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Since tidal marshes are low energy environments, protected from storms and wave energy, fine grain sediments carried by freshwater streams and rivers settle out, generating a rich soil for the growth of plants. A variety of grasses grow well in estuaries, and grass shoots slow water movement, trapping and accumulating even more sediment. As a consequence of the growth of grasses, the sediment level of estuaries rises, which in the past has allowed the elevation to match that of the rise in the sea level.
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The process by which present day estuaries are formed along the Connecticut coast involves the growth of grasses, accumulation of sediment, introduction of new plant species and animals. Botanists tell us that in a new estuary, one of the first grasses to grow is the Smooth cord-grass (Spartina alterniflora). Smooth cord-grass thrives at elevations between mean high tide and a little below mean sea level, roughly the upper two-thirds of the mean tide range, where it is flooded by tidal waters twice a day. Smooth cord grass stands create ideal habitats for Ribbed Mussels (Guekensia demissa) and fiddler crabs (Uca spp.), both of which, in turn, enhance the growth of the Cord-grass. Denser stands of grass stems slow tidal water even more, further increasing sediment deposition. With the right conditions Cord-grass can expand seaward, encroaching over mudflats, and also landward, over flooding uplands. Continued sedimentation and rhizome growth raises elevation on the landward side of these new marshes above the mean high tide level, creating the less frequently flooded high marsh habitat. Salt Meadow Cord-grass (Spartina patens), Black Grass (Juncus gerardii) and Spike Grass (Distichlis spicata) became the high marsh dominants. These plants are shorter in height than their low marsh counterpart, with finer stems and leaves and, unlike Smooth Cord-grass, their roots and rhizomes tend to form a dense turf.
 
The estuary is a dynamic place and its boundaries change with changes in sea level and water outflow from inland sources. At the border between the estuary and upland, flooding usually occurs only during extreme spring tides. During most of the rest of the year, this component of the estuary is relatively dry. These conditions are ideal for the growth of other plants, e.g., shrub Marsh Elder (Iva frutescens), often mixed with or even replaced by Switch Grass (Panicum virgatum) or Phragmites (also called Common Reed, Phragmites australis).   
When sea level changes, many of the oldest plants die and their roots or rhizomes are buried under new sediment. Over a large area, this has the effect of further increasing the depth of seawater within LIS. Within this organic layer there is little oxygen (anaerobic layer) but a great deal of salt (from seawater). Both of these factors slow the decomposition of the dead plants. Instead of rotting away, the plants are preserved as peat and can act as a historical record of estuarine development and plant changes over centuries much like tree rings can provide a view of past meteorological changes. In Connecticut, the oldest salt marshes are more common toward the western end of the State, have about three meters (10 feet) of peat, which overlie either mud flats of marine clays and silts, or, less frequently, upland soils. More recently formed marshes have shallower depths, with peat found over upland soils and low marsh Smooth Cord-grass peat over marine sediments. Except for Phragmites, the roots and rhizomes of the upper border plants decompose fairly quickly, and their remains are usually harder to find. Scientists can use the information within peat and sediment cores to determine marsh development. Peat sampling tools can be forced down through the marsh, cutting out a plug or long tube of peat which is continuous from the current surface to the bottom layers (in the same way ice cores are taken in Antarctica). A scientist can then carefully examine each layer of the core to determine which plants are present now and in the past as well as microscopic animals. Since plants and animals occupy (and hence are characteristic of) different marsh environments, preserved remains can be used to reconstruct the historic marsh environment as one moves down through a core, and simultaneously back through time. This time line can be followed back many centuries and records unusual events, such as hurricanes, which alter the deposition of plants and or animals, e.g., by inclusion of a great deal of sand within the peat.
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