The Cliff at the Tunnel Mouth: Tilted Layers, a Fault and a Landslide

A Fresh Cliff in Pine Notch

1Pine Notch is a small mountain town where earthquakes rattle windows every few years and spring rains sometimes send landslides onto the highway. To keep trains off the slide-prone slopes, the state is digging a new rail tunnel through the ridge above town. Cutting the tunnel entrance left a fresh cliff about 60 m high, and engineering geologist Ms. Hiroko Abernethy studies it to find weak rock before the tunnel goes deeper, because tunnels through broken or crumbly rock can cave in. Eighth grader Matteo Ruskin shadowed her for a week as part of a school job-shadow program.

2Ms. Abernethy's notes describe seven layers, numbered from the bottom of the cliff to the top (Table 1). Layers 1, 2 and 3 all lean at about 40°. The top of layer 3 is cut off along a wavy surface, and layer 4 rests on that surface. Layers 4 through 7 lie flat. Layer 4 is a conglomerate, a rock made of rounded pebbles cemented together, and many of its pebbles are pieces of the same limestone that makes up layer 3. Layer 5 contains a thin bed of volcanic ash that a lab dated to 152 million years ago.

3A fault, a crack along which rock has moved, slices down through the cliff. It breaks layers 1 through 6, and on its east side those layers sit about 8 m lower than on its west side. Layer 7, a gravel, lies across the fault without a break. At the north end of the cliff, a heap of jumbled boulders and soil from an old landslide covers part of layer 7. Inside the heap, Matteo helped dig out a buried tree trunk that a lab dated to about 1,200 years old.

4To date the layers, Ms. Abernethy used a fossil guide that lists when each kind of fossil in the cliff lived (Table 2). The guide also notes that all trilobites had died out by about 252 million years ago, and that ammonites and dinosaurs other than birds died out about 66 million years ago. Layers 1 to 3 hold the remains of sea animals, while layer 6 holds dinosaur footprints pressed into what was once mud on land.