Gulf Islands National Park Reserve of Canada

Geology

 Cliffs of  Monarch Head The cliffs of Monarch Head on Saturna Island
© Parks Canada / Josh McCulloch 2006

Rock formations at Narvaez Bay Exploring rock formations at Narvaez Bay on Saturna Island
© Parks Canada / Josh McCulloch 2006

The Gulf Islands are underlain by folded and highly faulted sedimentary rocks. This area of southwestern British Columbia is seismically active, overlying a tectonic subduction zone. On the islands, north-south headlands and high ridges of erosion-resistant sandstone/conglomerate are separated by narrow valleys and bays underlain by more easily eroded shale.

Glacial ice as much as a mile thick carved the landscape over two glacial periods, with the ice most recently receding 12,000 years ago. Deposits of glacial sand and gravel form Sidney Island, and striated and fluted bedrock are common evidence of past glaciation throughout the islands. Although sea levels have remained the same for the last 5,000 years, they have fluctuated, in the more distant past, up to 150 metres higher than the present level.