BOISE ROCK, NOT THE GLACIAL ERRATIC THAT WE THOUGHT, BUT RATHER PART OF THE LANDSLIDE HISTORY OF FRANCONIA NOTCH, NEW HAMPSHIRE

Boise Rock (41.15313 °N, 71.67812 °W, 595 m a.s.l.), a large boulder composed of Conway Granite, lies on the northbound side of Franconia Notch Parkway (I-93), directly across the highway from Cannon cliff, from which the “Old Man of the Mountain’’ met his demise on 3 May 2003. Perhaps because of its size (~14 m across and 1 to 6 m high), odd sub-angular shape, and composition, Boise Rock has long been considered an archetype glacial erratic. The rock derives its name from a teamster, Tom Boise, who barely survived a mid-winter snowstorm during the early 1800s under the boulder’s overhang.

As part of an effort to determine the rate of vertical lowering of the last waning ice sheet to cover the New England area, we analyzed ¹⁰Be from a sample collected atop Boise Rock with the desire to provide a lower anchor point for our cosmogenic nuclide glacial dipstick reconstruction in Franconia Notch. Our surprising result was an age of 7.1 ± 0.4 ka, almost 6 ka younger than expected, based on ¹⁴C ages on organic material from lake sediment cores in the area. Profile Lake at the head of Franconia Notch, about 1 km north of Boise Rock, provides two calibrated ¹⁴C ages of 12.83 ka and 12.77 ka on wood and bulk organic matter at just above the transition from glaciolascustrine sediment to gyttja in two of several sediment cores (Rogers et al., 2010).

Although not useful for our glacial dipstick study, the likely Conway Granite bedrock cliff source for Boise Rock about 400 vertical meters directly upslope provides important information on the prehistoric landslide history in Franconia Notch. Of 20 landslides (debris flows) from the eastern slope of Franconia Notch as recorded by clastic lenses measured in Profile Lake sediment cores, six of the debris flows originated from the southern track zone between about 8.5 and 4.0 ka (Rogers et al., 2010). Although historic rock falls from nearby Cannon cliff, including the collapse of the Old Man of the Mountain, are not associated with extreme precipitation events, historic debris flows in Franconia Notch recorded by documentary data and clastic lenses in Profile Lake are. Precipitation-triggered debris flows and rock slides in Franconia Notch during the early Holocene are also consistent with pollen records from the White Mountains that indicate increased warmth and storminess beginning about 8.5 ka.