Gloria Gill Geo 305
104451458 Fall 2005
Characterizing the Boulders near the ESS Building
Within recent years, Professor Gilbert Hanson has acquired a remarkable
collection of boulders. As seen in Figure 1, this collection is located behind the ESS
building. These glacial erractics originate from different areas through out the Stony
Brook campus. During the construction of buildings and roads, workers excavated them
from the glacial sediments covering the campus. The purpose of this lab is to classify and
compare the boulders according to size, shape, roundness, breakage, and rock type. We
will attempt to name each boulder based upon our evaluations.
Figure 1: Map of Stony Brook University: The red area indicates
the location of the boulder collection.
We will also learn a new and time efficient technique for mapping the boulders
using a computer program and tape measure. This method of mapping takes advantage of
known coordinates of the buildings on campus. Very similar to triangulation used to
locate the epicenter of an earthquake; this method uses the intersections of circles to
locate a common point. In order to find the exact coordinates of a point of interest based
on its distance to points of known coordinates, one would need to use three reference
points. However, this is not time efficient and therefore we will use a computer program
that enables us to use only two points of reference.
The initial step in the field is to characterize each numbered boulder. The second
step is to measure the distance from the boulder to two reference points such as the
corners of the ESS building. It is important to record data in a consistent manner,
meaning that the reference point that is to the left is always the first reference point. The
computer uses each reference point as the center of a circle and the respective distance as
the radius. The computer will give the two sets of coordinates of where these circles
intersect. As long as the first reference point was to the left of the boulder, the boulder
will have the coordinates of the first listed set of coordinates.
I characterized each boulder in terms of roundness, shape and rock type.
Roundness refers to how rounded the edges of a boulder are rather than its shape.
Therefore, a boulder could be oblong and unsymmetrical yet still have a high degree of
roundness if it is smooth. Figure 2 shows the roundness scale, ranging from 0.1 to 0.9.
A roundness degree of 0.1 refers to boulders with very sharp edges and 0.9 refers to those
whom are extremely rounded. For example, the boulder labeled #1 in Figure 3 has
roundness 0.7. Although some of the cobbles have fallen out of this conglomerate making
it seem rough at first glance, it is evident by the smoothness of the undamaged rock
surface that a glacier rounded this rock to a high degree. The amount of rounding can
give many clues about the history of the boulder. Just as rocks in a rock tumbler become
spherical and smooth over time, boulders too will become more rounded and spherical
the longer they travel within the glacier.
Figure 2: The Roundness Scale: Objects with very sharp edges have a roundness
factor of .1 and the smoothest objects have a roundness factor of .9.
Figure 3: Map of Boulders behind the ESS Building: Each x represents a boulder.
I evaluated and analyzed the boulders represented by numbers.
Sphericity is the measure of the shape of the boulder based upon the comparison
of the three dimensional axes. Figure 4 shows the classification of shapes based on the
dimensions of the object. Note that a cubic boulder with similar height, width and length
is a sphere, yet it could have a very low degree of roundness due to angular edges.
Figure 4: The Classification of shapes: this diagram shows the four basic
categories of shapes based on the objects dimensions.
It is also important to distinguish between breakage and angular edges. Boulders
fall into three categories of breakage. Relative fresh surface breakage, worn surface
breakage and no indication of breakage. Boulders labeled #20, #31, #40, #45 and #5 in
Figure 2 are examples of relatively fresh surface breakage. Boulders labeled #36, #41,
#15, #2 and #3 have worn surface breakage and finally some boulders such as #26 and
#37 have little to no indication of breakage.
The main purpose of giving a rock a name is so that another geologist can
recognize it in the field. It is imperative that the name includes its color, grain size,
characteristic minerals besides quartz, and lineation or foliation if applicable. Through
the estimation of roundness and the evaluation of composition, a geologist can infer the
types of basement rocks that underlie the Long Island Sound. Stony Brook Campus is
located on the Harbor Hill Moraine. This moraine formed about 20,000 years ago during
the last glacial advance. The last glacial advance was a large flowing sheet of ice that
uplifted bedrock as boulders as it covered most of Long Island area. The ice carried these
boulders along the bottom or incorporated them into the ice. A boulder can only travel
about 20 miles or so before disintegrating into rock fragments, therefore we assume that
the least rounded boulders originate from the nearest bedrock.
The first field exercise had us describe each boulder in terms of roundness, shape
and rock type. Diorite, labeled boulder #3 in figure 3, shown in Figure 5, has a roundness
of .1 and I classified it as a sphere. Note that the shape of this boulder is not a sphere in
the traditional sense but rather has the dimensions of a sphere. This boulder is an igneous
fine-grained plagioclase phenocryst with fine-grained weathered mica as seen in figure
Figure 5a and 5b: Boulder #3 is a Diorite; 5b is a close up to show grains.
Hornblendite, labeled #20, has a roundness of .2. Figures 6a and 6b are
photographs of boulder #20. This disk shaped boulder is a uniform dark grey. I refer to
this rock as a “fine grained grabbro, biotite mica, chlorite, hornblende, igneous, disk-
shaped, angular Hornblendite.”
Figure 6a and 6b: Boulder #20 is a Hornblendite; 6b is a close up to show grains.
I call the rock labeled #26 in figure 3, a “Lineated, grey, rod shaped, medium to
coarse grained, hornblende, biotite mica, quartz monzodiorite, Beardsley gneiss with
metamorphic prophyroclastic potassium feldspar, dark, rod-like zenolith.” As shown in
figure 7a, this boulder has a lineated, pulled deformation and the orientation is elongated.
Figure 7b shows that its crystals have a preferred orientation and are inter-grown.
Figure 7a and 7b: Boulder #26 is a Beardsley Gneiss; 7b is a close up to show grains
The boulders labeled #31 and #41 are both Pumpkin Ground Gneiss. Figure 8a
shows that boulder # 31 is broken with a roundness factor of 0.8. I would call boulder #
31 a “sphere-disk shaped, rounded, light grey, lineated prophyroclast, agon gneiss, biotite
mica, hornblende Pumpkin Ground Gneiss with a zenolith.” This boulder has 20%
lineated (easily deformed) quartz, 30% longer plagioclase feldspar, and 30% potassium
feldspar as seen in Figure 8b.
Figure 8a and 8b: Boulder #31 is a Pumpkin Ground Gneiss. 8b is a close up
to show grains
Boulder # 41 on the other hand, has a roundness of 0.4. I named it “a Sub-
spherical fine-grained quartz matrix with coarse grains of agon gneiss, biotite mica,
hornblende and a pegmatite vain of Potassium feldspar.” Figure 9b shows that it actually
looks like two different rocks due to the presence of the vain. This lineated oblong gneiss
shown in figure 9a is the same as #31 but has a vein.
Figure 9a and 9b: Boulder #41 is a Pumpkin Ground Gneiss with vain. 9b is a close up
to show grains
Boulders labeled #15 and #36 are both Basalt. Boulder #15 is a disk shaped
boulder with a roundness factor of .6. This boulder shows evidence of worn surface
breakage. Its flat surfaces, evident in figure 10a, (referred to as ventifacts) go in different
directions. Striations accompany these ventifacts, shown in figure 10b. Its weathering
shell appears a different color than beneath due to iron oxides.
Figure 10a and 10b: Boulder #15 is Basalt. 10b is a close up to show grains.
Boulder #36 is very similar to #15; it too has a shell of weather product that is red
due to iron oxides and green due to magnesium oxides. The green is especially evident in
figure 10a. As shown in figure 10b, beneath this shell, it is a dark grey, fined-grained
grabbro, yet near the surface, it is a basaltic dyke.
Figure 11a and 11b: Boulder #36 is Basalt. 11b is a close up to show grains
Boulder #44, shown in figure 12a, is very small diabase that is spherical in shape
and has a roundness factor of 0.7. It too has an iron-magnesium weathered shell that
really masks the inside. This is evident by the break shown in figure 12b. I name this
boulder “a small coarse-grained plagioclase, hornblende/biotite mica diabase.” This sill
or dike, that undergone alteration is a textbook diorite.
Figure 12a and 12b: Boulder #44 is a Diabase. 12b is a close up to show grains.
The boulder labeled #37, shown in figure 13a is a called a Stony Creek Granite
and are very common on campus. It is composed of 40% potassium feldspar, 30%
plagioclase, 25% quartz and 5% muscovite and biotite mica. It has a roundness of 0.7 and
is spherical in shape. I named this boulder a coarse-grained pegmatitic granite. The dark
grey areas in figure 13b are biotite mica.
Figure 13a and 13b: Boulder #37 is a Stony Creek Granite. 13b is a close up to show
Two good examples of Branford Quartz Monzonite are boulders labeled #11 and
#45. Boulder #11 is sphere shaped with a roundness of 0.3 and composed of mica,
potassium feldspar and quartz. It also has a vain of pegmatite crystals, which is hard to
see in figure 14a. There are larger crystals in the quartz matrix. The sheared pegmatite
indicates that deformation took place after the vain developed therefore making it
difficult to see larger clasts. In addition, there are many little pieces of imbedded garnet.
The dark red spots in figure 14b indicate this common igneous garnet found in this type
Figure 14a and 14b: Boulder #11 is a Branford Quartz Monzonite. 14b is a close up to
Boulder # 45, figure 15a, is a spherical boulder with a roundness factor of .6.
This boulder is 50% potassium feldspar, 20% plagioclase and 20% smoky quartz. As
seen in Figure 15b it has an iron oxide, hornblende biotite matrix with pegmatitic, weakly
Figure 15a and 15b: Boulder #45 is a Branford Quartz Monzonite. 15b is a close up to
show the grains.
The next step was to map four of the boulders. I chose boulder #37 Stony Creek
Granite, #44 Diabase, #26 Beardsley Gneiss, and #3 Diorite. I entered Table 1, the
Boulder Mapping Data Sheet, into the computer program to get the coordinates for each
boulder. I then mapped the boulders using ArcGis. To improve this map I would like to
add in the walking trails that are in the area of the boulder collection. Without the trails, it
is difficult to assess the quality of the map by eye. However based on comparing this map
to the map in figure 3, I conclude that this method of mapping was successful. Figure 16
displays my finished map.
Table 1: I entered this table into the triangulation computer program to find the
coordinates of boulders base on their distance to SBU structures.
Boulder Mapping Data Sheet
Boulder ID Ref Pt 1 Distance 1 Ref Pt 2 Distance 2
40 Hp 2 19.764m HP1 16.19m
44 HP1 13.75m ESS8 24.20m
26 ESS8 13.589m ESS7 16.352m
3 ESS7 16.7m ESS4 20.262m
Figure 16: Boulders Behind the ESS Building: I created this map
using the resulting coordinates from the computer program and ArcGis.
It is very difficult to name and categorize boulders. A geologist must consider a
large amount of information when deciphering the origin and history of glacial erratics. It
is important to know that the difference between roundness and sphericity is that
roundness refers to the degree of smoothness and sphericity to shape. The shell of a
boulder can also be misleading since color and minerals may vary beneath. The
composition of the boulder is the deciding factor categorization and it is easy to be
mistaken about percentages. Most important, a geologist must study the boulder’s grains
as close as possible, preferably with a hand lens. In my studies, I did not use a hand-lens.
However to improve my results in the future I will use one.
Mapping the boulders using this modified method of triangulation was useful and
accurate. This method takes advantage of geometry and mathematics to achieve an exact
location. I find this method better than GPS since GPS can be 2-3 meters off. GPS would
be almost useless since the boulders are within 2-3 meters of each other. Pace and
Compass, another method of mapping would not be useful in this scenario because the
boulders are close to buildings which could interfere with the magnetism of the compass.
Therefore, I conclude that this is the most appropriate method of mapping.