ENHS Nature Trails November 2001 Article 3

by Peter Helzer

I'm gazing down on an unfamiliar landscape, passing over crags and deep winding ravines, craters and palisades. The colors keep changing: earth brown, blue, metallic gray--then we pass over vast areas of white; sometimes flat and powdery, sometimes rising up like jagged towers of ice. My window onto this world is not the curving Plexiglas of an airplane but the eyepiece of a microscope. My wife, Marge Helzer, repeatedly glides over this terrain, crossing in north/south transects. Marge is the expert; I am a tourist, along for the sheer fun of it all. The object of her attention is a piece of charcoal, slightly smaller than a pencil eraser. After an hour of study, she announces, "I know what it is not, but not what it is." The charcoal has been collected from a fire hearth at the entryway to Connley Caves near Fort Rock, Oregon. The caves are important for two reasons: they are very old, possibly dating to 11,000 years before present, and they have recently yielded a surprising cache of exquisite basketry. Only a handful of these early Holocene sites have been discovered. Marge joins others, each a specialist, in the analysis of bone, soils, pollen, lithics, and blood residue to add to an emerging picture of Oregon's earliest inhabitants. Dr. Dennis Jenkins, the archaeologist in charge of the excavation, has asked Marge to analyze a liter of soil taken from the site, hoping that she can extract and identify charcoals from this soil so that the materials can be radiocarbon (C-14) dated.

Archaeologists must know exactly what kind of material is being dated because there are a few species of wood that can give misleading information. One of these is juniper (Juniperus occidentalis). Because this tree can live over one thousand years, the dates retrieved through C-14 techniques can be skewed by 1150 years when the variability of the technique is factored in. Marge systematically eliminates all of the woods capable of giving false readings, then goes to her comparative collections hoping to determine what the charcoal is. She rounds up a couple of suspects. The task is different from traditional botany in that most of the usual features of the plant are missing: there are no leaves, buds, or bark. Often there is no indication of plant size. What's left after 11,000 years, is an incomplete structural architecture, a turbulent landscape of crags and valleys, sometimes forming patterns, sometimes trailing off into indecipherable fields of ash.

Marge drops a small twig in my hand from her collection and asks me to burn it to the proper consistency. Not so easy--if I apply too much heat, it will turn the twig to useless ash; if I apply too little, it will not show the desired structure. After a few tense minutes in the shop, I return to the lab with a piece of charcoal slightly larger than a toothpick. Marge slides it under the microscope and adjusts the focus. After a short flight over the newly charred landscape, she finds what she is looking for and replaces the freshly burned twig with her Holocene speciman. "It's Purshia" she says, Purshia tridentata, bitterbrush, a plant with a life span well under one hundred years and therefore an excellent candidate for a C-14 dating. She will give this charcoal to Dr. Jennings and another small piece of the puzzle will fall into place.

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