Field Trip to New York's Barrier Island, Fire Island

Information for Readers

This is a lab report for my Sedimentology and Stratigraphy class at SUNY Stony Brook. You may use this report as a guideline for any report you may write on this area, but I strongly suggest you read the original sources that I cited.

Introduction

Fire Island is located just south of Long Island in New York. Standing on the south shore of Fire Island, looking south, is the Atlantic Ocean. We went to Smith's Point Beach on the eastern section of Fire Island. We arrived at approximately 9am on October 15, 2005 during a high-to-low tide transition, which was evident because well-compacted, wet sand was no longer being reached by the waves. The waves were coming in at an angle of approximately 15° to the shoreline from the southeast. The prevailing winds in this area are generally light from the southwest (FIA, 2004), which causes sediment accretion towards Long Island; however, before our trip, there was a major eight day rainstorm which had relatively strong winds from the northeast, which eventually died down to a drizzle and light winds from the northeast on the last day of the storm which is when we arrived.

Beach Morphology and Sediment Description

Beaches tend to experience seasonal profiles, summer and winter. A general beach profile is shown in Fig. 1. A summer profile, which occurs during the summer months, is characterized by a wide backshore, a well-developed berm, and a steeply dipping foreshore. Low waves with long periods move the sand from the offshore bar onto the berm, allowing the beach to expand. A winter profile, which occurs during the winter months or during major storms, is characterized by a narrow backshore, a shallowly dipping foreshore, and a large offshore bar. Large waves with short periods tend to erode sand from the beach and deposit it on the offshore bar (NHBC). During the summer, there can be major tropical storms that cause some erosion, but they are rather infrequent and weak compared to the nor'easters that batter the northeast coast during the winter, so overall beach accretion during the summer is still likely.


Figure 1: Beach Profile (NHBC)

We were unable to do a beach morphology profile because there was very little beach left to dig trenches in (Fig. 2). The beach had a characteristic “winter profile” caused by the storm, about two months before it should experience it. The very shallowly dipping foreshore ended at the dune. The berm and backshore were, presumably, eroded away to the offshore bar during the current storm and the waves had reached as far up as the dunes. The dunes were approximately two meters high.


Figure 2: Beach profile (Rasbury, 2005)

Sedimentary Structures and Current Indicators

Although we could not dig trenches, we did dig a small hole (Fig. 3) using a spade. The surface layer consisted of relatively heavy, dark minerals, such as iron. During high tide, the waves reach further up the beach. As the waves retreat, they lose energy, lowering the water's competency, so the heavier minerals are dropped, leaving dark layers on the beach called lag deposits.


Figure 3: Mini trench displaying layering of sand and heavy minerals (Rasbury, 2005)

Underneath the surface layer of lag deposits is a layer of light-colored sand that is about 2cm thick. This sand could possibly be from the berm overlaying the heavy mineral deposits during the summer buildup of the beach or it could be sand deposited from winter waves that already deposited the heavier minerals further up beach. It would seem more likely to be sand from the berm during the summer months because of how thick the layer is. Since there is extensive erosion during the winter, I suspect that this layer would not be as thick as it is if it were formed by high waves during the winter. Underneath this layer is another layer of heavy minerals that is about 1cm thick in one area, but branches out into multiple layers separated by light-colored sand layers on the right side of Figure 3. The next lower layer is light-colored sand again, but there were heavy minerals sprinkled throughout it and some slightly larger sized particles. The coarser particles would suggest a higher energy environment than the upper layers suggest, so perhaps there was greater wave action in this area when this layer was deposited.

Ripple structures (Fig. 4) were found by the tidal pools near the dunes. The ripples are asymmetrical, indicating that the current is unidirectional. Since the current is unidirectional, it is less likely that these ripples would be caused by bidirectional flows like waves or tidal action, but I think tidal action does still play a role in forming these ripples. During high tide, the water reaches the tidal pools which are depressions in the beach. The water will flow down the gradient into the pool, but it will not flow back up the gradient, so it might be possible for asymmetrical ripples to form.


Figure 4: Ripple marks in the tidal pool (Rasbury, 2005)

In between the two tidal pools, there was a channel that led back to the ocean (Fig. 5). There are heavy mineral deposits in the shape of upside down Vs on the surface of the channel bed. It would appear that these are caused by a sudden decrease in water, perhaps around an obstacle in the water's path. The point of the V would be where the obstacle is and the legs of the V would point in the direction of water flow which would be out of the ocean towards the tidal pools. During high tide, the water probably flows through this channel and erodes the eastern bank in Figure 5. It is also evident that the water was probably flowing towards the eastern bank because the braided stream pattern of heavy mineral deposits curves around the bank.


Figure 5: Channel between tidal pools leading to ocean (Rasbury, 2005)

Discussion

Barrier islands are temporary, dynamic features that migrate laterally along the shoreline and also migrate onshore or offshore. Two of the main factors in barrier island dynamics are the change of sea level and sediment supply. Generally, a rising sea level will cause the barrier island to move landward, in our case Fire Island would move towards Long Island. If sea level falls, the barrier island will move further out into the ocean. The supply of sediment affects just how much onshore or offshore movement there is. Generally, a large supply of sediment will cause a barrier island to migrate offshore and a small supply of sediment will cause it to migrate landward. If there is a large enough supply of sediment, the barrier island can actually migrate offshore if sea level is rising slowly enough (Prothero, 1996).

It is unlikely that many of the features that we saw will be preserved in the rock record. Since our visit was in mid-October, there is still much erosion to take place during the winter months which will probably destroy most of the features we saw, such as the deer tracks, the channel, and the ripples. As for long term preservation, it still seems unlikely that many features will be preserved. Sea level is currently rising due to the melting of the polar caps. Barrier islands that migrate landward are not preserved as well as those that migrate offshore during transgressing seas. Rising sea level tends to rework the barrier sequence that has previously been deposited, destroying the sequence, whereas falling sea level allows the sequence to be buried by later depositional processes before it has a chance to be reworked (Prothero, 1996).

Some preservation is still possible during rising sea level times. The backbarrier sediments of Fire Island have not been reworked because of the stepwise retreat of the surf zone. If sea level rises rapidly enough, shoreface erosion will not get a chance to extensively rework the sequence, allowing it to be buried by other depositional processes underwater. Some of the backbarrier Fire Island sediments have been radiocarbon dated to be between 7,000-8,000 years old (Rampino, 1981).

If we were to go to back to Smith Point's Beach on the same day next year, it is hard to say what the beach would look like. Assuming sand has not been artificially added to the beach and there is not another major storm prior to our arrival, I would suspect that the backshore and the berm would exist; however, they would still be relatively diminished. I have visited this beach occasionally for the past 15 years and every time I see it, the berm has diminished more and more, particularly after the strong nor'easters of 1993.

Works Cited

Photography Cited

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