Beginning Stages to Determining the Primary Erosive Force that Shaped the Pebbles on Long Island, Site Location: Suffolk County Farm, Yaphank, NY

Alexandra Danz Stony Brook University, Stony Brook, NY

Abstract:

The pebbly loess known to cover much of Long Island has very little research to date. Research of pebbly loess or gravelly sand has been conducted in Iowa, Ohio, Minnesota and Alaska. Studies of the soil horizons on Long Island have been completed and findings show the pebbly loess is a diamict. Research on the pebbles in the loess on Long Island has not been conducted until now. Collection and analysis of the pebbles tells a small part to a big story; how did the pebbles get here and what was the dominant force that shaped them? Extensive work was done to obtain quantitative and qualitative data such as sphericity, roundness, composition, and presence of fractures. The strong presence of quartz suggests resistance to a powerful erosive force, but the variation of roundness is telling of a low cyclicity. From the data collected, the question of formation and shaping still remains, but landscape features and pebble shape make active glacial transport with later fluvial reworking a viable option. This process would involve long term reshaping where glacial debris would 1.) have the ability to break down and fracture into pebbles and 2.) minerals with a low resistance would weather away. Undergoing these processes would result in pebbles composed mostly of quartz that are sub-angular to sub-rounded and show rounded fractures. Introduction:

The unsorted nature of the pebbly loess at Suffolk County Farm in Yaphank, NY brings forward many questions regarding the presence of pebbles. After studying the nature of pebbles found in the pebbly loess, more specifically a diamict (Dominguez, 2014) analysis was conducted to begin understanding the driving question; what were the geological processes that were involved in the formation of the pebbles?

Exposures in Iowa, Ohio, and Minnesota, contain a similar pebbly loess, but disagree on the primary erosive force that shaped the pebbles. Coastal locations in Greenland, Baltic Sea and Hudson Bay have large spans of gravelly sand on cliffs. For the locations in the United States, some geologists suggest reworking by wind over a short period of time, while others suggest reworking by water over a long period of time. Even locations that agree on glaciers as the primary erosive force, cannot agree on the role the glacier played in shaping the pebbles. Pebbles could have been created by outwash, sheet erosion, or active transport within the glacier itself.

Melrose believes that the modes which are common throughout sediments on Long Island are not connected to the weathering process, but rather may be indicative of the bedrock material (Melrose, 2014). It was previously believed that from a uniform deposition with equal parts sand, silt and clay, a dominant force would separate out sediments. Nieter infers that the sediment is indicative of the mode (Nieter, 1975). Confident that the source of sediment was nearby, Nieter considers eolian weathering post glacial deposition and bases this conclusion off of surface features. Further, Nieter

states that the presence of ventifacts and dreikanters are evidence for a lag deposit before the deposition of the silt layer.

A random sampling of 200 pebbles at each of the 3 sites, including David Weld Sanctuary in St. James, Suffolk County Farm in Yaphank and Dwarf Pine Plains in West Hampton was conducted to rule out bias collection of data. This paper goes into detail about the pebbles collected at Suffolk County Farm in Yaphank, NY.

Figure 1: Map of location where data was collected

Figure 2: DEM of collection site. Samples collected just past rim of Carolina Bay.

Methods: To avoid bias sampling, we laid down a badminton net with 1’’x1’’ spaces. We

observed each interception site and collected the pebble that was closest to the interception under the prerequisite that the major axis was greater than or equal to 1 centimeter in diameter. We collected 200 pebbles from the Suffolk County Farm site, coordinates ranged from 40.828930°N, 72.924532°W at one side of the net to 40.828919°N, 72.924607°W. The altitude of the sampling site was 17m. Post collection consisted of washing the samples in a gentle soap and water solution and scrubbing the samples to remove soil allowing a better visualization of fracture and striations. Each sample was grouped and numbered with others in sets of 10 based on similar observable characteristics (e.g. presence of fracture, composition, angularity). To determine sphericity, the major (A), intermediate (B) and minor (C) axis were measured using a vernier caliper. Roundness was assigned to each sample using Power’s Roundness Scale.

Figure 3: Sampling Site, 7/21/2015

Figure 4: Badminton net laid over collection site used for objective sampling

Results: Figures 5-7 focus on sphericity

Type of pebble b ÷ a c ÷ b

Sphere > 0.67 > 0.67

Disc > 0.67 < 0.67

Rod < 0.67 > 0.67

Blade < 0.67 < 0.67

Figure 7: Zingg's 2/3 breakdown shows the sphericity limits which characterize a pebble as sphere, disc, rod (roller) or blade.

Figure 5: Side by side view of Zingg’s Shape Classification and Krumbein and Sloss, 1963 intercept sphericity chart. It is easy to see the shape that is yielded by the ratios of the intercepts.

Figure 6: Zingg’s shape classification is based on Krumbein and Sloss, 1963. Using the sphericity formula, values were plotted and fitted into shape categories. Most of the samples from Suffolk County Farm are classified as disk and sphere with a small amount of roller and even less of blade.

Figures 8-10 focus on roundness (angularity)

Figure 8: A classification developed by several authors based on Hawkins 6 classes of roundness (Hawkins, 1993). Each degree of roundness has its own limits. Roundness measurements developed by Wadell (1935).

Figure 9: Roundness according to Powers. Very angular was assigned a value of 1 trending to well-rounded which was assigned a value of 6. Numerical values were converted using Fig. 8 arithmetic midpoint column (Powers, 1953).

The subjective nature of measuring roundness was considered throughout the process. To minimize error, multiple people were asked to assign a pebble’s roundness value. Power’s roundness scale was

Figure 10: Roundness based off Power's Roundness Scale

Figure 11: Roundness vs. Sphericity gives a comparative view of 2 observable and measureable characteristics. Data points are grouped in columns due to Powers’ assigned values for very angular to well rounded. Images from Powers roundness scale have been overlaid for easy association of numerical values of sphericity and roundness.

used for consistency with Figure 9.

Figure 12: Samples from Suffolk County Farm were 95% quartz. 200 samples were collected and analyzed.

Discussion:

Evidence for glacial activity across New England and Long Island is apparent in landscape features such as moraines, presence of striations and erratics in the area. The source material is most likely predominantly carried from Vermont, Massachusetts, Connecticut and New Hampshire seen by the orientation of the striations. “In New England, the ice reached its maximum extent at Long Island around 21,000 years ago (ages presented here have been obtained from radiocarbon dating).” “Ice in New England extended out onto the continental shelf, which at that time was not covered by water due to low sea level. While at this maximum position a series of terminal moraines were deposited.” (Rittenour, 2015).

From the samples collected, 95% were of quartz composition. The sub-rounded to sub-angular nature suggests a low cyclicity. Rocks that were transported by the glacier were not limited to quartz, the bedrock of New England is mostly gneiss and schist. Quartz veins are probably the parent rock for the pebble material due to its resistant nature. While other compositions are present in the samples collected: anthracite coal, chlorite, granite, they are very low in number which aligns with their low resistance. The high presence of quartz suggests major reworking and resistance to weathering. New England and Long Island are known to have glacio-fluvial deposits. (Oldale and O’Hara, 1984). The quartz suggests that it was worked by a glacier and the rounded corners suggest that it was finished with fluvial weathering.

The average sphericity for

the 200 pebbles was 0.75. Figure 6, Zingg’s Shape Classification, gives a visual breakdown of the shapes that the pebbles were classified as derived from sphericity values. About 50% were classified as disc and 40% were sphere, with the remaining 7% blade and 3% roller. Figure 6 helps to visualize the relationship between sphericity and roundness using Krumbein’s formula for sphericity and Power’s Roundness Scale. It must be stated that validity of this graph is in question due to the subjective nature in assigning pebbles a roundness value. Power’s roundness scale is a picture scale and has received a lot of criticism, but remains the most used method for determining roundness. Using the 1-6 values allowed us to convert the values to numbers using Figure 8: Degrees of roundness, Wadell values. Determining sphericity was a very objective process due to the method of calipers. The lack of guesswork rules out significant error. Basing shape off of sphericity values allows confidence in the results. The sphericity was able to be plotted against the roundness because they both shared values between 0 and 1. Davis and Fitzgerald state that state that “sphericity is more strongly influenced by the origin of the particle than is roundness” (2004).

Similar exposures of pebbles in loess have been studied in Iowa, Ohio, Minnesota, Alaska and Greenland. The following paragraphs will address the above type locations and make comparisons to the Long Island pebbly loess better known as a diamict. They are listed in increasing order of probable forces that shaped the pebbles in the loess on Long Island, with eolian being the least dominant and glacial being the most prevalent in terms of shaping.

Figure 13: Approximately 1/2 of quartz showed rounded fracture and less than 2/3 showed no fracture. The samples of anthracite showed fresh fracture, whereas chlorite and granite samples showed no fracture.

EOLIAN: In Iowa, geologists have recorded the presence of a thick pebble band near the surface of the till (Kay, 1931). Theories regarding the origin of the pebbles are still disputed. One theory states that the band of pebbles “is the result of wind action in a marginal area during the retreat of the Iowan ice. The presence of the Laurentian Ice Sheet on Long Island 21,000 years ago would fit well with the wind action theory, however, few ventifacts were found in the area. This may also be because most pebbles were too small to see possible presence of ventifacts.

FLUVIAL: The second theory stated in Kay’s paper suggests that the pebble band is the product of erosion of the till by moving water. It should be acknowledged that this mode would take far longer than the previous mentioned view. Very few of the pebbles collected showed a roundness of 0.81 or well-rounded as assigned by Powers. This suggests that they were not eroded by a river for very long. Further, the discrepancy of roundness amongst samples could be due to pre-existing weathering pre-transport via the glacier.

GLACIAL: The Iowan drift exposure in Minnesota contains a concentrated band of pebbles beneath the loess. Alden and Leighton are confident the “pebbly concentrate” is the result of a “residual deposit from the wash that occurred before the till had become protected by the loess cover” (Leverett, 1932). The exposure is on a small slope which would align with the slow process which eventually transported smaller sediments and somewhat rounded the pebbles that remained. Sheet erosion, would conflict with the melting glacier and outwash due to the difference in time each process would take. Predominantly believed to be of glacial origin, Alaskan till and loess deposits in the eastern area of the Little John site show a 60 centimeter layer of pebbly loess (Hoffecker, 2007). Neiter lists evidence for glacio-fluvial events in his thesis (1975). He states that there is observable evidence found at the boundary between the silt and sandy-gravel. Nieter saw ventifacts, not present at Suffolk County Farm, which support strong winds coming off of the glacier. The pebbles may have been exposed to wind before the deposition of the silt, creating a lag deposit of wind eroded sediment.

The majority of the pebbles were categorized as a disc shape, based off the A, B and C axis used for the sphericity value. Zingg used a 2/3 rule that separates pebbles into 4 different shape categories: disc, sphere, roller and blade. His chart is based off of sphericity studies by Krumbein and Sloss (1963). Davis and Fitzgerald state that it is common to find the disc pebbles on the elevated portions of gravel beaches. Due to the long A and B axis, but short C axis, the pebbles are light for their size and were able to get to these beaches most likely by storm waves (Davis and Fitzgerald, 2004). In their book, Beaches and Coasts, Davis and Fitzgerald explain the presence of elevated beaches through glacial rebound. The weight of the ice sheet, in this case the Laurentide, causes the lithosphere and surface features to temporarily depress. When the ice sheet melted, the Hudson Bay and Baltic Shorelines sustained re-equilibrium and rebounded shorelines almost 300m. Sediments transported and deposited by the glacier are seen on these high beaches due to the effects of glacial rebound. While Suffolk County Farm lacks much relief in the topography, the glacial rebound supports the presence of pebbles in the loess. The 300m relief was seen at locations that were beneath the middle and therefore heaviest part of the glacier. Being that Long Island was the furthest approach of the glacier, it would make sense that there is low if any relief in the topography.

Many of the samples showed a rounded fracture which can be interpreted as having been rounded within the glacier as particles interact with other particles and the bed of the glacier itself (Benn and Evans, 1998). Benn and Evans give two outcomes for sediments that are transported actively in the glacier, which would require them to be in the basal shear zones: 1.) debris can either be broken into smaller pieces 2.) debris can be changed into another form or morphology as less resistant minerals break away (Benn and Evans, 1998). The pebbles that display rounded fracture align with the first outcome. The second effect on debris is supported in the data with 95% of the samples having the composition of quartz. If New England bedrock was glacially transported, it would make sense that the minerals in the igneous and metamorphic such as feldspars and micas would weather out and quartz would remain.

Conclusion: The question of the pebbles’ origin in the loess is a very big undertaking and having just first order, primary data, the following can be stated. Pebbles from Suffolk County Farm are located in the loess and are deposited on the surface. They are mostly quartz, consistent with glacial material and mostly deriving from the quartz veins in New England bedrock. Davis and Fitzgerald state that glaciated coasts, such as the coasts of Long Island, have sediments of similar composition to the source material (2004). In these environments, there is a wide range of grain size and compositions. The exposure at Suffolk County Farm includes pebbles mainly of quartz sitting in a silty loess. The stable and chemical makeup of quartz allow it to resist weathering during transport (Davis and Fitzgerald, 2004). The edges generally range from sub-angular to sub-rounded with an average sphericity suggesting low cyclicity. Acknowledging that some rounding can take place in the glacier, roundness and fracture suggests transport by glacier. Samples that are well rounded may have been fluvially modified before they were transported. Because the act of rounding and fracture balance out a rock’s physical appearance, samples rarely fall into the very angular or well-rounded categories in an active glacial environment (Benn and Evans, 1998). Benn and Evans’ claim that actively transported debris typically show intermediate roundness, specifically sub rounded and sub angular, is verified amongst the data from Suffolk County Farm. The pebbles in the diamict on Long Island most likely derived from fracture and rounding within the glacier followed by deposition with further reworking by water and wind. Overall, the transport, deposition and later reworking of the pebbles in the loess was most likely a time intensive process.

References

Benn, Douglas I., and Evans, David, J.A., 1998, Glaciers and Glaciation, 203-207, pp. Davis, Richard, Jr., and Fitzgerald, Duncan, 2004, Beaches and Coasts, 332, p. Dominguez, Catherine, 2014, Grain Size Analysis and Soil Stratigraphy Across Suffolk County: Proxy for Classification of Sediment as a Diamict, 18, p. Field Studies Council, http://www.geography-fieldwork.org/ice/fluvioglacial/4-data-analysis.aspx, July 2015. Hanson, G., 2014, Stony Brook University, oral communication. Hoffecker, John, F., and Elias, Scott, A., 2007, Human Ecology of Beringia, 158, p. Hubbard, Bryn, and Glasser, Neil, F., 2005, Field Techniques in Glaciology and Glacial Geomorphology, 229, p. Index, Iowa Geological Survey Annual Report, vol. 26 issue 1, Iowa Geological Survey Annual Report: Vol. 26: 447-457, pp. Kay, George, F., May-Jun., 1931, Origin of the Pebble Band on Iowan Till, The Journal of Geology, Vol. 39, No. 4, 377-380, pp. Leverett, Frank, and Sanderson, Frederick, William, 1932, Quarternary Geology of Minnesota and Parts of Adjacent States. Melrose, Courtney, 2014, Polymodal Grain-size Modes in Long Island Sands, Silts, and Weathered Bedrock, Stony Brook University Master Thesis. Nieter, William, 1975, A Late Wisconsian Loess Deposit in Southeastern Long Island, New York, Queens College Master Thesis. Oldale, R.N. and O’Hara, C.J., 1984. Glaciotectonic origin of the Massachusetts coastal end moraines and fluctuating late Wisconsinan ice margin. Bulletin of Geological Society of America 95, 61-74. Rittenour, Tammy Marie, Ice Ages in New England, http://www.bio.umass.edu/biology/conn.river/iceages.html, July 2015. Rodrigues Zavala, Juan, Manuel, 2012, Particle Shape Quantities and Influence on Geotechnical Properties, A Review.

APPENDIX 1: PEBBLE DATA

Link to Excel File with data