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Coasts

Waves form when the wind blows over a body of water William. In the open ocean waves look like a series of swells. When a wave gets near the shore the wave bottom drags against the sea floor while the top keeps moving. The wave gets narrower and higher and eventually topples over crashing onto the beach face.

A series of waves are crashing onto a beach. © Abigail Burt

A beach forms when waves deposit sand and gravel along the shoreline.

Beaches, Strathy Bay, Sutherland

Some coasts are dominated by shoreline erosion. Waves pound the shore with water, sand and even rocks Larry. This can undercut cliffs causing large sections of rock and sediment to fall into the water.

Waves are undercutting the cliffs along the shore of Lake Erie, Ontario, Canada.  The sediment that falls into the lake gets washed away.  Can you see the white field drain sticking out of the cliff? © Abigail Burt

Tides are the daily or twice-daily rise and fall of the oceans. Tides are caused by the gravitational pull of the moon and sun on the ocean.

Many beaches are narrow at high tide and wide at low tide. Man o' War Rocks and Cove, and St Oswald's Bay, from Durdle headland, Dorset, UK

Waves that hit the beach at an angle carry sand and gravel up the beach face at an angle. When the water washes back the sediment is carried straight back down the beach face. Individual particles are moved along the beach in a zig zag pattern. This is called longshore drift.

Walls are built to try and stop longshore drift.  Sand and gravel gets piled up against the walls.

Mean sea level is the average sea level surface. It is half way between high tide and low tide. Geologists look for evidence like old wave cut cliffs and beaches to see how sea level has risen and fallen over time.

The large notch shows that sea level has changed over time.  The flat spot half way down the cliff used to be at sea level. © Abigail Burt

Beaches and spits that formed along the shore of glacial Lake Agassiz are shown in dark blue on the superficial geology map Manuel of the Red River Valley area of Canada and the United States. There are six beaches on the western side of the map. The beaches formed as water levels slowly dropped.

Can you find matching beaches on the other side of the large purple area?


 

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