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Why dig at Cooper’s Ferry?

Posted by: | February 15, 2011 | 1 Comment |

People sometimes question the wisdom behind our choice to excavate at the Cooper’s Ferry site. Mainly this questioning is based on information about the recent history of the site and on things that people know about the archaeology of the Salmon River canyon. In this entry, I attempt to dispel a common misconception about the work that we’re doing at the site by addressing an important question that I’ve heard many times over the years.

Don’t you realize that you’re only digging through road fill?
In the 20th century, road construction took place in the lower Salmon River canyon in the vicinity of the Cooper’s Ferry site. To make the modern road, construction workers removed gravelly sediments and basalt bedrock deposits, cutting a notch into the toe of the canyon slope. The geological materials that they removed to create the road needed to be moved elsewhere and some of this material was pushed over the edge of the bank toward the river in the area of the Cooper’s Ferry site. Because road fill is a recent kind of deposit that will not contain intact archaeological evidence of ancient human occupation at the site, working only in these deposits would be a bad thing. So, before we spend our time and effort to carefully excavate at a potential site, we must know more about what the site contains. We can gain this advance knowledge of what’s in a site in two ways: first, we need to understand what’s already known about the site from any previous archaeological or geological excavations; second, we can dig a small hole into the site to quickly see what’s under the ground.

During several summers in the 1960s, an archaeologist named B. Robert Butler excavated a trench toward the site’s eastern end. Butler noted that they encountered thick deposits of fill dirt, likely from road construction; however, intact archaeological deposits were found toward the bottom of their excavation units. In the summer of 1997, Loren Davis directed the excavation of a single test pit (measuring 2 x 2 m or about 6 x 6 feet in surface area) on the western edge of the site to a depth of 2.5 meters (a little over 8 feet deep). This test pit gave us a window into the ground, so to speak, showing a series of different layers made of varying colors and kinds of geological materials—nearly all of which contained prehistoric artifacts. The 1997 test pit did encounter a layer of fill, which measured less than a foot thick, at the site’s surface. In addition to finding artifacts, our 1997 test pits also found the intact forms of several ancient campfire hearth and two different pits. One of these pits contained a collection of stone tools, including beautifully crafted spear points, which were placed in the ground for safekeeping. We also sent small fragments of animal bone and charcoal to scientists at a commercial laboratory called Beta Analytic (you can learn more about this lab at www.radiocarbon.com) who conducted radiocarbon analyses on our samples. The results of these radiocarbon analyses showed that organic items held in the layers at Cooper’s Ferry range in age between about 8,500 and 13,000 years old.

So, there clearly is road fill at the site, but the thickness of this deposit varies across the site. On the basis of information from prior excavations, we know that both disturbed road fill material and undisturbed archaeological deposits exist at the site and that the road fill deposits appear to be thicker on the upstream (east) end of the site’s landform. Based on our study of the site, we also understand how to tell the difference between road fill and undisturbed archaeological deposits. Unlike undisturbed deposits in the site, road fill deposits show swirled coloration (like a calico cat), lack fine sedimentary layering produced by ancient floods of the Salmon River, do not contain intact archaeological features like pits and campfire hearths, and commonly contain items from the 20th Century.

Geological cross section through the Cooper’s Ferry site from south (left) to north (right) through the site’s western (downstream) end.

This cross section model shows our expectation of what we would see if we cut a large trench through the site, based on previous archaeological and geological studies. The 1997 excavation unit is shown as the light orange rectangle and extended through deposits 3 and 5. Our recent excavations were built as an extension of the 1997 unit and have progressed through about half of the depth of the 1997 unit. The excavation units are actually oriented along an east-west line but are shown in a north-south orientation here in order to best illustrate how they relate to the site’s deposits.

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