USGS:Science for a changing world
Woods Hole Field Center


A Poster session presented at the Annual Meeting of the Geological Society of America, October, 1997

L. J. POPPE, J. F. DENNY,K. F. PAROLSKI, H. J. KNEBEL (USGS Woods Hole Field Center, Woods Hole MA 02543) and R. S. LEWIS, and M. L. DIGIACOMO-COHEN (LISRC, Avery Point, Groton, CT 06340)

contact Larry Poppe at


Falkner Island is part of the Connecticut National Wildlife Refuge, an array of five federal tracts set aside to protect unique ecosystems, to manage biodiversity, and to benefit environmentally sensitive fauna and flora. We have conducted continuous-coverage sidescan sonar, bathymetric, and CHIRP subbottom surveys over a 12.5 km2 area around Falkner Island to assess the variability of the seafloor and to improve our understanding of the processes that control the complex distribution of bottom sediments and benthic habitats.
          The seafloor geology around Falkner Island is extremely variable over short distances and controlled by a combination of the subsurface stratigraphy and the modern hydraulic regime. Areas of high backscatter on the sidescan image, which reflect bouldery outcrops of glacial till and are environments of erosion or nondeposition, occur adjacent to Falkner and Goose Islands and over shoals extending northward and southward from the islands. These coarse deposits overlie an irregular bedrock surface which generally dips southward. The strong oscillatory nature of the tidal currents around the islands is demonstrated by the linear accumulations of finer-grained sediments that extend both eastward and westward from isolated boulders. Areas of lower backscatter, which surround the outcrops of till, represent relatively fine-grained Holocene sands and silty sands. The finest-grained sediments occur just west and southeast of Goose Island and in other protected sites characterized by lower-energy depositional environments. Sand waves occur in small patches both north and south of the islands, but they never exceed 0.5 m in height or 20 m in wavelength.

map of study area

This map shows the location of the study area (striped polygon) around Falkner Island, Connecticut. Map also shows the locations (open polygons) of the Norwalk Islands, Milford, Hammonasset, Roanoke Point, Niantic Bay, New London, and Fishers Island Sound sidescan sonar surveys which are part of this map series, the other components of the Stewart B. McKinney National Wildlife Refuge, and the major onshore moraines.

Click on the figures for larger images.

bathymetric map

A bathymetric map of the study area. Depths are contoured in 2-m intervals, have been corrected for tides, and are adjusted to mean sea level. Small dashed lines are the tracklines along which the geophysical data were collected. Figure also shows the locations of seismic profiles shown in the next figure.


CHIRP subbottom profiles show the stratigraphy and different echo character types.

(A) Profile from south of Falkner Island showing an example of the prolonged acoustic facies interpreted to be glacial drift and the opaque acoustic facies interpreted to result from gas-charged sediments.
(B) Profile across the shoal north of Falkner Island showing the infilling of topographic lows in the irregular surface of the prolonged acoustic facies by glaciolacustrine sediments.
(C) Profile across the shoal north of Goose Island. Boulders are clearly present on the surface of the prolonged acoustic facies.
(D) Profile across the north-central part of the study area showing the infilling of topographic lows in the irregular surface of the prolonged acoustic facies by glaciolacustrine sediments. Examples of the transparent and layered facies are also labeled on these profiles. Locations of the profiles are shown on the bathymetric and echo-character maps.

echo character map

A near-surface echo character map shows the distribution of the prolonged seismic facies (depth to acoustic basement, m) and the acoustically-opaque seismic facies interpreted to be the result of gas-charged sediments. The map also shows the areas within which the seismic facies of the Holocene sediments are predominantly layered or transparent. The prolonged facies outcrops on the sea floor within the 0-m contour. This Figure also shows the location of subbottom profiles shown above.


Sidescan sonar mosaic of the Falkner Island study area. Light tones on the image represent areas of high backscatter (usually coarser-grained sediments); dark tones represent areas of low backscatter (usually finer- grained sediments).

Click on the figures for larger images.


Interpretation of the Falkner Island sidescan sonar mosaic shows the distribution of bedrock, gravel and boulders, areas characterized by low backscatter, and sand waves. Heavy dashed line marks the extent of the sidescan sonar coverage; the coastline is stippled.

Bottom sampling

Surficial sediments were sampled and bottom photography was attempted at 50 locations during March 1996 aboard the RV John Dempsey by means of a Van Veen grab sampler equipped with video and still camera systems. The photographic systems were also used to appraise intra-station bottom variability and to observe boulder fields and outcrops where sediment samples could not be collected.

Sediment types

Map showing the distribution of surficial sediments and the locations of the sampling and bottom photography stations. Interpretation of the sediment distribution is based on data from these stations, on tonal changes in backscatter on the sidescan sonar image, and on the correlation of the textural and backscatter data with the bathymetry.


The strong, oscillatory nature of the tidal currents in the study area is demonstrated by the linear accumulations of sediment extending both eastward and westward from isolated boulders on the sidescan images. These accumulations, which form in the lee of the boulders, are best developed southwest of Stony Island.

obsticle marks

The accumulations on the westward side of the boulders are usually longer in this area and around Falkner Island than on the accumulations on the eastern side. This asymmetrical distribution is evidence for a predominantly westward direction of net bottom sediment transport. The near absence of these features north of the islands suggests that the currents there are not as strong.


Falkner Island has decreased by 2.8 acres or 50 percent since 1818, when accurate observations on island size were first conducted. Rapid erosion continues because the surrounding bluffs are steep, only partly covered by vegetation, composed of unlithified glacial drift, and fronted by narrow beaches.

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