Rock Size Distribution at Boulder Field in Alexandria, Pennsylvania
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The JUNIATA JOURNAL of GEOLOGY, 1, 1-7 (2014) Original Article Rock Size Distribution at Boulder Field in Alexandria, Pennsylvania Joseph Orso Boulder fields are areas of land in which boulders, or large rocks, are richly deposited. These fields are mostly found in mountainous areas and are commonly on ridges and along valleys. In Pennsylvania, boulder fields can be easily found along the Appalachian Mountain range. Concurrently, I observed a boulder field along Waterstreet in Alexandria, Pennsylvania that was around 700ft x 900ft. Rock size and composition vary from different boulder fields. The distribution of sizes was an important aspect of my observations. If my hypothesis is correct, larger, heavier rocks will be found near the bottom of the slope. Gravity, talus creep, and rock fall support this idea. Larger boulders will be distributed at the bottom during deposition, or over time larger boulders will shift and slide down slope. The methods in which these boulders are deposited are unimportant in this experiment. However, the arrangement at which different sized boulders are found is what drove this process. After measurements were collected on the boulder field, I conclude that larger boulders were found at all levels on the slope of the hill. However, there was a greater concentration of large boulder, some even 120 inches long, near the bottom of the ridge. With the presented data, my hypothesis was supported, but with more measurements, my experiment would be more comprehensive and elaborate. Keywords.—Boulder fields; Appalachian Mountains; Rock distribution; Size gradient; Tuscarora Formation Department of Geology, Juniata College, 1700 Moore Street Box#1103., Huntingdon, PA 16652. E-mail: [email protected] INTRODUCTION frequently range in size and rock composition. The boulder field that this report is based on is In the heart of Pennsylvania, boulder fields seem located in Alexandria, PA along the Frankstown like a common geological structure. Boulder Branch Juniata River. An image of this boulder fields, occasionally referred to as talus slopes, field is depicted on page 9 (Figure 1). are areas in which boulders have been Alexandria is a small town located distributed and appear to be stacked on one approximately 35 miles southwest of State another. These boulder fields, which are College, Pennsylvania. The field I observed was abundant along the Appalachian Mountains, nearly 700ft by 900ft and is relatively steep. BOULDER SIZE DISTRIBUTION wooded edge, the field actually extended into the woods in nearly every direction a few hundred feet. What event deposited these boulders is unknown at this point. Many scientists claim that boulder fields such as this one were a result of a recent glaciation event (Kochel). However, others argue that the boulders were formed solely by freeze-thaw cycles and lithology. Some specifically suggest that lithology alone is the reason boulders fields in the Tuscarora Formation exist. Either way, Figure 1. Boulder field in Alexandria, Pennsylvania. these sandstone boulders must have underwent This picture was taken on January 30, 2014. extreme temperatures and pressures to form. The steepness and uneasiness of the boulders at The lithification process of a sandstone from this site cause this area to be hazardous. This loose sediments, also known as diagenesis, can boulder field is part of the Tuscarora Formation. take on the upwards of millions of years to As determined by the U.S. Geological Survey, complete (Pettijohn). the Tuscarora Formation formed during the What intrigued me about this boulder field was Silurian time period (USGS). Therefore, this whether there was a gradient at which certain boulder field was created 419 to 444 million sized boulders were deposited or located. If years ago. Most boulder fields in Pennsylvania there were a gradient, it would be supported are made up of sedimentary rocks, more through the collection of data based on the sizes specifically sandstones. Sedimentary rocks form of the boulders. This experiment can possibly when existing rock weathers and erodes, and the exemplify the size gradients present at other sediments of these rocks accumulate and boulder fields throughout the Appalachian become lithified to form a different rock Mountains, more specifically, the Tuscarora (Tarbuck, Lutgens). The sandstone present at Formation. the boulder field was more specifically quartz arenite, which is a characteristic of the METHODS Tuscarora Formation. According to the Wentworth scale for the classification of In order to devise an applicable experiment to boulders, a boulder is defined by having at least test this size gradient hypothesis, I had to visit one linear dimension of 10.1 inches or greater the area multiple times. To locate this site I (Columbia). The boulders at my experimental needed to park past the field in a flea market location were poorly sorted and had no general parking lot, and then walk back up along the shape. Some boulders were round, while others highway till I reached the boulder field. The were block shaped. These boulders range in size first time I visited the site, I simply took a pen and can be as big as a car or a small as a and a notebook and recorded all the visible basketball. A closer picture of these boulders aspects that can be seen at this location. I can be seen on page 11 (Figure 2). Each boulder observed many features, but a feature that was to supports the weight of the boulders above it. In my benefit was that an old trail ran along the certain areas these rocks are shielded from the base of the mountain. This feature made this sunlight and algal growth has formed. Although site easier to access. Another important feature the boulder field looks like it ended at each of my first visit was the large accumulation of 2 ORSO measurements on the far left side of the field that was much safer. At this area, trees exist, which aided me on the climb up. Then by using a tape measurer, I measured the longest side of six different boulders. I measured one of the exposed surfaces and generated the results with this single measurement. I measured six different rocks a quarter of the way up. Then I proceeded up the mountain face till I reached approximately half way up. I simply determined if I was halfway by comparing the visual Figure 2. This is a picture of the boulder field up close. distance between myself and the bottom and top The size and shape of these boulders can easily be seen. of the boulder field. Six more measurements were taken half way up and then again three snow and ice on the steep slope. This proved to quarters of the way up. I figured measuring at be another way that this area was dangerous to three different increments would suffice and be climb. After that visit, I brainstormed and useful for this experiment. With these 18 created my hypothesis. After research, I realized measurements, I determined the mean rock size that there was little written on the distribution of for all three levels. I also calculated the range of boulders and the size gradient that may exist. I rock sizes to display any possible outliers that then created a plan to perform this experiment. existed. The results are portrayed in (Table 1) By walking along the path at the bottom of the on page 10. boulder field, I would determine the halfway mark of the field. I would then walk up RESULTS approximately a quarter of the way toward the top and make six measurements of six different Just at a glance one can see the ranging of sizes rocks around myself. I would also do this half of these boulders at Waterstreet. However it way up and three quarters of the way up. may be hard to visibly comprehend the gradient. However, I was unable to make measurements After conducting this experiment, the mean in this manner. The steep terrain and icy boulder size at a quarter of the way up was 74.83 conditions nearly ruined my developed plan. inches and had a range of 69. At half way up the Also snow accumulations at the base of the field boulder field, the mean size was 35.33 inches measured nearly four feet in areas, because the and a range of 22. At three quarter of the way snow seemed to simply slide down the slope. up the average sized boulder was 31.16 inches and had a range of 23. The results support the Due to the weather and ice present on the hypothesis that larger rocks would be boulders, I thought it would be easier and safer to walk up the left side of the field. I would then Table 1. Gradient of sized boulders present at the walk into the mass of boulders and make my boulder field in Alexandria, PA. It is obvious that the measurements in this manner. Again, this plan larger boulders were located at the bottom of the slope. was unsuccessful. When I walked into the mass of boulders I fell and slipped numerous times ½ up slope 37, 41, 44, 22, 39, 29 35.33 22 which resulted in tearing my pants and myself feeling uncomfortable. After these failed ¾ up slope 44, 21, 33, 26, 40, 23 31.16 23 attempts, I realized I would have to make 3 BOULDER SIZE DISTRIBUTION concentrated near the bottom of the boulder for the far left side. The fact that the field. Larger boulders were generally found measurements were taken on one particular side near the bottom, although boulders of all sizes of the boulder field might have skewed my were present at all levels. measurements. Each location of the boulder field might have a different gradient of sizes.