​​​​Snow Productions BI 

Bainbridge Island Geology: Video Previews, Field Trips

Discover how a small island in Puget Sound can reveal the larger picture.

​​Fun Field Trips

Geology in Public Places
Esperance Sand
 photo fossil from Blakeley Formation
Blakely Harbor
Blakely Harbor Formation
photo Pt Monroe sandspit

Bill Point, Pritchard Park

Fort Ward Park

Blakely Harbor Park

Rockaway Beach Park

Fay Bainbridge Park

On the east side of the point at the opening of Eagle Harbor, an embankment lines the beach. Glacial till forms the upper layer of the bluff under the soil. The bluff ramps up as it moves south, revealing an under layer of deep, pure sand. These two layers were left by the last glacier that descended into the region about 17,000 years ago. First, rivers coming from melt water moved tons of sediment out in front of the glacier. The sand represents part of an outwash plane that covered the Puget Sound Lowland. As the glacier began to melt more rapidly and retreat, it left rocks, sand and clay compressed as till.

You will find a good example of glacial till in the waist high embankment near Bill Point.
The shoreline around the south end of the island displays ridges of sandstone and siltstone, some places riddled with marine fossils. The fossils derive from marine animals that burrowed into the seafloor between 25 and 30 million years ago. The ridges show us the edges of old layers of seafloor, lifted up and tilted 90 degrees by movement on the Seattle Fault.

​You can walk on these ridges at Fort Ward Park.
The east-west orientation of the harbor suggests influence from the Seattle Fault Zone which cuts through the south end of the island. Compressed sawdust, pilings and other remnants from a timber mill characterize the surface around the harbor. The road bordering the harbor reveals pieces of the Blakely Harbor Formation. (See Rockaway Beach.)

​From Blakely Harbor you can see the uplifted terrace on the south end.
 Scuba divers enjoy diving here because of the sea life that clings to a line of rocks visible from the shore. Those rocks and the conglomerates along the beach are part of the Blakely Harbor Formation. Within the conglomerates, dark smooth pebbles of basalt testify to an origin in the Olympic Mountains. About 13 million years ago, these sediments descended in rivers to a floodplane where they were buried and compressed into hard rock, then, movement on the Seattle Fault raised and tilted them close to 60 degrees.

You can see this formation in the embankment. Every year the exposed uplift erodes several feet towards the road..
Wind and tides constantly move sediment. The prevailing direction south to north has creating a long spit, large enough to support houses. Winter storms can change the direction and eat some previous gain away.

You can see the spit extending from the north end of Fay Bainbridge Park.