Reconstructing a shoreline history takes skill. Today we’re using altimeters to establish the elevation of Lago Cardiel’s former shorelines. We also continue to look for shells to help us date the lake’s past shorelines, a task that requires strong powers of observation.
In one short stretch there might be a dime-sized snail shell almost indistinguishable from the millions of white, rounded pebbles on this ancient beach. A few meters away, we might have to readjust our eyes to search for tufa, or smoothed glass fragments (people left bottles on the shore, which wore down like sea glass). Most often, we don’t know what we’re looking for, so we shuffle around looking staring at our feet as if trying to find a lost contact lens.
Barbara, Jay’s wife, is a master at finding stuff. She spotted a piece of nacre—the shiny part of an oyster shell–about the size of a quarter. I have no idea how she picked it out from the grayish-blue rocks around it. The fragment must have come from a pretty big oyster. Could giant freshwater oysters once have lived here?
As the hunt continues, Mike finds an oyster chunk as long as a stick of gum, and I discover a saucer-sized fragment, and another, the size of my palm.
What are these things? I picture a filter feeder big enough to eat a flamingo. With each fragment we find, it becomes obvious that these oysters probably originated from the sea. Jay says we’re on the edge of a drainage delta, so these oyster fragments may have been carried downstream from their fossil beds and deposited here.
These bivalves also appear to be ancient. Mike remembers finding similar oysters several years ago near a rock outcropping at least as old as the Miocene, the geological period that spanned from 23 to 5 million years ago.
Such is the nature of fieldwork: a constant gathering, synthesizing and imagining of information. How can I build a story about this place that incorporates all of the latest clues?
