Three hundred forty boulders on the campus of S.U.N.Y. in Stony Brook on Long Island were analyzed in order to find more specific information about the rock types to expected among the basement rocks of Long Island Sound. The study was based on the previous work made mainly by Drake (1972), Lewis and Stone (1991), Krumbein (1941) and others. These studies showed that:
1. Less then 0.1% of any rock type remains beyond 22 mi of its source.
2. The roundness and breakage distribution of the rock fragments indicate a state of dynamic equilibrium between the process of abrasion and crushing and it can be compared to the distance of transportation of this material.
3. The basic geological units of the Long Island Sound are known from seismic – reflection studies.
The most possible path of transportation of boulders was chosen from the direction between Stony Brook and Pine Orchard, east of New Haven on the Connecticut shore. On this line, the closest to the Long Island shore, is granite S followed by gneiss S of the same petrographical characteristic. The boulders closest to Connecticut, starting mid-way of this distance, are granites A with all the characteristics of the Avalonian rock types. The lineated leucogranite could came from two places, one in area of granite A and second in Connecticut. The basalt has its intrusions close to the Connecticut shore in the Hartford Basin. The approximate location of augen gneiss PG, gneiss B and amphibolite could not be estimated because of the small number of samples
Based on mean roundness .5 on the Krumbein scale, there is a relationship between the mean size of particles and the distance of transportation of these particles in a glacier.
The general information about bedrock of the Long Island Sound basin is derived from seismic – reflections studies; but specific data about types of rocks and their location are not known. Based on previous studies, it is believed that the bedrock that underlies the Long Island Sound Basin northeast of Stony Brook is similar to the crystalline rocks of Eastern Connecticut. These crystalline rocks are mapped on the Bedrock Geological Map of Connecticut (Rodgers, 1985) as Precambrian and Paleozoic gneiss and granites of the Avalonien terrane. In the New Haven area, Rodgers (1985) showed an off shore extension of the eastern border fault of the Hartford Basin. The extension of the Hartford Basin is also shown in the seismic study of Lewis and Stone (1991) as a deep bedrock anomaly (fig. 6).
The crystalline bedrock surface of Connecticut generally dips southeastward from the Hartford Basin (Pierce and Taylor, 1975). The bedrock structure of southwestern Connecticut, according to the aeromagnetic map of Hartford by Ziemtz (1977), extends offshore.
The crystalline bedrock surface in Long Island Sound is closest to the surface along the eastern side of the Eastern Border fault taking into consideration that surfaces of bedrock generally dip southeastward from the Connecticut coast (GRIM et al., 1970). West of New Haven, Paleozoic schist gneiss and granites of Iapetos terrane (Rodgers, 1985) are exposed along the north shore of Long Island Sound. Similar rocks probably extend under the western Sound because these types of rocks are known also from the extreme northwest corner of L.I. (Veatch, 1906). The Bedrock surface under western L.I. dips southeastward (Newmen, 1977).
The study of boulders from Stony Brook provides more specific information about the petrographical localization of rock types in the area between Stony Brook and Pine Orchard, east of New Haven, by analyzing the shape, roundness and breakage distribution. The direction from which the boulders have been transported can be inferred from the direction of the striations and grooves in the bedrock surface of Connecticut, the surface morphology, the shape of Harbor Hill-Roanoke Point - Fisher Island - Charlestown moraine and the map of seismic studies of Long Island Sound bedrock (Lewis and Stone, 1991).
Link to Map of the campus of S.U.N.Y. in Stony Brook with the area of study inside the red lines.
Petrography of boulders from Stony Brook
The 340 boulders were divided into 16 different types.
1. Granite S (18% of sample population) is medium – grained, very often slightly lineated biotite granite.
2. Gneiss S (10% of sample population) is medium – grained biotite gneiss.
3. Granite P (4.7% of sample population) is fine to coarse grain, two-mica granite, with gronoblastic structure and no indications of lineated distortions.
4. Lineated leucogranite (11.5 % of sample population) is two mica with abundant muscovite, moderately foliated, fine grain to medium grain with quartz flowing around white feldspar structures granite. These granites can be compared to the Ansonia or Shelton granites.
5. Granite A and gneiss A (11 % of sample population) are medium grain, garnet bearing, double mica granite and gneiss with creamy K-feldspar and white plagioclase. This granite and this gneiss closely resemble Precambrian and Paleozoic rocks of Avalonian terrain.
6. Pegmatite A (10 % of sample population) is coarse grained with very often mea crystals of creamy pictographic K-feldspar around 20-cm size, garnet bearing with white plagioclase pegmatite. This pegmatite seems to be associated with granite A and gneiss A.
7. Pegmatite K (13.5 % of sample population) is coarse grained with mega - crystals of creamy K-feldspar of the very often criptographic types (7 boulders contained pink K-feldspar), white plagioclase pegmatite. In the boulders, contact of the pegmatite with lineated leucogranite, granite S and gneiss S has been observed.
8. Pink gneiss is the medium grain, well - foliated, biotite type, and contains pink feldspar gneiss.
9. White quartzite (3.2 % of sample population)
10. Green low temperature quartzite (0.6 % of sample population)
11. Basalt (8.2 % of population)
12. Amphibolite (2 % of sample population)
13. Gneiss PG (2.3 % of sample population) is coarse-grain, lineated, well-foliated, containing biotite and large size augen-type K-feldspar gneiss. This gneiss has all the characteristics of Pumpkin Ground ortogneiss.
14. Gneiss B (0.8 % of sample population) is medium grained, strongly lineated gneiss. This gneiss has characteristics of Beardsley Gneiss.
15. Red conglomerate (0.6 % of sample population) is similar to Hartford Basin conglomerate.
16. Meta-metamorphic green schist (0.6 % of sample population) is similar to the schist at the bottom of the Hartford Basin.
The length in inches, of long (a), intermediate (b) and short (c) diameters was measured. The roundness of each boulder has been specified using Krumbein’s visual technique (Krumbein, 1941). The breakage has been classified in three categories of:
1. fresh break
2. worn break
3. no break
The measurements of boulders along three perpendicular axes were used to classify their shapes according to Zingg’s classification of particles (Zingg, 1935).
The sphericity was calculated according to Wadell’s definition of sphericity (Wadell, 1932).
The results are presented in Table 1. This is a large file containing the data for 340 rocks. Be patient while it downloads.
Ice motion in the glacier in the direction of Stony Brook
The ice in the structure of the Wisconsin Glacier moved into Long Island in three lobes. When the glacier reached the Long Island Sound Basin, the middle, Connecticut lobe seemed to be predominant and as a result, it pushed to the side in a fan- like way the two neighboring lobes of the Hudson and Narragansett. The evidence of this type of interaction in between the lobes is the shape of the coastal line of Long Island and the shape of Harbor hill – Roanoke Point – Fisher Island – Charlestown Moraine. The central part of this moraine has an arc- shaped band, which can be explained by the fan- like transgression of the Connecticut Lobe. The location of Stony Brook seems to lie on the border of interaction of these two streams of ice, wherein the fan on the Connecticut Lobe could be pushed back and forth to each side according to the gaining or losing strength by either side of the ice stream. This behavior of the glacier may explain the surface feature called the Wall and the boulders associated with Iapetos terrain like a garnet schist boulder on the path in Ashley Schiff Preserve on the campus of SUNY in Stony Brook. It can also explain presence of boulders, which have their most possible origin in rocks from western side of the Hartford Basin like Pumpkin Ground ortogneiss, amphibolite, or Beardsley gneiss.
The second type of evidence for the fan -like motion of the Connecticut Lobe comes from analyzing the lithology of boulders in Stony Brook. The bedrock immediately north is Stony Brook is anomalously deeper due to the extension of the Hartford Basin and only the Connecticut shore in this direction exposes the crystalline types of rock. This means that if the ice in the glacier moved mainly from this direction, all the boulders of crystalline type would be very well rounded because of the long distance of their transportation. The types of rocks and their percentage distribution would also be very different because there would be more rock types of western Connecticut. The rock sample from Stony Brook generally resembles the rock of east of the Hartford Basin (table 1).
This evidence shows that the boulders were picked up by ice on their way somewhere around Pine Orchard, and then plucking was continued on a path parallel to the eastern border of the Long Island Sound part of the Hartford Basin. Some rocks like boulders of basalt, conglomerate and meta – metamorphic schist were pushed from the Long Island Sound Hartford Basin into the main stream of the ice by side streams from the north.
Size as a dependent factor of roundness in determining the distance of the transportation of the rock fragments
Drake (1972) observed that pebble and boulder distribution from known sources of rocks decrease at similar distances of 21 miles in east central New Hampshire. His and Barrett’s (1986) observations suggest that shape parameters like roundness, sphericity and 4 categories of shapes (blades, rods, spheres and disc) should be independent of size (Barrett, 1986).
In Drake’s (1972) studies, the roundness, shape and breakage of pebbles was described versus the change in distance from the source of rock. If size were an independent factor, then the boulders should follow the same patterns of changes in roundness and breakage as they move by the same distance as pebbles from their source. It should be possible to estimate the distance to an unknown source of the boulders by matching the patterns of the changing in roundness and breakage of the pebbles through each mile from their source, to the patterns of the distribution of roundness and breakage of boulders from Stony Brook.
To make this comparison possible, Drake’s results (1972, table 1 and 2) were recalculated by summarizing the numbers in each column which represent the specific distance to the source of the rocks and then changing these numbers to percentage. Then, the patterns of the percentage of distribution of roundness and breakage in every successive mile from the source of rock (table 2 and 3) were compared to the distributions of roundness and breakage in all types of boulders (table 4 and 5).
The patterns of the percentage of the distribution of roundness and breakage of different types of boulders from Stony Brook do not match any patterns of distribution of those parameters in the roundness and breakage distribution of the pebbles in every successive mile from their source. The distance to the sources of the boulders can not be found by using this method. The shape parameters are not independent of size.
The size seems to be influencing the roundness and shape because bigger particles can sustain much higher pressures without breaking and bigger surface area needs a longer distance of transportation to abrade them to the same degree of roundness.