South Whidbey State Park and the Trillium Community Forest
Driving along Smuggler's Cove Road, from its intersection with Lagoon Point/Westcliff Road, to the entrance to South Whidbey State Park, and a little farther south to the trailhead for the Trillium Community Forest, the road stays on a shelf 200 feet above sea level. The shelf may be a wave-cut terrace left when relative sea level was higher. In any case, it is a level natural spot for a park and homes along Smuggler's Cove Road.
Seeing the geological framework here is not easy. You need to use LIDAR images of the area (see example in figure below) to see the landforms and deposits left during the last glacial episode (18,000 to 13,000 years ago). That and earlier glacial episodes left till and sediments with a cumulative thickness of about 3,000 feet; we are seeing only the remnants of the last glaciation. During the last glacial episode Whidbey Island was buried by up to 2,600 feet of ice.
The State Park is located mostly along the western slope of a ridge that is a north-south-oriented, 4,000-foot-wide, drumlin, which was formed during southward ice flow. The highest point on the drumlin, reached along the Ridge Trail, is 480 feet above sea level. The highest point in the Trillium Community Forest, along its trail complex, is 400 feet. The drumlin is dissected by stream drainages cut perhaps during, or most likely after, the glacial retreat.
While climbing the Ridge Trail you can see patches of the deposits that make up South Whidbey State Park. Near the beginning of the trail is sand and gravel left by outwash from melting glaciers. Above that is up to 70 feet of glacial till, sculpted by the overriding ice.
The projected southern margin of the NW-SE trending Whidbey Island fault zone clips the southern margin of the state park and the northern part of the Trillium Community Forest. This fault zone has been identified primarily by geophysical surveys in Saratoga Passage and Admiralty Inlet. On land it is difficult to identify in that much of the offset along the fault is buried by younger glacial deposits.
Cliffs along the western park margin were formed
during wave erosion and thousands of years of
landsliding. Landslides are a major hazard for cliffs
along most of Puget Sound shorelines.
In 2013 a landslide north of the State Park at
Ledgewood-Bonaire buried a beach, carrying along
a beachside cabin and leaving a steep headwall that
cut into many backyards. The "Beach Trail" in the
State Park is frequently closed when damaged by
LIDAR image of central-western Whidbey Island.
North-south drumlins were left by glacial scouring