Counting Stripes in Glimmer Lake: Shaking, a Landslide and Deep Time

A Core From Glimmer Lake

1Glimmer Lake sits high in a steep mountain valley. Last July, a team from the state geological survey, led by Dr. Lucan Pereira, anchored a raft over the deepest part of the lake and pushed a long tube down into the lake floor. Bryn Oduya, an eighth grader at the lake's summer science camp, helped carry the pieces of the 10.5 m core to the lab tent and watched the team split them open.

2Most of the core was striped like a barcode. Dr. Pereira explained that in spring and summer, melting snow washes fine silt into the lake and a light layer settles on the bottom. In winter, when ice covers the lake, only very fine dark clay settles. So each pair of one light and one dark layer stands for one year. The team photographed the core and counted the pairs from the top down. Two people counted each section on their own and then compared their totals, and where a pair was hard to see they checked it under a magnifier. The pairs get thinner deeper in the core, where the mud has been squeezed by the weight above it.

3In a few places the stripes were interrupted. The team found jumbled layers of sand and mud a few centimeters thick, with bent and broken stripes just below them. Dr. Pereira said that strong earthquake shaking can send mud sliding down the lake's underwater slopes, and the mud settles again as a jumbled layer. In one place, a layer of coarse sand full of broken twigs and pine needles lay directly on top of a jumbled layer, with no stripes between them. The team thinks that sand came from a landslide off the valley wall. Table 1 lists these event layers and how many pairs were counted above each one.

4Below the striped mud, the core held gray clay with no stripes and scattered pebbles. This clay formed when a glacier still filled part of the valley and dropped stones as it melted. At the very bottom, the tube hit solid bedrock: gray limestone holding fossils of ammonites and belemnites, which were sea animals with shells. The team used the fossil guide in Table 2 to date the limestone.