Abstract. Introduction

Size: px

Start display at page:

Download "Abstract. Introduction"

Cornelia Freeman
6 years ago
Views:

1 Analysis of pebbles from loess of David Weld Sanctuary, St. James, NY: What process brought these pebbles to the loess? Breanna Podmore Stony Brook University, Stony Brook, NY Abstract The purpose of this study was to gain a better understanding of the nature of the pebbles found within the loess across Suffolk County. Sampling of 00 pebbles from three sites in Suffolk County and pebble analyses of these samples was done. One sample site, the David Weld Sanctuary, found in a tunnel valley, provided data that suggests glacial processes being responsible for these pebbles. This sample had 8% quartz pebbles, which resist weathering. Both striations and flattening, typical of glacial pebbles, were also found within this sample. Lastly, many disk-shaped pebbles that are flattened, perhaps by glacial movement were present. These analyses lead us to believe that glacial movement was the original process that brought these pebbles into this loess material across Suffolk County. Past studies called this pebbly loess a diamect providing even more insight into this hypothesis. A compilation of these analyses and past research efforts helps us to gain a better understanding of these pebbles and their origin. This study and its efforts can help scientists to account for these pebbles in sediment that is does not typically have pebbles. Introduction The purpose of this research was to study the nature of the pebbles in a diamect pebbly loess. Loess material does not typically have pebbles, so their existence in Suffolk County Farm (SCF), Dwarf Pine Plains (DPP) and David Weld Sanctuary (DWS) is currently somewhat of a mystery to geologists. Our main focus was to find out how these pebbles got there, as wind would not be strong enough to carry many of them. Pebbles have been studied quiet thoroughly and many telltale signs of different processes have been identified. Sediment in these areas has also been studied by using grain size analysis to gain a better understanding behind the distribution of different grain sizes. This study lead by a Stony Brook University research team concluded that the sediment across Suffolk County would be better described as a diamect. A diamect by definition is poorly sorted, and can form by a variety of processes (Dominquez, 0). As Dominguez goes on further to discuss, being a diamect correlates to the unsorted nature of the sediment and the presence of pebbles. This study looks to build off of this information and gain a better understanding of the pebbles themselves. By collecting samples from all three sites and carefully analyzing each pebble from these samples, we can gain a better understanding of their history. We chose three locations where this pebbly loess was known to be and did our sampling unbiased. When conducting this study it is important to ensure randomness to eliminate any bias. This was done by using a net to guide our collection of pebbles. This method essentially chose the pebbles for us. Analyses after collection gave us further information about this pebbly material that can be further studied in the future. Having grain size analysis from past studies and now pebble analysis from this study can provide future efforts with the foundation of this material and its origin. Based on the studies completed it is likely that these pebbles were brought here by glaciers.

Methods Pebble samples were collected from three locations in Suffolk County: Suffolk County Farm, Dwarf Pine Plains and David Weld Sanctuary. At each location a net with boxes of x inch was laid out.

2 Methods Pebble samples were collected from three locations in Suffolk County: Suffolk County Farm, Dwarf Pine Plains and David Weld Sanctuary. At each location a net with boxes of x inch was laid out. The nearest pebble to every cross-section was collected to ensure randomness. No pebble under cm was collected. 00 pebbles from each site were collected in this manor and labeled by location with the following data: David Weld Sanctuary (DWS) 00 N,.00 E, Altitude m (Fig.) Fig. : David Weld Sanctuary site of sampling with net laid out for pebble collection. After collection from all three sites each set of pebbles was washed in soapy water and any soil remaining was scrubbed off. Pebbles were then rinsed and laid out to dry overnight. Once pebbles were clean, each pebble was then numbered from -00 and sorted by rock size and type (Fig. ).

Fig. : David Weld Sanctuary pebble samples sorted by size and mineral type. Each pebble was then analyzed to find the following information (Appx. A):. Sphericity. Angularity. Fracture. Composition.

3 Fig. : David Weld Sanctuary pebble samples sorted by size and mineral type. Each pebble was then analyzed to find the following information (Appx. A):. Sphericity. Angularity. Fracture. Composition. Ventifacts. Glacial striations. Glacial flattening. Sphericity was found by using a Vernier Caliper to measure axis A, B, and C for each pebble. These values were then plugged into both Wentworth () (Fig. ) and Krumbein () equations for sphericity (Fig. ). Diameter was also taken from these values by finding the mean of each set.

Fig. : Direction of the axis to measure with caliper shown on the left. Wentworth s equation for sphericity shown on the right. Fig. : Krumbein equation for sphericity.

4 Fig. : Direction of the axis to measure with caliper shown on the left. Wentworth s equation for sphericity shown on the right. Fig. : Krumbein equation for sphericity.. Angularity was measured using a two-step process. First, a roundness value of - was used referring to Powers () image (Fig. ). Second, those values of - were matched up to an arithmetic midpoint value also from Powers () ranging from.-.8 (Fig. ). Fig. : Powers image for roundness scale of -.

$. Fracture was determined by closely analyzing each pebble and placing it under one of the following categories: FF- Fresh fracture (shiny flat face, rigid sharp edges) RF- Rounded Fracture (dull$

5 Fig. : Highlighted in red is Powers table for roundness scale arithmetic midpoint.. Fracture was determined by closely analyzing each pebble and placing it under one of the following categories: FF- Fresh fracture (shiny flat face, rigid sharp edges) RF- Rounded Fracture (dull flat face, rounded edges) N/A- No fracture evident. Composition was determined by examining the pebble with the naked eye and microscope if needed. Hardness test was also used. The following abbreviations apply: GR- Granite QZ- Quartz MI- Mica. Ventifacts were noted on larger pebbles if a flat face with a subtle dip in the center and a distinct edge was found on + sides of the pebble. This was recorded as a Y for YES for every pebble with ventifacts.. Glacial striations were noted when parallel or sometimes intersecting scratches cut into the surface of the pebble. This was recorded as a Y for YES for every pebble with striations.. Glacial flattening of pebbles were also noted when + side was worn flat and/or for disk shaped pebbles. This was also recorded as a Y for YES for every pebble with evident flattening. Results The focus of this research component was The David Weld Sanctuary in St. James. Sphericity values for this site using Krumbein () equation all fell below.0 cm. When data was plotted to match up with Zingg s chart (Fig. ) most values landed in the spheroid and disk regions as shown in figure 8. Figure shows most of the pebbles were angular with lower values. Angularity vs. Sphericity in figure 0 shows most pebbles with a sphericity of.-. with ranging angularities. Fracture types were pretty evenly distributed in the David Weld

6 Sanctuary with the majority of pebbles having no fracture at all as expected with quartz pebbles (Fig. ). Fig. : Zingg s chart to match up data from Krumbein sphericity values on.

7 Fig. 8: Krumbein sphericity values plotted onto Zingg s shape graph. Most points found within spheroid and disk regions. Fig. : Angularity shown on a histogram showing most pebbles to be angular.

8 Fig. 0: Angularity vs. sphericity shows concentrated angularities with varying sphericities. Fig. : Most pebbles did not have any fracture sites. This is expected with quartz materials. 8

$Composition in DWS was predominantly quartz followed by granite (Fig. ). When composition and fracture were compared, the majority of quartz was not fractured.$

9 Composition in DWS was predominantly quartz followed by granite (Fig. ). When composition and fracture were compared, the majority of quartz was not fractured. Granite and other samples were so few that the data for this component is not reliable when comparing mineral type and fracture type (Fig. ). Fig. : Composition chart showing mostly quartz then granite and other (mica). Fig. : Fracture vs. Composition. Most quartz pebbles were not fractured, granite samples so small they are not reliable.

As there were very few pebbles large enough to have ventifacts, no pebbles in the DWS sampling were found to have ventifacts. Some glacial striations and flattening in this site were present (Fig.).

10 As there were very few pebbles large enough to have ventifacts, no pebbles in the DWS sampling were found to have ventifacts. Some glacial striations and flattening in this site were present (Fig.). The lack of striations and flattening in the other pebbles may be due to weathering that further occurred after glacial movement. Fig. : Glaciation striations and flattening limited, but present. Discussion Among all three sites that the research team sampled from, there were pebbles within the loess material at all of them. Where these pebbles came from was the original reason for this research. Pebbles from each site were examined as described above to try and attribute a process to their arrival in the loess. It is is very likely that more than one process plays a role in a pebbles journey. The location, David Weld Sanctuary, of the sampling suggests glacial pebbles. The site of collection was on the wall of a tunnel valley. Tunnel valleys form as a result of melting from the glacier. The melt water from the glaciers causes erosion as it flows since it is under hydrostatic pressure forcing the water to flow upward (Menzies, 00). To this end, other characteristics such as flattening, striations, and disk shaped pebbles found in the David Weld Sanctuary are evidence of glacial movement (Wentworth, ). Figure showed about 0% of the pebbles having either striations or flattening. Striations in rocks are a clear indicator of glaciers. The scratches found on the pebbles show ice movement along the rock. Direction can even be determined when carefully examining some striations (Menzies, 00). Glacial pebbles also more commonly have flat facets, often times on both sides creating the disk shape (Wentworth, ). This flatness can be attributed to the great pressure it was under by the glacier or harder rocks (Charles, 8). Referring back to figures 8 &, many of the pebbles from this sample were disk shaped with flat sides. Very angular pebbles are also an indicator of glaciers. Figure 0 shows that most of the pebbles from this sample were lower in angularity values meaning more angular.a higher quantity of quartz is also an 0

11 indicator of glacial deposition since softer rocks would be ground into soil by the time the glacier was through with it (Charles, 8). The angularity of the pebbles also suggests glaciers as the majority of these pebbles were on the more angular side with values falling largely from - (Wentworth, ). Processes that weathered pebbles after the glaciers dropped them off, such as glacial melt water or a river, could account for the more rounded pebbles. Based on the data presented above it seems likely that glaciers were the original source of the pebbles in this loess material. What processes the pebbles have undergone since the glaciers in still a question we do not have all of the answers to. It is no surprise that these pebbles went through other processes and forms of weathering and erosion after the initial glacial movement. We can speculate that wind is probably not the process responsible for transport. Even the strongest winds do not typically carry larger particles, but rather more in quantity of smaller particles (Press, 00). Press stated, Strong winds rarely move large pebbles and cobbles in the way that rapidly flowing rivers do. Conclusion The pebbles found in the loess throughout Suffolk County are a topic of interest to some geologists. The fact that the pebbles are found within the layers of sediment makes their processes even more intriguing. It is safe to say that these pebbles were a product of glacial movement followed by another process or two. Wind can be ruled out to a degree so that leaves flowing water. This would be an interesting aspect to take this topic further with. More research on wind and water transport may give us a better understanding on what occurred after the glaciers. There are still many questions to be answered about the pebbles across Suffolk County as well as other parts of the world

12 Appendix A Site: sphericity Wentworth sphericity Intermediate long Short intermediate sam ple # A B C A+b/c b/a (y axis) c/b (x axis) Krumbein sphericity angularity - diameter fracture sqrt((b*c)/a^ ).-.8 (a+b+c)/ FF, FR n/a qz n/a qz 0.. rf qz compo sition wind glacial glacial QZ, GR, MI ventifact striations. n/a qz Y.0 n/a qz.8 n/a qz. n/a qz Y. rf qz Y. rf qz.0 n/a qz. rf qz. n/a qz Y. n/a qz. rf qz flattened

13 rf qz. n/a qz Y rf qz n/a qz Y n/a qz. rf qz. rf qz Y. n/a qz 0..8 n/a qz n/a qz Y n/a qz rf qz ff qz Y.0 n/a qz ff qz rf qz Y.08 rf qz. ff qz

14 n/a qz.0 rf qz.0 rf qz ff qz n/a qz ff qz rf qz n/a qz Y n/a qz.8 rf qz Y Y. ff qz.8 n/a qz. n/a qz. n/a qz ff qz rf qz 0.. ff qz n/a qz

15 rf qz.0 n/a qz Y n/a qz Y Y ff qz n/a qz.0 n/a qz. n/a qz rf qz ff qz Y 0..8 rf qz n/a qz.0 rf qz. rf qz ff qz n/a qz n/a qz ff qz n/a qz

16 n/a qz ff qz.8 rf qz. rf qz.0 n/a qz.8 rf qz. n/a qz. rf qz. rf qz. n/a qz. n/a qz. n/a qz ff qz. ff qz. n/a qz rf qz n/a qz n/a qz Y

17 rf qz n/a qz n/a qz Y Y n/a qz Y Y n/a qz Y ff qz Y. rf qz n/a qz n/a qz Y.0 n/a qz rf qz 0..0 rf qz rf qz rf qz.0 n/a qz rf qz ff qz n/a qz

18 n/a qz.08 n/a qz rf qz Y rf qz. n/a qz rf qz n/a qz rf qz ff qz Y Y. n/a qz.0 n/a qz.0 n/a qz.00 n/a qz.8 n/a qz.0 rf qz 0 rf qz n/a qz Y n/a qz 8

19 n/a qz 0.8 ff qz.0 rf qz n/a qz Y n/a qz rf qz n/a gr n/a qz Y 0 0. n/a qz rf qz rf qz rf qz.0 rf qz rf qz 0.8 ff qz ff qz rf qz Y ff qz

20 rf qz 0. n/a qz rf qz 0..0 ff qz n/a qz ff qz 0 0. ff qz ff qz.0 ff qz rf qz ff qz. ff qz.0 rf qz 0.8 rf qz.8 rf qz. ff qz ff qz 0. 8 ff qz 0

21 rf qz rf qz rf qz. rf qz 0. n/a qz ff qz ff qz ff qz ff qz.8 ff qz rf qz ff gr 8 n/a qz rf qz Y n/a mi rf qz ff qz n/a mi

22 n/a qz n/a qz. n/a qz. n/a qz.0 n/a qz n/a qz Y n/a qz Y rf qz. ff qz rf qz rf qz Y. n/a qz. n/a qz. n/a gr Y n/a gr n/a gr n/a gr.8 ff gr

23 n/a gr Y n/a gr ff gr ff gr rf gr n/a gr

24 References Charles, F.L. 8. How to read a pebble. A guide in nature study. l+flattening+of+pebbles&source=bl&ots=pqogeeqiv0&sig=hclwuehvzkmo AVSPsguA&hl=en&sa=X&ved=0CCIQAEwAWoVChMIiNgCfxwIVAyQeChnIgG U#v=onepage&q=disk&f=false (accessed August 0). Dominguez, K. 0. Grain size analysis and soil stratigraphy across Suffolk county: proxy for classification of sediment as a diamect. Stony Brook University. (accessed August, 0). Menzies, J. 00. Modern and past glacial environments. Butterworth-Heinemann. pgc&pg=pa8&lpg=pa8&dq=how+does+a+tunnel+valley+form&source=bl&ots=q qnlnpaznu&sig=jr0f0mzizgib-opgv-w- Np&hl=en&sa=X&ved=0CDcQAEwAoVChMIqygrLWfxwIVQZmAChlnQCX#v=o nepage&q&f=false (accessed August 0). Press, F. 00. Understanding earth. Susan Finnemore brennan. p. d+speed+needed+to+carry+pebbles&source=bl&ots=qlwaviavt&sig=qopphmuc_ OVHcMPgKkq0Me0jdg&hl=en&sa=X&ved=0CDgQAEwBGoVChMIzIHeqClxwIVihc- Ch0yQU#v=onepage&q=wind%0speed%0needed%0to%0carry%0pebbles&f =false (accessed August 0). Rodriguez, J. 0. Particle shape quantities and influence on geotechnical properties- a review. Lulea university of technology. (accessed august, 0). Wentworth, C.K.. Quantitative studies of the shapes of pebbles. State university of Iowa. (accessed August 0).

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

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