During the period from maximum submergence to maximum emergence, rivers, 

 streams, and other erosion agents dissected the coastal area upland and 

 generated a land surface topography similar to today's (atlas map 3). 

 Weathering and reworking of surficial glacial and glaciomarine sediments took 

 place during the period of sea level rise from 8500 years ago to the present, 

 creating the present shoreline. These processes have formed, and continue to 

 form, the coastal marine sedimentary environments along the present shoreline 

 (atlas map 3) . 



Holocene sea level rise . Sea level has been rising for the past 8500 

 years. It occurs through a combination of eustacy (world-wide rise of sea 

 level due to the addition of glacial meltwater to the ocean's volume) and a 

 depression of the coastal New England region land mass (Grant 1968). 



Using C radiometric dating of buried salt marsh peat, Thompson (1977) found 

 sea level rise 3000 years ago to be approximately 0.45 inch (1.15 cm) per 

 year. This relative rise decreased to a rate of about 0.11 inch (0.06 cm) per 

 year over the past 1000 years (figure 2-19). 



Sea level rise rates for the past 40 years have been determined from tide- 

 gauge records along the Maine coast (Hicks 1972; figure 2-20). Apparent 

 trends of sea level rise at Portland and Eastport from 1940 to 1970 are 0.06 

 inch (0.16 cm) per year and 0.11 inch (0.34 cm) per year respectively (Hicks 

 1972). A minor portion of the present rise is due to eustatic sea level rise; 

 most of the rise is apparently due to crustal subsidence of the Maine coast. 

 The reason for this subsidence is unknown. 



For the past 8.5 millenia, the sea has been advancing upon coastal regions. 

 Concomitantly, normal subaerial weathering, both chemical and mechanical, has 

 continually eroded the Maine upland region. The end result of upland erosion 

 is the delivery of riverborne and streamborne sediment to the shoreline or to 

 closed depressions (lacustrine and palustrine systems) on the terrestrial 

 landscape. 



In summary, the geological framework of the Maine coast is a mosaic of two 

 geological elements: (1) a consolidated and unconsolidated complex outline of 

 bedrock and glacial deposits, and (2) recent sedimentary deposits on the 

 bedrock and glacial deposits. Both elements are being continually changed by 

 erosion and deposition resulting from coastal forces and a rising sea level. 



Erosion and sedimentation . Rapid erosion and sedimentation of nearshore 

 environments alters the substrata and creates stresses on plants and other 

 organisms, including people, that utilize these substrates. Natural erosion 

 and deposition create beaches and dunes for human recreation, intertidal areas 

 for marsh and shellfish growth, and natural channels for navigation. 



Fine-grained sediment is introduced to the nearshore zone by rivers 

 discharging into estuaries, by sediment reworking within the basin, and by 

 floor sediments from the inner Gulf of Maine. Sediment loads of rivers 

 discharging into heads of estuaries during normal discharge periods vary from 

 <0.9mg/l to about 12 mg/1 (U.S. Geological Survey 1976). Sediment loads after 

 spring freshets may be 40 mg/1. These loads correspond to daily suspended 

 sediment discharges of from 29 tons (26 t) per day to 3270 tons (2943 t) per 

 day. Calculations based on sediment yields from soil erosion indicate that 



2-41 



10-80 



