Fort Ebey State Park and Island County Kettles
17,000 years ago, along the west coast of North America, the last continental glaciation extended southward from British Columbia into the Puget Sound lowland, reaching as far to what is now Olympia. Ice covered the region for about 1,000 years, then began to melt and retreat.
The ice thickness near Coupeville was 1,200 meters (3,900 feet). 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 mass 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. The combination of lowered sea level and depressed land placed the island about 150 meters (500 feet) below what is today's sea level.
With a warming climate the glaciers' terminus began to recede to the north at 340 meters (1,100 feet) per year. As 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.
Meltwater rushing out of the sediment-laden glaciers into surrounding shallow seas and lakes constructed deltas that fanned out from the glacial margins. One of these deltas is present as the northern bluffs above Ebey's Prarie.
The ice sheet began to advance southward 13,000 years ago and stopped near Penn Cove before it began its final retreat. Just south of Coupeville the glacier left behind a low ridge that is an east-west terminal moraine made of gravel . As the glacier melted it also left large ice blocks that had "calved" (broken off ). The blocks were buried by sediment carried in the meltwater. After the ice blocks were buried they slowly melted. , leaving large depressions known as "kettles." Within the two adjacent parks there are 25 kettles, ranging from 700 meters (2,300 feet) long [need to add a 2nd length to have a range] and 48 meters (160 feet) deep to 250 meters (820 feet) deep.
East-west trending Penn Cove may have been formed by fast-moving rivers of subglacial melt water, which carved the trough through sediments north of Coupeville.
Image of the Kettles, Coupeville
and the land north of Penn Cove.
The Kettles and the Coupeville moraine mark the glacial terminus from which the glaciers retreated to the north.