'Arch of Janus'
A massive marble structure 16m. high, 12m. wide, the four-way arch stands with its piers astride the Great drain (Cloaca Maxima) which runs down the valley to the Tiber beside the Round Temple. Possibly an 'Arch of the Deified Constantine' which is listed in this area in the Regionary Catalogues, it must date from the early C4 AD (the quadripartite concrete vault over the crossing incorporates terracotta storage jars in a technique typical of the period) and probably replaced an earlier one on the same spot. It seems not to have been a triumphal arch but a Janus of the boundary marker type found in the Roman Forum (which may also have coincided with the Drain. Much of the marble shows signs of having been reused, but the rather rough appearance and squat proportions are misleading. The holes at every joint were made in the Middle Ages to extract the iron dowels which held the blocks together and all four facades have also been robbed of the small freestanding colonnades which projected in front of the niches on both levels. Whether the niches were intended to contain statues is uncertain but their semidomes are elegantly carved in the form of scallop shells. A figure of Minerva is carved on the keystone of the archway on the north side; Roma appears on the east. A doorway in the SE pier led up to a suite of chambers and corridors in the upper reaches; the arch must have had an attic storey, which some would restore as a pyramid but is more likely to have been rectangular, echoing the podium-like footings. A substantial superstructure in brick-faced concrete was removed in 1830 in the belief, perhaps mistaken, that it was the remnant of C13 fortifications by the Frangipane family.
Amanda Claridge, Rome: An Oxford Archeological Guide (Oxford: Oxford University Press, 1998), p. 258.
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