On the uplands of Crane Island, the bedrock is generally massive metamorphosed sandstone of the Constitution Formation.
Chert is primarily composed of ribbon chert, alternating layers of silica rich
1 to 2 inches thick with thin layers of shale. The chert is derived from the
accumulation of silica diatoms that accumulated on the ocean floor. The shale
was derived from fine dust and silt that also reached the ocean floor. The
layering developed after deposition when the ooze of silica rich and silty
sediment was compacted. The age of the Orcas Chert is between 180 and 280
million years old. The Orcas Chert is part of terrain consisting of the related
Deadman Bay Volcanics, which are ocean floor pillow basalts that are somewhat
older than the Orcas Chert and likely formed the basement that the silica rich
sediment that formed the chert unit was deposited on. The original depositional
setting was an ocean floor area far from
The Constitution Formation was also originally an ocean floor assemblage; however, this unit is on the order of 130 million years old. The Constitution Formation consists predominantly of metamorphosed fine sandstone derived from a volcanic arc. There is some chert and basalt in the unit as well. This formation was also accreted to the edge of North America. A lack of older aged sediment, North America sediment, suggests the sediment may have been derived from an ocean island arc.
The Rosario Thrust Fault and the juxtaposition of the Orcas Chert and Constitution Formation took place after accretion -- note that the younger Constitution is thrust over the older Orcas Chert.
The map pattern shows that Crane Island is klippen of Constitution Formation on the underlying Orcas Chert. Parts of the Constitution Formation have been stripped away leaving the Constitution Formation as an 'island' on the Orcas Chert and as well as the rock formation that makes up most of Crane Island.