Jessica L. McEachern* and Daniel Davis
Department of
Geosciences, SUNY Stony Brook, Stony
*now at MC Environmental, LLC,
Introduction
Boulders on the beach appear to be distributed non-randomly, and we have performed statistical tests to verify this hypothesis. The manner in which the boulders are distributed may give one insight into their history and mechanism of transport. We have devised three hypotheses to attempt to explain this distribution. The purpose of this study is to trace the history of these boulders from transport away from their source by studying their pattern of distribution at the park; to attempt to locate the source of the boulders; and to deem any one of these hypotheses as valid. We have obtained results from this study via three basic types of observations: boulders on the beach, boulders on top of the bluffs (in campground and on trails), and boulders uncovered via ground-penetrating radar (GPR) surveys.
Proposed Hypotheses for Distribution
The first hypothesis suggests that there are isolated clumps
of boulders scattered in the park and on the beach, with no real pattern or
trend to suggest anything about transport mechanisms.
The second hypothesis suggests the presence of linear trends of boulder clumps in the direction of propagation of the glacier (in this case a roughly north-south trend). This would imply that there are distinct sources for each of the clumps which are seen. This model gains support if the clumps on the beach and in the park have a linear connectivity in the direction of glacier movement as discussed in literature. Once again, a method such as circumnavigating the clumps found via GPR is a good way to test for this. The model fails if the GPR surveys do not show evidence suggesting this situation, or if the directions of glacier movement do not fall within a reasonable range.
The basic motivation for the final hypothesis is tectonic
process. This model suggests that the
boulder clumps are situated along strike, or normal to the direction of glacier
movement, which may give clues about the transport mechanisms involved. For example, it may be possible that the
glacier ran into obstruction and had to ramp up, thus dumping its load of
detritus. Evidence of a boulder
distribution suggesting such a trend, or any indication of tectonic process
where a change in the decollement can cause the
glacier to dump its load, uncovered via GPR surveys would make this model very
favorable.
Results
We studied 647 boulders on the beach along a line parallel to the shoreline and over a distance of 1.3 kilometers. We classified them according to several important characteristics, including roundness (measure of how smooth or angular the edges are; Pacholik and Hanson, 2001), apparent diameter, lithology, and position along the beach. Officials at the park maintain that they are either native to the beach or had been previously located in the bluffs overlooking the beach (Nellen, 2001). As the coastline eroded back, the boulders would tumble down out of the cliffs into gullies or cascade down the sides of gullies, particularly during a storm, and end up on the beach. One question is whether the distribution of boulders is the result of a beach process, or if it is related to their history and mechanism of transport, as shown by one of three models we have devised. The results of this study clearly show that the distribution of boulders on the beach is non-random, apparent from the distinct clumps in which they are found (fig. 2).
Furthermore, there is not a significant amount of northward transport in this process. That is, when the boulders tumble out of the cliffs, they do so more in a z direction (down) than in either the x or y directions. This results in the final position of the boulders being within approximately 10 meters of the original position. This lends credence to the idea that this distribution is not the result of beach processes, and may reveal something about their transport.
We conducted a GPR survey on the park trails from May to June 2003. 200 MHz antennas were used for most of the GPR lines run, and several were conducted using 50 MHz antennas. Both yielded a presence of boulders within a restricted layer near the surface. This layer appears to be a capping diamict that rises upward. It is possible that the boulder-bearing layer rises up with it. The radargram in figure 3 shows boulders occurring in clumps at a depth of about 5 meters beneath the surface.
We compared the boulder density for clumped areas on the GPR line to that for clumped areas on the beach. The results were within very close range of those for the boulders on the beach. Clumped boulder densities for beach boulders and radar boulders ranged from 0.007 to 0.05 boulders per m2, and 0.013 to 0.03 boulders per m2, respectively. These results suggest that the radar clump is of the same order as the beach boulder clump. It may be reasonable to describe the radar clump as a “4th millennium clump.” That is, as the cliffs are eroded back, this clump would be visible on the beach in the distant future.
Discussion
The distribution of boulders on the beach is non-random. This is reflected in their location along a line parallel to shore, as well as in the statistical results. The ground-penetrating radar surveys have uncovered clumps of boulders located below the surface in the park, thus the distribution of these boulders also appears to be non-random. The boulder densities of these clumps fall within the same range as those on the beach, meaning that they are on the same order as the beach clumps.
At this time, hypothesis 3 does not appear to be very viable. The fact that the boulders appearing below the surface in the park are found in a capping diamict layer makes this tectonic model unlikely. Furthermore, if this model was viable, one would expect to see a broader band of boulders on the beach. Hypotheses 1 and 2 are both viable. However, we do not know yet whether the clumps on the beach and in the park subsurface have a linear connectivity, or are isolated. In order to differentiate between the two models, a circumnavigation of the radar clumps is suggested. If no other boulders are found, hypothesis 1 may be more favorable. If more boulders are encountered along the line between beach clump and radar clump, hypothesis 2 would seem more likely. In order to make a more clear distinction between the two unresolved hypotheses, it is important to conduct further GPR surveys in the park.
References
Hanson, G.N., 1996, personal communication.
__________, 2001, personal communication.
Lewis, R.S., and Stone, J.R., 1991, Late Quaternary Stratigraphy and Depositional
History of the
Coastal Research, Special Issue 11, p. 1-23.
McEachern, J.L., 2003, Analysis of Boulder Distribution: Implications for Glacial Processes in the Vicinity of Wildwood State Park, Wading River, New York MS Thesis Stony Brook University
Nellen, R., 2001, personal communication.
Pacholik, W., and Hanson, G.N., 2001, Boulders on Stony Brook Campus May
Reveal Geology of
Long Island Sound Basement. Geology of
Metropolitan
Suffolk County Department of Public Works, 1974, Topographic Map - Five Eastern
Towns -