Recycling the Earth: Archaeology and Reconstruction at El Presidio de Santa Barbara SHP

by Michael H. Imwalle

In October 2015 the Santa Barbara Trust for Historic Preservation received a grant from the California Missions Foundation to investigate the architectural remains of the second outer defense wall. A small excavation unit was opened beneath the floor of the 1887 Bonilla house. The primary purpose of the investigation was to evaluate the condition of the wall. The exposure successfully identified in-situ adobe melt on the interior and exterior of the wall, identified the historic grade outside the wall, and recorded an episode of roof collapse on the interior of the wall. Lucas Nichols carefully exposed the wall including in-situ lime plaster on the interior surface.

All of the soil excavated consisted of adobe melt. Adobe melt is the soil that constituted the adobe blocks used to construct the wall. Over time, gravity and the weather cause the adobe to dissolve or melt and it is redistributed across the ground surface. Once the adobe melt has been archaeologically removed from the unit, artifacts, gravel, and other debris are removed from the soil with 1/8″ sieves. The sterile sandy loam that falls through the screen is recycled into new adobe bricks for reconstruction. Today the soil Lucas is screening is going right back into the walls as mortar. The walls are being rebuilt using soil recycled from the melted bricks originally laid on the second defense wall in the late 1790s.

This week we began the last phase of the Presidio Northwest Corner project. Trust Board member Tim Aguilar and Restoration Specialist Isidro Ruiz are laying adobe blocks to complete the three corral or yard walls behind the recently constructed adobes at the Northwest Corner.  These backyard areas will provide outdoor exhibit space for the new Presidio Visitor Center. Watch the progress as the last walls of the Northwest Corner project are completed!

Mike Imwalle is the archaeologist at the Santa Barbara Trust for Historic Preservation

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