GROUND-BREAKING TECHNIQUES: MATERIALS & METHODS OF HISTORICAL EXCAVATION AT RANCHO LA BREA, CALIFORNIA

The fossils of Rancho La Brea (RLB) are known for their abundance, asphaltic preservation, and diversity; excavation at the site began over a century ago and is still ongoing. We reviewed field notes and historical materials to improve practices and understand how new research foci and collection methods impact excavation techniques at RLB.

The Los Angeles County Museum (LACM) excavations of 1913-1915 extracted large fossils with miners’ picks, railroad pickaxes, and shovels, using dynamite and sledgehammers to break through the hardest matrix. In the 1969 reopening of Pit 91, a partially excavated LACM deposit, focus shifted to include the collection and study of microfossils, necessitating more refined practices. Hammers were paired with wood chisels or flat-headed screwdrivers for breaking up sterile matrix. Precision use of dental picks became the dominant protocol for densely fossiliferous areas. These practices continue in the current salvaged deposits of Project 23, where excavation is aided by small amounts of solvent, softening oxidizing asphaltic matrix, increasing pliability of sediments, and reducing damage to fossils. Plastic tool components are avoided, as asphalt and solvents can cause them to degrade. All current solvent use follows Cal/OSHA and institutional PPE protocols including gloves, safety glasses, and ventilation.

Some aspects of excavations at RLB have hardly changed. LACM pits were often flooded by rain and groundwater, requiring hand- and gas-powered pumps and bailing with buckets. This remains an issue in Pit 91, where a sump and mechanical pump prevent water from damaging unexcavated fossils. Turkey basters are used to remove water around surface-level fossils. Trenches were added along the exterior shoring of the excavation, channeling water to the sump.

Documenting changes in methods and materials over time at a specialized locality like RLB can lead to a better understanding of the site and why these changes have been adopted. By sharing this history, RLB aims to provide technical guidance and develop best practices with researchers at asphaltic localities around the world.