Geology Cafe

Coastal Environments

Beaches, Barriers, Bays, Lagoons, Estuaries, and Rocky Coastlines
Shoreline environments are rapidly evolving landscapes. Wave action is a nearly constant force of erosion, with incredible power during storms. Changing conditions in nearshore setting, places of deposition can change to places of erosion, and vice versa. Daily tidal flow into large bays can carry as much as a large river in flood. Nearshore currents transport sediments to beaches, offshore bars, and offshore onto the continental shelf and beyond. Coastal environments also support diverse habitats for marine, brackish water, to freshwater and terrestrial environments which can change from season to season. Shoreline environments are subject to sea level changes; a change of a few feet can cause major changes to shoreline geometry.
A beach and barrier island complex in cross-section profile. The landward migration of sedimentary "facies"is a result of sea level rise and deposition progresses over time.
Click on thumbnail images for a larger view.
Aerial view of Bolinas Bay, Stinson Beach (barrier) and Bolinas Lagoon in Marin County. Prevailing northwesterly winds drive waves that are refracted around the headlands at Agate Beach (located at the southern end of the Point Reyes Peninsula. The refracted waves drive longshore currents that that transport sediments northward along the coast onto Stinson Beach. Sea level rise caused by the melting of continental glaciers of the last ice age resulted in the partial flooding of the valley now occupied by Bolinas Lagoon. That stream valley developed along the San Andreas Fault. The fault segment that ruptured in the 1906 earthquake runs under the west side of the lagoon and the beach.
This view of Bolinas Bay, Stinson Beach, Bolinas Lagoon, and the southern end of the Point Reyes Peninsula. This west-facing view is from Bolinas Ridge in the Golden Gate National Recreation Area, Marin County. The dark character of the back beach area reflects the urban development of the barrier island. Limontour Beach and Estero (lagoon) is part of a barrier island complex at Point Reyes National Seashore. Daily tidal cycles drive currents that flow into to the channels that flood and drain salt marshes in the lagoon and back barrier area. Dense vegetation that cover the salt marshes trap sediments and provide both food and shelter for an abundance of wildlife that utilize the marshes.
Moss Landing Beach is at the eastern end of Monterey Bay. The coast along the bay is dominated by large coastal dunes that provide a natural barrier to storm waves, and potentially, tsunamis. This view is looking to the southwest toward the manmade jetty built to protect the entrance to Moss Landing Harbor at the mouth of Elkhorn Slough. Elkhorn Slough is an estuary along the border between Monterey and Santa Cruz counties. The slough and surrounding mudflats and wetlands fills a drowned stream valley that during the last Ice Age flowed into Monterey Bay at the head of Monterey Canyon (an undersea canyon). This image was taken near Kirby Park.
A summer beach profile at Cove Beach at Ano Nuevo Beach State Park in San Mateo County. Note that sand has accumulated on the beach (relative to the winter profile below). During the relatively quiet summer weather conditions, wave action and currents move sediments from offshore bars onto the beach. A small spring-fed stream channel is in the foreground. A winter beach profile at Cove Beach at Ano Nuevo Beach State Park in San Mateo County. Note that the beach sand has mostly vanished, and that cobbles and gravel cover much of the remaining surface. At high tide, winter waves reach the base of the sea cliffs. During winter storms, waves actively erode the seacliffs, making this beach, and most locations along the coast, extremely dangerous places to humans.
Beach and coastal dunes along Point Reyes National Seashore Headlands and seastacks at Point Reyes National Seashore. The Point Reyes Headlands are a part of a ridge of granitic rock that rises above the surrounding ocean.
A massive sea cave on Santa Cruz Island, Channel Islands National Park.

Left: A boulder- and cobble-covered beach in Monterey, California.

Below: headlands and coves with sandy beaches are at Montara Beach State Park in San Mateo County. Granitic rocks that form the core of Montara Mountain are well exposed in the coastal headlands. Most of the California coastline consists of rocky shorelines with intermittent beaches with seacliffs and exposed rock reefs (not calcareous organic reefs like in tropical regions). In places along the California Coast, the steep slopes along the shore do not continue beneath the ocean. Along the coast of California the 100 meter water-depth contour ranges from several miles to many dozens of miles offshore.
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