Active Coastal Processes in the Lubec Embayment
Coastal Setting
Figure 1 |
Between West Quoddy Head and the town of Lubec, Maine are some of the most
interesting beaches and coastal features in the region (Figure 1). The
tides in the Lubec Embayment exceed 6 meters on full and new moons (spring
tides), providing the greatest tidal range on the East Coast of the United
States. Water rushes in and out past West Quoddy Head State Park twice
a day at up to 1.96 meters per second (4 knots) and alternately covers
and exposes the extensive tidal flats of the Lubec Embayment. Waves are
kept to a minimum by the sheltering effects of land masses on all sides.
This is called fetch restriction, and waves cannot become larger than about
0.7 meters high because there is not sufficient water area for the wind
to blow across (Walsh, 1988). Thus, geologists call the Lubec Embayment
a "tide-dominated" beach system in reference to the kind of energy available
to move sand and gravel on beaches. |
Geologic History
Figure 2 |
The bedrock of the area includes the Quoddy Formation, consisting of dark-colored
shale and argillite, and abundant gabbro and diabase intrusions (Bastin
and Williams, 1914; Gates, 1977; Osberg and others, 1985). The rocks
date back to the Silurian Period, approximately 415 to 440 million years
ago. Glacial sediment rests on the bedrock and was deposited by melting
ice around 14,000 years ago (Thompson and Borns, 1985). Till, in the form
of a moraine, makes up Lubec Neck and is well exposed at the town gravel
pit on Route 189 (Kelley and others, 1991). Till is a glacial deposit composed
of a mixture of boulders, cobbles, sand and mud. Glaciomarine muddy sediment,
containing fossils from 14,000 to 12,000 years old, covers the till in
most places indicating the flooding of coastal Maine by the sea as the
glacier retreated. This glaciomarine mud is now widely exposed along
the coast in the faces of eroding bluffs. In a very few places, a layered
sand and gravel deposit overlies the glacial-marine sediment. This material
is thought to represent a coastal beach or nearshore deposit formed when
sea level fell across the present shoreline between about 11,000 and 10,000
years ago. Where the marine sand and gravel is absent, a peat deposit often
covers the glaciomarine mud as at Carrying Place Heath (Figure 1 and Figure 2).
The youngest features in the area are the beaches, salt marshes and tidal
flats. They are Holocene in age, or less than 10,000 years old. |
Sea level has risen all along the coast of Maine for the past 10,000 years
(Kelley and others, 1991). The rate of sea-level rise has varied in the
past, but the rate recorded by the Eastport tide gauge since 1930 has averaged
2.7 millimeters per year (Lyles and others, 1988). In response to rising
sea level, coastal bluffs of glacial sediment erode and beaches and marshes
move landward. The exact mechanisms by which these environments shift location
can be complex, and we shall focus below on the beaches.
Historic Shoreline Changes in the Lubec Embayment
Figure 3 |
The earliest maps of Lubec Embayment were prepared by the British and depict
a shoreline very different in appearance from that of today. In 1785 a
long spit extended to the north from the Carrying Place Heath (Figure 3).
By 1805 the large spit had changed into a smaller spit connected to
the mainland on the north and a barrier island beach (Figure 3). In the
1830 map these two beaches are reconnected in a more landward position
and a salt marsh had formed in the sheltered area behind the beach (Figure
3). "Marston's Dike" refers to an embankment built on the marsh for agricultural
purposes, and the "Basin" was a natural harbor. |
Figure 4 |
By 1840 the spit had again divided into a small south-trending spit
and a barrier island (Figure 4). The whole system had also moved moved
landward, and only a small salt marsh remained. The barrier island disappeared
by 1862, and the present spit began to grow toward the south (Figure 4).
Also, a new spit began to form on West Quoddy Head, possibly the same one
depicted as intertidal in 1805 (Figure 4). The 1919 map shows that the
southward growth had paused and a pronounced curve developed at the south
end of the spit (Figure 4). The pause in growth and protection afforded
by the beach allowed an extensive salt marsh to fill in the area behind
most of the beach. On the 1919 map buildings, roads and piers are shown
near the northern end of the beach. |
Figure 5 |
The spit has grown several hundred meters south since 1919 (Figure
4 and Figure 5) and retreated landward so that only some supporting posts remain
from the buildings that had existed earlier. So rapid has been the recent
growth of the beach, that salt marsh has not yet developed south of the
1919 spit terminus. As the spit continues to grow to the south, its tip
is forcing the tidal creek to adjust to a new position (Figure 5). In addition,
growth of the beach to the south will eventually project it into the Carrying
Place Heath, which is already eroding at a rate greater than 0.5 m/yr (Walsh,
1988). |
Modern Coastal Processes
In an embayment where waves are very small, and in a tidal regime where
waves only reach the beach for a brief period every day, what processes
occur that have led to the profound rearrangement of this system over the
past two centuries? The most important process today is tidal movement,
and if "the present is the key to the past" (the motto of geology), tides
must have been important in earlier years.
Figure 6 |
Close examination of the actively growing tip of the spit reveals a
large accumulation of seaweed. Each plant has a "holdfast" or large stone
that it attaches to when it begins to grow. Seaweed has air pockets along
its length (bladders) that allow it to remain erect when it is underwater.
These bladders also provide buoyancy to the plant and stone. When strong
currents rush over the stone with attached seaweed, the buoyancy of the
plant allows the current to drag the stone with the current. On flooding
tides, the water rushes towards the beach and especially to the end of
the spit, and stones and seaweed accumulate. After a short while the seaweed
dies and breaks off, leaving behind a stone and some sand newly added to
the beach. Evidence for this process can be found in the many drag marks
left by the stones and seaweed over the tidal flat (Figure 6). |
Figure 7 |
In the winter, ice freezes to the tidal flat at low tide. Some sand
and gravel is frozen into the bottom of ice blocks. As the tide rises,
the ice floats and carries sand and gravel off the flat. Ice blocks are
generally carried by the tides into the salt marsh area where they are
stranded on the marsh and back side of the beach. In springtime, the ice
melts and large quantities of mud, sand and gravel are left on the beach
and marsh (Figure 7). |
The Future of the Beach
The history of the Lubec Embayment is recorded in vintage maps as well
as in many deposits on the modern tidal flat. A shipwreck exists in the
"Basin" location of the 1830 map, and Marstons Dike is still visible on
the salt marsh. The bottom of an earlier beach that moved landward is also
recognized as a low ridge in the central part of the tidal flat (Walsh,
1988). Peat deposits crop out on the beach, testifying to the formerly
greater extent of the heath.
The future will probably bring still more changes to this beach system.
The growth of the spit to the south may lead it into the Carrying Place
Heath and accelerate erosion of that peat deposit. Alternatively, the narrow
part of the spit may become breached, as occurred twice in the history
of this system. Then two beaches would possibly migrate to a more landward
location. Finally, the beach attached to West Quoddy Head (Figure 4) may
continue to grow to the west and link up with the other spit.
References
Bastin, E. S., and Williams, H. S., 1914, Eastport folio, Maine:
U. S. Geological Survey, Folio 192, 15 p.
Gates, O., 1977, Geologic map
and cross-sections of the Eastport quadrangle, Maine: Maine Geological
Survey, Geologic Map GM-3.
Kelley, J. T., Borns, H. W.,
Jr., and Walsh, J. A., 1991, Late Quaternary evolution of the Lubec, Maine
coastal region, in Ludman, A. (editor), Geology of the coastal lithotectonic
block and neighboring terranes, eastern Maine and southern New Brunswick:
New England Intercollegiate Geological Conference, 83rd meeting, p. 169-185.
Lyles, S. D., Hickman, L. E., and Debaugh, H. A., 1988, Sea level variations
for the United States, 1855-1986. National Ocean Service, Rockville, MD,
182 p.
Osberg, P. H., Hussey, A. M., II,
and Boone, G. M., 1985, Bedrock geologic map of Maine: Maine Geological
Survey, scale 1: 500,000.
Thompson, W. B., and Borns, H.
W., Jr., 1985, Surficial geologic map of Maine: Maine Geological
Survey, scale 1: 500,000.
Walsh, J.A., 1988, Sedimentology and late Holocene evolution of the
Lubec Embayment: Unpublished Masters Thesis, University of Maine,
Orono, 434 p.
Originally published on the web as the August 1998 Site of the Month.
Last updated on October 6, 2005
