Bass Harbor Ferry Terminal
Bass Harbor Ferry Terminal
Mount Desert Island is a unique and beautiful spot along the coast of Maine. However, this island was not always so pretty. Roughly 420 million years ago, there was a supervolcano here (Braun and Braun, 2012). This volcano, and the tectonic activity associated with it, formed the bedrock we see today that makes up this island (Figure 1).
The Bass Harbor Ferry terminal is a small spot along the southwest corner of Mount Desert Island (Figure 2). People at this site are typically too busy boarding and entering the ferry to notice a sand beach with an abundance of geologic features just to the left of the pier (Figure 3).
Figure 3: Ferry Terminal Beach.
Figure 1: Geologic bedrock map of Mount Desert Island (Holly, U.S. National Park Service, 2017)
N
= Ferry terminal
Figure 2: Bass Harbor Ferry Terminal field site showing diabase dike cutting across the beach (left) and a gravel pit East of the beach (right). Map and legend modified after Braun, 2017 (left) and the Holly U.S. National Park Service, 2017 (right)
18,000 years ago the Laurentide ice sheet, a continental glacier that covered MDI, deposited sediments as it receded (Braun & Braun 2012). A gravel pit northeast of the ferry terminal was created from deposits of sediments carried by the glacier that were left as it receded (Figure 2). The sand higher up on the beach is a medium to coarse grain and was most likely created from rocks that came from the gravel pit that were eroded by the tides as well as human activity (Figure 4). Lower on the beach where the tides come in the sand is a finer grain and contains shells (Figure 4).
Figure 4: Medium-coarse grains of sand found higher up on the beach (left) and finer sand found lower on the beach where water covers at high tide (right).
The first rock we observed at this site is a grey salt and pepper bedrock with fine to coarse grain. This rock has some small crystals and is a light grey color when weathered (Figure 5). On the updated draft of the Geologic Bedrock Map of MDI this rock is identified as a diabase dike which is a type of intrusive microgabbro (Figure 2). Veins of epidote are present in this rock, orienting 86 degrees East (Figure 6). We measured two sets of fractures each with four different measurements. Fracture set 1 has measurements of 107º, 104º, 111º, and 122º averaging 111º NW / SE. Fracture set 2 has measurements of 201º, 211º, 213º, 220º averaging 211º NE / SW (Figure 5). There is a basalt intrusion present on this rock, which is 17 CM wide with baked sides and orients 220º NE / SW (Figure 7). Glacial striations measure 355º NS (Figure 5). These are from the same glacier that helped form the sand.
Figure 5: Diabase dike with fracture sets and glacial striations.
Figure 6: Epidote veins in the diabase dike.
Figure 7: Basalt intrusion in the microgabbro.
In the center of the beach there is a medium to coarse grained, tan-grey colored rock. This rock is weathered with noticeable grains and has inclusions of another rock (Figure 8). The first rock we identified as Tuffs of the Cranberry Island Volcanic Series (Holly, 2107). Geologist Hobart King describes a tuff as “an igneous rock... (composed of) ejecta produced by a volcanic eruption” (King). This ejecta may be ash, magma, rock, etc. The inclusions are coarse grained and a green brown color because of weathering but underneath are pink with small black and white crystals. We know the inclusions are older than the Tuffs of the Cranberry Island Volcanic Series because whenever there is an inclusion, the included rock is older than the surrounding, due to the Law of Inclusion (Marshak, 2011). At this location on the beach there is another intrusion of a fine grained, dark grey rock, which is basalt in the diabase dike. The intrusion is roughly 2-3 feet wide (Figure 9).
Figure 8: Inclusion in the Tuffs of the Cranberry Island Volcanic Series.
Figure 9: Basalt intruding microgabbro.
Towards the end of the beach, the final rock we observed is a coarse grained, light grey tan color. This rock we identified this as Tuffs of the Cranberry Island Volcanic Series (Holly, 2017). There are also two dike intrusions present that both trend NE/SW and are between 3-5 feet wide.
Figure 10: Basalt intruding Tuffs of the Cranberry Island Volcanic Series.
References:
Braun, Duane, Braun, Ruth, A Guide to the Geology of Mount Desert Island, 2016, North Atlantic Books.
Holly, M., 2017 Mount Desert Island U.S. National Park Service
King, R.M. Tuff: Geology, https://geology.com/rocks/tuff.shtml.
Marshak, Stephen. Earth: Portrait of a Planet. 4th ed., W.W. Norton, 2011.