Wednesday, November 23, 2011

Rain Effects on the Platform Mound

Here in the desert, we are always hoping for rain and very grateful when it comes. However, rain has negative impacts on cultural resources that are exposed to the elements. The platform mound and compounds at Pueblo Grande are open for public visitation and education, but as such, they are also relatively unprotected from the effects of weather. Jim Britton, and his volunteer group of PGM Mudslingers, have done their best to counteract the effects of rain and other forms of weathering on the Pueblo Grande ruins for the past 18 years.

Moisture at the base of the platform mound
Rain seeps into the bases of the ruin walls and moves upward through the mortar by capillary action. The moisture reacts with the salt in the adobe, changing its composition and causing erosion, most commonly at the base of the walls.

Erosion of mortar between wall stones, caused by rain
Heavy rain also creates runoff channels, scouring prehistoric room fill and wall fabric, as water flows across the site, sometimes exposing sensitive resources and mixing and disturbing archaeological deposits.

Jim and the Mudslingers have created a series of drainage gutters along the top of the platform mound, the purpose of which is to channel the rain water to specific routes that drain it away from the sensitive parts of the ruin. The gutters themselves are composed of solid amended mud that is less easily eroded.

Runoff gutter created by Mudslingers
This past Saturday, stabilization activities were performed on the Pueblo Grande platform mound. Walls that were damaged from rain over the Veteran’s Day weekend were patched with amended mud, and cracks in some of the adobe were filled with a thinner “slurry” and then capped. The rain early Monday morning, following these efforts, was relatively light, and overall, the resource was minimally affected. But more rain is anticipated over the Thanksgiving weekend…
Maintaining the architecture of the site of Pueblo Grande is a never-ending task. Only a few areas of the site can be worked on at a time during the monthly Mudslinger activities. It can be difficult to keep up with the need for amended mud, when there are only one or two people hauling dirt to the site. Mixing up the material is strenuous work, and carefully delivering it to the specific work areas across the site keeps Jim very busy. If stabilization activities occurred continuously, there would still always be repairs needing to be made.

Posted By Laurene Montero, City Archaeologist

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