Silver Sands State Park, Milford, CT1.
Is this a Tombolo?
Aerial photograph of Charles Island
with it’s “tombolo”, taken in 2004.
A tombolo is a depositional land-form that creates a narrow neck of land connecting an offshore rock or island with the mainland. Sand is deposited in such a fashion because waves refract (bend) around both sides of the island and thus wash sand toward the middle, where it is deposited. Charles Island at Silver Sands State Park is connected to the mainland beach by a gravel bar. Although it is referred to as a tombolo, it appears more that it is left behind from erosion rather than deposited. We will explore that notion in this EarthCache.
The “tombolo” is submerged most of the time and Charles Island is only accessible at low tide. The “tombolo” is flooded by as much as 4 feet of water during high tide. People venturing out onto the island should be acutely aware of the tides. The “tombolo” is only above water for about two hours per tidal cycle. Currents over the flooded neck can be tricky and an undertow develops during some conditions. Therefore, to complete this EarthCache, the smart cacher will look up the tide charts2 and will plan the trip to start about and hour or 90 minutes before low tide and head back to the mainland no later than an hour after low tide. If you are late in returning you may have to wait 12 hours for the next low tide (it will be dark and cold) or you may have to swim; the currents are said to be tricky. You may wish you had a kayak.
Purpose: This EarthCache is published by the Connecticut Geological and Natural History Survey of the Department of Environmental Protection. It is one in a series of EarthCache sites designed to promote an understanding of the geological and biological wealth of the State of Connecticut.
Location: N. 41o 11.9016’, -073o 03.9708’
Directions: Off I-95: take Exit 35. Follow Schoolhouse Road south to Route 1 (Bridgeport Avenue). Turn left onto Route 1 then right at first light (Silver Sands Park Way). Follow Park Way across Meadowside Road and continue down hill to main parking lot. There are no park entrance or parking fees (as of Summer, 2008).
Silver Sands is Connecticut’s newest shoreline State Park. It was created by reclaiming an old land-fill and sewage treatment plant. According to Joseph Leary (2005), “Extensive clean-up work has turned Silver Sands into one of the state’s prettiest beaches,” and the area wild-life is coming back. A broad salt-marsh separates the beach from the
1. A description of the geology of this state park has been written and is awaiting posting at the State Parks section of the DEP website.
mainland. The marsh was larger prior to the land-filling operation. The large mound just to the west of the road is the covered land-fill. A large box-culvert at the east end of the beach drains the salt marsh
Some of the beach sand is natural, but its coarseness suggests that some has been added during the clean-up. The sand on the beach-face is noticeably coarser than the sand that is uncovered at low tide. In addition, beach-sand east of the park contains more shells and slightly finer grained than sand inside the park. It is also narrower than the beach in the park.
Figure 1. A. Box culvert draining the salt-marsh behind the beach. Notice more sand on the park side (left) of the culvert relative to that of the east. B. Beach sand east of the park contains more shells and is finer grained. C. Coarse sand on the park beach. D. Fine sand on beach to east of box-culvert.
Find N. 41o 11.5195’, -073o 03.4014’. Charles Island is a little more than a half-mile off shore from the beach. Access is over a rocky “tombolo” at low tide. BE AWARE OF THE TIDES and do not get caught on the island after the tide has begun to submerge the “tombolo” (reread the warning at the beginning of this EarthCache). The tombolo is relatively straight and covered with pebbles, cobbles and boulders. At its landward end there is a sand and shell veneer. At the island connection there is a “tail” of shell sand. The rest of the tombolo is covered by a armor of stones that do not move around much during the year. These stones are not wave-deposited but rather are left behind. We refer to them as a “lag-deposit”.
Figure 2. Left: Tail” of shelly sand deposited on north side of island at its connection to the “tombolo” Right: Central part of “tombolo” has no sand deposits; rather it is a lag of cobbles that were too large to be eroded by storm waves. Note in distance the sand veneer on the “tombolo” at its attachment to the mainland.
The beaches of Charles Island itself are of interest. The interior of the island is a nesting area for egrets and herons. PLEASE DO NOT ENTER THE INTERIOR OF THE ISLAND AND DISTURB THEM! Indeed, access is restricted during nesting season (May 12 to September 10) by the Parks Department. The beaches are littered with a great variety of large and small stones that were brought and left by the glacier during the last ice age. Many are a porphyritic mafic-gneiss, others are porphyritic granitic-gneiss. Some however, are quartzite, basalt, and biotite schist. That they all are concentrated here suggests that this
Figure 3. Typical rocks composing the end moraine of Charles Island. Left: Porphyritic mafic gneiss. Center: Porphyritic granite gneiss. Right: Fine-grained basalt.
is an end moraine left by the glacier during its melt-back history (Stone and others, 2005). The melting stopped for several years and the glacial front remained in this position long enough to deposit a moraine.
The source of the rocks in the end moraine is of interest. The porphyritic mafic- gneiss, which makes up a considerable proportion of the boulder and cobble population, is identical to the outcrops along the road entering the park. It is mapped as a metavolcanic rock by Rodgers (1985). The porphyritic granite-gneiss is similar in appearance to the Harrison Gneiss that crops out in the Shelton-Ansonia area, about five miles north-northwest of the park. The basalt is problematic. The closest outcrops of basalt/diabase to the north-northwest of the park are in the Ansonia-Seymour area (the Bridgeport Dike). It is possible that the source for the basalt is the Buttress dike, but the closest known outcrops are northeast of the park. The orientations of streamlined (drumlin-like) hills and glacial grooves and striations in this area are toward the southwest (Flint, 1968; Stone and others, 2005), suggesting that glacial movement could have brought basalt boulders from the known areas of outcrop. Alternatively, because the trend of the Buttress Dike is southwestward, possible outcrops of the Buttress Dike could be just north and northwest of the park and today are covered by glacial soils.
By definition, a tombolo is a depositional landform created by wave refraction around the island or rock to which the tombolo is connected. This is problematic because the “tombolo” at Charles Island does not appear to be deposited by wave refraction. Rather it appears that it was deposited as glacial till and subsequent wave erosion has removed sand and silt leaving behind a concentration of cobbles and small boulders that form the land connection. It is noteworthy that Patton and Kent (1992), in their description of Connecticut shoreline environments, omit Charles Island in the discussion and illustration of tombolo landforms.
Flint, R.F., 1968, The surficial geology of the Ansonia and Milford Quadrnagles, with map
State Geological and Natural History Survey of Connecticut, Quad. Rpt. 23, 36p
Leary, Joseph, 2004, A Shared Landscape: A Guide and History of Connecticut’s State
Parks and Forests. Friends of Connecticut State Parks, CT D.E.P, and CT Forest and Park Association, Hartford, CT, 240p.
Patton, Peter and James Kent, 1992, A Moveable Shore, the Fate of the Connecticut Coast.
Duke University Press, Durham, N.C., 143p.
Rodgers, John, 1985, Bedrock Geological Map of Connecticut. State Geological and
Natural History Survey of Connecticut, Nat’l. Resource Atlas Series, 1:125,000, 2 sheets.
Stone, J.R., Schafer, J.P., London, E.H., DiGiacomo-Cohen, M.L., Lewis, R.S., and
Thompson, W.B., 2005, Quaternary Geologic Map of Connecticut and Long Island Sound Basin (1:125,000). U.S. Geol. Surv. Sci. Invest. Map # 2784.
To log this EarthCache:
1. Why do you agree or disagree with the above assessment of the Charles Island “tombolo”?
2. Submit a picture of yourself or your family/companions at the Charles Island location (see above) with the “tombolo” in the background.
3. Submit a picture of yourself or your family/companions that illustrates westward drift of beach sand on the beach west of the “tombolo”. Include an explanation of how this picture demonstrates westward drift of the sand.
Terrain difficulty: 2