Holocene marine coastal evolution of the United States

The Holocene began about 10.7 ka following the glacial readvance of the Younger Dryas (Valders) interval that terminated the Pleistocene. World sea level had fallen to about ?54 m. Nearly half the present United States continental shelf was then a coastal plain with vegetation ranging in nature from subarctic tundra to coniferous woodland. A fluctuating transgression ("Flandrian" stage) followed, accompanied by rising temperatures. Reaching the present coastline about 6 ka, the transgressing seas drowned river valleys, creating estuaries and dendritic embayments. As barrier spits and islands developed, the estuaries and embayments became lagoons. During the later Holocene, world sea level was modulated by numerous negative fluctuations, signaling cool intervals that are indicated by pollen analyses and neoglacial advances. Extra-warm cycles were characterized by higher storm frequency; in areas favorable for preservation, these conditions are recorded by distinctive sets of beach ridges. Local paleogeography was dominated in most places by a tectonic "groundswell," such as is presently found throughout the collapsing forebulge regions along subducting plate margins of southern Alaska and on the taphrogenic coast of California. More or less stable platforms are found only in northwest Alaska and Florida. Present coasts are widely affected by the post-Little Ice Age warming, which has led to steric and glacio-eustatic sea-level rise. The warming has also slowed the velocity of the Gulf Stream and other geostrophic currents, reducing the Coriolis-controlled dynamic tilt and causing further sea-level rise along the mainland coasts.