THE MURALLA PIRCADA--A 2000 YEAR OLD NATURAL HAZARDS DAM NEAR THE SANTA RITA B ARCHAEOLOGICAL SITE, NORTHERN PERU

The Muralla Pircada is a northwest-trending, ~2.5 km long stone wall that forms the eastern boundary of the Santa Rita B archaeological site in the Chao Valley, northern Peru. This recently discovered Moche-Chimu site is 65 km southeast of Trujillo, near the town of Chao, and was an important center that provided control over the movement of resources between the cordillera and the coast.

Well-sorted, well-rounded, 20-30 cm and larger, volcanic boulders and cobbles comprise the main part of the Muralla which is 1-2 m high and up to 5 m wide. However, the Muralla turns sharply northeast at ~2.2 km and is generally smaller, ~1 m high and 0.8-3.0 m wide. Boulders and cobbles on the upstream face of the main part of the Muralla are imbricated, however, the downstream face is less organized. Long axes of the cobbles and boulders are inclined and perpendicular to the northwest trend of the structure. Case-and-fill construction was used in wider parts of the Muralla with smaller cobbles, pebbles, and sand used as fill. Meter-sized boulders are exposed in breaches and these may have been used in-place for construction. The Muralla is not symmetrical in cross-section, the base is thicker, to 5 m in the downstream direction and tapers to the top with a steep upstream face. The Muralla is perpendicular to and crosses the majority of the now abandoned NE-SW drainages that traverse the site.

At several locations on the ground and perpendicular to the Muralla are elongate clusters of 10-20 cm, well-rounded, and sorted cobbles. These clusters are on the upstream and downstream sides of the Muralla, are 1-1.5 m from the wall, and may have been used as stockpiles for construction or repair.

We interpret the Muralla Pircada to be a natural hazards dam that was constructed to protect the settlement from El Nino-related, or possibly earthquake-induced, debris flows originating in the cordillera ~2000 years ago.