In chapter one, “The Coastal Zone and its Origin,” Downing discusses the Puget Sound region’s unique geological history and the various coast features that this history has yielded. The geological origins and development of coastal features, which Downing separates into glacial legacies, beaches, coast deposition, and coast erosion, explain their heterogeneity and establish several patterns that serve as an important starting point for further analysis of Puget Sound geology.
The importance of the Puget Sound area’s glacial legacy is reflected in two inherent constituent features of shore processes. The first and most obvious such feature attributed to glaciation is the excavation of long, narrow valleys that now constitute Hood, Canal, Puget Sound, Lake Washington, and Lake Sammamish, to name a few. In shore, the glacial legacy is important for shore processes because it has created the waterways themselves. Additionally, cycles of glaciation produced an enormous amount of sediment. Sediment supply, another necessary component of shore processes, plays an important part in the determining the features present in a particular area of the coastal zone.
Beaches are the most prevalent coastal landforms resulting from the last period of glaciation. Features of typical beaches along the Puget Sound shoreline vary according to topography, wave activity, and sediment supply, but all beaches have certain elements in common (e.g., all have intertidal zones, although they vary in size). One feature linked to sediment supply that varies more significantly throughout the Puget Sound is the existence of a backshore. Backshores are generally regarded as the most desirable recreational areas of beaches, but exist in only a minority of Puget Sound coastal areas.
Coastal deposition of sediment occurs in areas where the supply of sediment exceeds the amount of sediment removed by waves, resulting in features such as river deltas, tidal flats, spits, cuspate forelands, and sand dunes. Factors important in the creation of these features include wave direction, wind strength, seasonal changes in transport patters, river and stream presence, and wave energy.
Coastal erosion, or the removal of coastal material, is characteristics of parts of the coastal zone with high sandy cliffs or even steep slopes of bedrock. Processes that contribute to erosion include quarrying (extraction of rock or sediment due to direct impact of breaking waves), abrasion (grinding of coastal rocks by wave-agitated sand and gravel), water-layer weathering (rock die) and biological factors.
The coast of the Puget Sound is a result of many geological events, including the advances of glaciers, the subsequent changes in sea level, the formation of beaches and other sedimentary deposits, an the erosion of these deposits. The major lakes, canals, and basin of the Puget Sound were excavated by glacial ice, which also scoured numerous smaller depressions to form bays, inlets, and passages. These features were established by the time the ice retreated 13,000 years ago. When the ice retreated, the Earth’s crust under the Puget Lowland lifted up and sea level rose. The land ceased to rise about 6,000 years ago, but the sea level continued to rise and still rises a bit each year. The glaciers deposited large amounts of sediments on the coast, which waves and currents began to rework after the ice retreated. Where there was a supply of loose sand and gravel along the coast that could be moved by the waves, a beach developed. Beaches have constantly changing character and consist of a backshore, a beach face, and low-tide terrace. The backshore is the area that remains dry except during severe storms and includes the flat portion where sediment accumulates. The beach face is the area that is continually modified through sediment movement as the tide rises and falls. The low-tide terrace is the area that extends out to about where the water reaches at low tide and often consists of sandbars formed by nearshore currents and the movement of waves. These parts may or may not exist to some degree on an individual beach. Sedimentary deposits form in coastal areas where the supply of sediment exceeds its removal by waves. Beaches and other sedimentary features were shaped over the last 5,000 years. Parts of the coastal zone with high sandy cliffs or steep slopes of bedrock are subject to erosion from the impact of waves and other natural processes. Beaches in these areas reflect an imbalance between the supply of eroded material and its removal by waves. Coastal erosion affects conditions on nearby beaches and can lead to a retreating shoreline.