Erratics of
granite S and gneiss S
(Images of Boulders can be found at this
link.)
The largest erratics found in the area of study are granites S, gneiss S and pegmatite K. they are characterized by an enormous size, angular shapes with no indication of rounding and the occurrence of extensive fractures. These characteristics suggest that these rocks were freshly quarried somewhere in the vicinity. According to seismic – reflection profiles (Lewis and Stone 1991), the closest location that these rocks could come from is six miles north of Stony Brook.
The key to explaining the phenomena of such low degree of abrasion in glacial transportation lies in the morphology at the bottom of Long Island Sound. Coming from the north of Long Island Sound, the bottom surface declines gently down until it reaches the trench- like feature located six to eight miles off the LI shore. In a depression like this the pressure on the northern wall was built up by incoming ice. Due to the curve of the trench, this force changed the direction of the ice flow and its speed in the bottom layer. As a result, the bottom layer moved faster in an upward direction and entered the inside of the glacier sliding over the slower moving ice ramp underneath it. The layer of faster moving ice contained boulders plucked from the bottom of the trench, which were saved from abrasion by sub-glacial transportation. In a similar way, valley glaciers make cirque lakes
This motion of the ice also explains why we do not observe Cretaceous rocks in the boulders of Stony Brook with the exception of one boulder in the area of the Ashley Schiff Preserve. Slow moving ice between LI and the trench mostly did not erode the Cretaceous conglomerates, which were covered by sediments of previous glaciation.
Link to Fig.
4 The proposed mechanism for transport of granite S and gneiss S
Mean roundness of 0.5 versus the distance of the transportation of the boulders.
In a glacier environment, large particles have greater durability. These large fragments have the ability to sustain higher pressure without cracking and can survive collisions without breaking into pieces. It also takes a longer distance of transportation to abrade boulders because of the presence of a larger surface area. For example, in a distance of 5 meters, boulders with a mean axis of 2 meters can be rolled over only once when a pebble of a diameter of 20 mm can be rolled around 100 times. This should explain why small particles grind up faster than boulders.
According to Drake’s (1972) study, pebbles reach a mean roundness of 0.5 in 1 mile of transportation. Based on the study of boulders from Stony Brook, boulders do not reach this degree of roundness in 15 to 20 miles of transportation. Sand, on the other hand, reaches its mean roundness of 0.5 in a distance shorter than one mile. It is very unlikely to find angular sand.
Based on a mean roundness of 0.5, there is a relationship of the mean size of the particles and the distance of transportation of these particles in a glacier (fig. 3). After the mean roundness of 0.5 is reached, the mean roundness of 0.5 is sustained through the whole process of transportation of these types of particles due to existence of dynamic equilibrium between breakage and roundness. The point where the mean roundness of 0.5 is reached by the specific size of studied lithology can be used for the estimation of the distance to the source of the rocks. The studied size of the particle will just reach its 0.5 equilibrium if the majority of particles will still have a roundness number less than 0.5, like granite A in table 5. In using the graph (fig, 3) for estimating the distance of transportation, the best results will give study of the same lithology in the size of pebbles, cobbles, and boulders. Also, the graphs should be made for each different lithology separately. The durability of dolomite is much different than the durability of igneous rocks.
Link to Fig. 5. This graph can be use to
estimate distance of transportation of rock particles. It is based on the mean
size of the particles and the mean roundness of 0.5
