Table 2 Distribution of the Roundness of Pebbles versus Distance based on Drake’s (1972) studies

Roundness #

.9

0.16

-

-

-

-

-

-

-

-

-

-

-

-

-

.8

0.65

-

-

-

-

-

-

-

-

-

-

-

-

-

.7

2.29

-

1.75

-

-

-

3.03

6.06

-

-

-

-

-

-

.6

14.54

12.5

21.05

19.05

71.43

33.33

36.36

31.43

33.33

60

33.33

-

66.67

-

.5

45.26

47.22

53.51

33.33

28.57

66.66

46.97

48.51

58.33

40

33.33

66.67

 

100

.4

21.56

23.61

21.05

38.1

-

-

13.64

11.43

8.33

-

33.33

33.33

33.33

-

.3

8.01

9.7

2.63

9.52

-

-

-

-

-

-

-

-

-

-

.2

4.74

4.16

 

 

 

 

 

2.86

 

 

-

-

-

-

.1

2.78

2.78

-

-

-

-

-

-

-

-

-

-

-

-

Angular distance miles

1 mi.

2 mi.

2.5 mi.

3-4 mi.

4-5 mi.

5-6 mi.

7 mi.

7-8 mi.

8-9 mi.

9-11 mi.

12-13 mi.

13-14 mi.

16-17 mi.

20-21 mi.

 

Table 3  Distribution of the breakage of pebbles versus distance based on Drake’s (1972) study

Group Breakage

3

48.27

41.77

57.52

52.17

100

100

27.0

28.21

33.33

100

100

21.43

100

100

2

23.84

35.44

23.84

34.13

-

-

55.41

64.10

66.67

-

-

35.71

-

-

1

27.84

22.78

18.58

8.69

-

-

17.57

7.69

-

-

-

42.86

-

-

Distance

0-1 mi.

1-2 mi.

2-3 mi.

3-4 mi.

5 mi.

6 mi.

7 mi.

8 mi.

9 mi.

10 mi.

12 mi.

14 mi.

16-17 mi.

21 mi.

 

Table 4  Distribution of roundness for boulders from Stony Brook.

Round-ness no.

Granite S

Gneiss S

Granite P

LineatedGranite

Granite A

Gneiss A

Pegmatite A

Pegmatite K

Quartzite

Basalt

.8

-

2.86

-

-

-

2.94

2.56

7.61

-

.7

1.61

5.71

-

7.64

-

2.99

2.56

15.38

14.29

.6

14.52

11.43

57.14

17.95

16.22

5.88

35.24

7.64

17.86

.5

12.90

5.71

28.51

35.90

37.83

23.53

23.08

23.08

42.86

.4

19.95

28.57

7.14

7.64

18.92

38.24

15.38

 

21.43

.3

30.65

20

7.14

10.26

21.62

20.54

25.64

38.46

-

.2

16.13

22.86

-

20.51

5.40

2.94

-

7.69

3.57

.1

4.84

2.86

-

-

2.70

2.94

-

-

-

 

Table 5 Distribution of breakage for boulders from Stony Brook

Category of breakage

Granite S

Gneiss S

Granite P

LineatedGranite

Granite A

Gneiss A

Pegmatite A

Pegmatite K

Quartzite

Basalt

3

30.65

25.71

64.29

43.54

35.14

14.71

43.54

23.08

89.28

2

45.16

37.14

21.43

30.77

43.24

58.82

43.59

46.15

7.14

1

24.19

37.14

14.28

25.64

21.62

26.47

12.82

30.77

3.57

 

Table 6  Distribution of shapes of different types of boulders in Stony Brook

Shape

Granite S

Gneiss S

Granite P

LineatedGranite

Granite A

Gneiss A

Pegmatite A

Pegmatite K

Quartzite

Basalt

Spheres

38.71

28.57

50

30.74

51.31

41.18

74.35

46.15

60.71

Blades

11.29

8.57

 

5.13

8.11

2.94

 

 

 

Discs

37.04

57.14

27.57

35.90

32.43

17.65

20.51

30.77

25

Roundness

12.90

5.71

21.43

28.21

8.11

38.24

5.13

23.08

14.24

 

Change of the roundness in the process of transportation

            According to previous studies (Drake, 1991, Goldthwait, 1968), less then 0.1 % of any lithology remains beyond 22 mi of its source. Assuming that the most possible direction of ice movement to the area of our study was Pine Orchard – Stony Brook direction, it gives us the distance of 28 mi of maximum of transportation through LIS Basin. It means that almost all boulders have their source in the bedrock of LIS.

            Observation of boulders indicates that boulders were rounded to a roundness 0.5 or higher on Krumbein’s scale and then were crushed again. This observation agrees with Drake’s study of pebbles in which a dynamic equilibrium was observed between the process of abrasion and crushing. Average pebble was abraded to a roundness 0.5 and then crushed to start the process of rounding over again. For pebble size particles, the equilibrium around the mean roundness of 0.5 was reached at a distance of 1mi from the source and then remained through all 21mi of transportation. In general, the roundness of the pebbles increases with the distance.The boulders present the analogical characteristic of roundness and breakage like pebbles. They also develop dynamic equilibrium between breakage and roundness around mean roundness number of 0.5 but after a longer distance of transportation respectively to their bigger size. Higher durability of bigger particles lets them survive through much longer distance than pebbles can. To reach dynamic equilibrium by boulders between roundness and crashing around mean distribution of roundness 0.5, the distance of, approximately 15 to 20 miles is needed based on the roundness distribution of granite A (table 4). The granite A just reached mean roundness of 0.5 because there is still a big percentage of boulders below the roundness of 0.5. The basalt for example carried its equilibrium of 0.5 through many miles before it had been deposited in the Stony Brook area, which is indicated by a big percentage of boulders with roundness above 0.5 (table 4).

 

The breakage of the boulders Link here to images of boulders.

          The boulders fall into one of three categories of breakage similar to Drake’s (1972) categories for pebbles:

1.       Relatively fresh surface breakage, less then or equal to 0.3 on Krumbein’s roundness scales.

2.       Worn surface breakage, worn to roundness 0.4 or greater.

3.       No indication of breakage.

Boulders from category 3 have covered the longer distance of transportation because of their rounded shape. Also, because they would break and would not appear in the category of boulders, due to their smaller size. Table 5 illustrates that basalt and granite P indicate long distance of transportation.

The shape of the boulders

            Boulder shape was classified according Zingg’s (1935) classification (table 1 and 6). The analyses of distribution of spheres, blades, rods and discs show certain regularities:

1.       The rocks, which have covered a higher distance of transportation like basalt, quartzite, granite P and granite A show a higher percentage of spheres.

2.       The Blades do not exist in rocks, which came from far distances (table 5 and 6).

3.       Discs are more abundant than rods.

These observations agree with Drake’s (1972) study of pebbles. In Drake’s study, the blades disappear within 9mi from the rock source, rods and discs extended to 14mi, and spheres persist to 21 mi.

The estimation of the location of the sources of the Stony Brook boulders

          On the base of data from tables 5, 6 and 7, the estimation of distance of transportation of Stony Brook boulders was made. According to this data, the longest distance of transportation was covered by granite P. The mean roundness of these rocks is higher than 0.5.These boulders mostly do not show breakage (table 5). They are mainly spheres. Blades do not exist in the population of this rock.  Taking into consideration the small percentage of these rocks (4.7 %), these boulders should have come from a distance greater than 25 mi somewhere in Connecticut.

The basalt came from the second most distant source. A large percentage of these rocks have a roundness above 0.5, in general there is no indication of fresh breakage, mostly they are spheres. Blades are not present in the population of these boulders. If the basalt is associated with the Hartford Basin, then there is a possibility that the source for this lithology lies near the shore of Connecticut in the range of 20 to 25 mi 

            21 % of the studied rocks are represented by granite A, gneiss A and pegmatite A. According to table 4, it looks like these rocks just reach a roundness of 0.5 because a large percentage of these boulders have their roundness below the mean roundness of 0.5. A study of roundness suggests that these rocks covered a distance of 15 to 20 mi. The large percentage of these boulders in our sample population suggests a large source for this lithology. Similar conclusions can be inferred by analyzing the breakage and shape of these boulders (table 5 and 6). Spheres make up about half of the studied population, blades still appear among the samples, and fresh breakage is common.

            The closest rocks to the LI shore are granite S and gneiss S. Since, they are 28 % of the studied sample this makes them the most abundant rocks in the boulder population. Their mean roundness number is lower than 0.5 (table 4). They did not yet reach equilibrium between breakage and roundness. Gneiss S has a mean roundness number around 0.4 (table 4) which suggests a slightly longer distance of transportation than granite S with a mean number of roundness of 0.3. According to a seismic reflection map (Lewis and Stone, 1991 fig. 6), the closest possible location of those rocks is situated 6 mi northeast from Stony Brook.

            The lineated granite (11% of sample population) has a bimodal population of roundness (table 5). This may be an indication of a double source of these rocks. One source should be located close to the Connecticut shore as indicated by a mean roundness of 0.5 with higher numbers of 0.6 to 0.7. The second source is close to the LI shore as indicated by a roundness of 0.2 and the existence of blades.

            Bimodal populations are also observed in quartzite and pegmatite K. This situation may be possible because pegmatites may be associated with any rock described above with an exception of basalt. Pegmatite K has all the characteristics of pegmatite A except that it lacks garnet. On the other hand, in boulders this pegmatite shows contact with pegmatite S. This observation suggests that part of the population of pegmatite K is actual pegmatite A without the garnet. This explanation agrees with the bimodal distribution of roundness for this lithology (table 4).

            The other types of boulders found on the Stony Brook campus are not discussed because there were not enough samples found of these rocks.