The Basics of Boulders:
For the past four weeks, we (Abby Rec and Ilana Sobel) have been investigating boulder fields in south-central Pennsylvania and Northern Maryland. Boulder fields are exactly what they sound like…
…a field of boulders. Our advisor, Dr. Sarah Principato, decided to begin studying them because of how little is known about their origin and process. Boulder fields are anomalous in the field of geomorphology but carry important information about ancient climate change. These fields are examples of how the topography changed in response to the last glacial maximum 20,000 years ago when the Laurentide Ice Sheet (blue) extended into most of Northeastern U.S. (below):
This summer’s research sought to identify whether the boulder fields were created by periglacial activity, or if their distribution and orientation is more heavily impacted by gravity. The term ‘periglacial’ refers to areas adjacent to glaciers/ice sheets that undergo periods of freezing and thawing due to extreme cold. When water enters fractures in bedrock and freezes, the water expands and cracks the rock, resulting in macrofractures such as in this boulder (bottom left):
While macrofractures caused by freeze-thaw action are good indicators of cold climate and possible periglacial activity, it doesn’t completely explain the orientation of the boulder field. In order to quantitatively investigate the features of boulder fields, we chose to focus on two boulder fields (Hawk Mountain and Raven Rock Hollow) and two talus slopes (Thurmont Vista and Waggoner’s Gap) in order to distinguish any trends in geomorphological process. *Talus slopes are bedrock piles that collect at the base of a slope or cliff—caused by gravity pushing the rock downslope* (see image top right).
We expected that boulder field area will increase approaching the ice margin. In addition to size changes, we also compared and contrasted boulder fields and talus slopes. We hypothesized that talus slopes would have a stronger fabric (orientation of long axes) than block fields.
Getting the data…
Throughout this study, Ilana and I have certainly learned that field work does not come without its fair share of trials (no pun intended), laughs, and brief flickers of frustration when moving at one transect per hour. Being a pilot study, we were eager to organize and implement an efficient method for measuring these boulder fields. Surprisingly, all it took was two tape-measures and a Brunton compass. Hiking boots recommended.
At our first site at Hawk Mountain, Sarah, Ilana and I quickly learned why so few studies have been conducted on boulder fields. They are NOT easy to walk on. But, braving the deep crevices, snakes and spiders, we got to work laying transects in 5 meter intervals and measuring axis length, orientation (direction of long axis), parallel dip (angle of the rock face along the long axis) and perpendicular dip (angle of rock perpendicular to the rock face) with a Brunton compass.
Ilana was absolutely in her element, navigating the boulders with the skill and agility of a mountain goat. I, however, was ill-equipped for the rough terrain and required much hand-holding as I dodged large spider webs and sought refuge on intermittent flat rocks. We divided the responsibilities so that Ilana was in charge of the tape-measures (transects and axis length) and that I was responsible for the temperamental, but very useful, Brunton compass.
Transect and dip measuring requires some athleticism (see above)—as you can see, we have crouched in some very unnatural positions. I have also had to place my hand into several dark crevices of a boulder field nicknamed ‘Devil’s Racecourse’ in order to achieve a perfect perpendicular dip. No pain, no gain, ladies and gentlemen.
Having since made my peace with the Brunton compass, Ilana, Dr. Principato and I managed to get a good sample size from Hawk Mountain, Raven Rock Hollow, Thurmont Vista and Waggoner’s Gap. Our team suffered only one fatality: our single field-pencil, which fell—I swear, in slow motion—from Ilana’s hand into a dark, cavernous hole between boulders. We continue to mourn the loss. (Later that same day, Ilana suffered her only fall of our field experience. She was fine.)
Above: Ilana and I on a foggy, mysterious day at Raven Rock. We were a little unsettled by a noise in the woods with an uncanny resemblance to an Ewok from Star Wars. Neither of us would’ve been surprised if this lil guy marched out of the trees with a tiny hunting spear.
After collecting the data, it was time to work on forming conclusions. After a bunch of t-tests and correlation analyses, we came out with some interesting results that compels further investigation. We found that long axis length of the rocks in boulder fields were statistically significantly larger than rocks in talus slopes. Axis length of rocks in Hawk Mountain boulder field were found to be significantly similar to axis length in Raven Rock Hollow boulder field. There was also a significant difference in perpendicular dip between boulder fields and talus slopes—boulder fields having a larger dip than talus slopes. These results suggest that there is something special going on with boulder fields: they’re different from talus slopes which are influenced by gravity, and their erratic dip angles indicates that the fabric of the fields were not created merely by falling rocks. There were no significant correlations between axis length and parallel/perpendicular dip in both boulder fields and talus slopes, which suggests that process greatly impacts the distribution and orientation of the boulders, as rock angle is highly variable in each site and shows no distribution trends.
We’re planning on continuing our study to investigate boulder fields further north, approaching the ice margins using GoogleEarth in order to gain a better understanding of boulder field area in relation to geographical location. Stay tuned.