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Geological Definitions

Continental Glaciation

Over the last two million years, with multiple periods of glacial growth and retreat, continental glaciers moved south from Canada into the Puget Sound lowland, reaching as far as what is now Olympia. During the last glaciation, ice covered the Puget Sound area for about 1,000 years, then began to melt and retreat.



The most evident features

are the north-south-oriented

drumlins on Camano Island

and the southern two-thirds

of Whidbey Island.

Drumlins are elongate hills

of packed glacial till and

sediments that were sculpted

by the south-flowing ice.



A rock fragment carried by glacial ice, or by floating ice, deposited some distance from the outcrop from which it was derived.


A long, narrow glacially eroded inlet or arm of the sea. Many of the fjords in Puget Sound were submerged by rising seas (for example, Admiralty Inlet west of Whidbey Island)



Glacial Outwash Delta

The slopes down to Ebey's Prairie are the fronts of marine deltas that were deposited by sediment-laden glacial melt water flowing into a shallow sea.





Isostatic Rebound

The ice near Coupeville was 1,200 meters (3,900 feet) thick.  The weight of ice on what was to become Whidbey Island was about 576 billion tons and depressed the land (the Earth's crust is flexible and responds to overlying masses like the glaciers). At the same time the growth of glaciers caused a lowering of global sea level with water locked up in glacial ice.



A retreating glacier leaves large ice blocks that had "calved" (broken off ). The blocks are buried by sediment carried in water from the melting glacier. After the ice blocks are buried they slowly melt, leaving large depressions known as "kettles."  These are most evident in the "Kettles" of Fort Ebey State Park and the adjacent county park.



A mound of poorly sorted glacial till deposited as ridges around the toe or along the margins of a glacier.


Wave-Cut Terraces

As the glaciers melted the land surface rose (rebounded) and sea level simultaneously rose. On Whidbey Island the rate of rising land exceeded that of the rising sea and as it rose, terraces were cut into the land by waves; the terraces are now visible around the island like bathtub rings.

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