Currents, geology, and water depth control the deposition and erosion of sediment in Long Island Sound. Stronger currents are able to move heavier particles than weaker currents. As currents slow down, the larger particles being carried by the water tend to settle out. This slowing of currents may represent the cyclical changes in velocity that occur during the tidal cycle with the approach of the time of high or low tide. A decrease in velocity also may occur as water proceeds into restricted embayments or enclosed and vegetated tidal marshes. For a discussion of the classification of sediment particles by size, see
Wikipedia: Sediment. As described in that article, boulders > cobbles > gravel > sand > silt > clay, in terms of particle size.
The attached KMZ file contains map overlays that indicate the types of sediments that are found on the surface that underlies the open waters of Long Island Sound. This surface lies above the surfaces of older sediment, such as Pre-Tertiary material, that is now buried. The map images were downloaded from
USGS Studies in Long Island Sound: Geology, Contaminants, and Environmental Issues.
Following are some quotes from that article that describe the sediment distribution in Long Island Sound and the environments associated with this distribution of sediment. Note that, in general, current velocities tend to decrease from east to west within the Sound, and that sediment particle sizes trend from coarse to fine in the same direction.
Surficial Sediment Distribution
The regional surficial sediment distribution is extremely patchy, but exhibits textural trends which are related to sea-floor geology, bathymetry, and the effects of currents. In general, gravelly sediments are dominant in easternmost Long Island Sound, where tidal currents are strong, and in areas characterized by glacial tills. Sand occurs across the east-central Sound and along most of the nearshore margins. Silty sand and sand-silt-clay mark transitions within the Sound from higher to lower energy environments, such as on the flanks of bathymetric highs. Clayey silt and silty clay are predominant in low-energy environments, such as on the floors of the central and western basins.
Sedimentary Environments
The circulation in Long Island Sound, which is controlled by an east-to-west weakening of tidal-current speeds coupled with the westward-directed estuarine bottom drift, has produced a succession of sedimentary environments. The succession begins with erosion or nondeposition at the narrow eastern entrance to the Sound, changes to an extensive area of coarse-grained bedload transport in the east-central Sound, passes into a contiguous band of sediment sorting (where the estuary noticeably widens), and ends with broad areas of fine-grained deposition on the flat basin floor in the central and western Sound. The distribution of these environments provides insight into the long-term fate of contaminants, and is a guide to the future utilization of the sea floor. This information also helps to define the general locations and variability of benthic biological habitats, and gives a long-term perspective of sea-floor conditions in the Sound.
See also
Google Earth Community: Long Island Sound and
Google Earth Community: Pre Tertiary Surface of Long Island Sound.
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