Gard was a technical work far from the city, no aesthetic considerations were applied when finishing the stones. They were cut out of the quarry as close to their finished size as possible and only the joint surfaces were dressed to provide the tightest dry fit. Because of this, telling marks of the tools can still be seen and appreciated. Archaeological digs in 1985 and 2000 uncovered previously unknown parts of the quarry and the marks left by the escoudes here are especially clear.
The Romans had an ingenious component block system for efficiently building the 64 original arches with great uniformity; arch stones were cut to predetermined sizes and labeled with numbers and directional symbols. This way, any block with the correct label should fit in the same place in any arch of the same radius. The bottom row of arches was composed of three stone widths and the middle row only two. This juxtaposition and its resulting lack of staggered vertical joints were only carried out for the arch stones. The rest of the structure had random vertical joints with stones custom-fitted. All stones were dry-fit.
The uniformity of the arches allowed reusable wooden forms to be used for laying up the stones. The projecting keys inside the arches were designed to support these forms, which were slid in from the face side and then shimmed up. When the stones were all in place, the shims were knocked loose and with the weight off, the forms could be removed easily. Similarly, the protruding stones along the face of the arch pillars and the gaps along their sides were built to bear scaffolding and lifting devices, and were left in place because of the utilitarian nature of the edifice. Man-powered cranes, including large ones driven by a human hamster wheel, were used to elevate the stones, which weighed up to 6 tons.
The walls above the top row of arches were laid up with much smaller stone blocks, which enclosed the 4-foot-wide-by-6-foot-tall waterway along with flat capstones. The water was contained by a 3-inch-thick lining of mortar consisting of lime cement mixed with fine gravel, broken tile, and pottery shards, and covered with an iron oxide-based waterproofing mixture known as malthe. When cleaning the incessant mineral deposits off the walls of the aqueduct, the malthe's enduring red color served to show generations of cleaning crews where to stop.
Carved markings on the Pont du Gard, inscribed by the builders and now long left to history, display three appropriately Roman qualities: The superstitious, a phallic symbol thought to ward off the Evil Eye during construction; the classist, a depiction of the librator who was master of the site; and the dutiful, the official inscription mens totum corium that declares approvingly "entire structure measured." Those three inscriptions and 100 million silent hammer marks bear witness to the humanity of those creators.