Cortes Island Natural History

Floral History
Geological History

Vegetation Cover

With the exception of the North Peninsula, Cortes Island lies within the Coastal Western Hemlock Very Dry Maritime Biogeoclimatic subzone. The dominant tree species in this area are Douglas-fir, western red cedar, western hemlock, and red alder. Scattered western white pine are also found, and arbutus and lodgepole pine grow on exposed, rocky slopes.

The great majority of Cortes was clearcut in the early 1900’s, and post-logging fires killed many scattered trees and forest patches left behind after logging. As a result, the landscape is dominated by young (80 to 100 year old) Douglas-fir forests, with occasional red alder stands in moist hollows or on heavily disturbed sites. Hemlock and red cedar are commonly understory trees, although they are part of the main forest canopy in places. Some moist depressions support growth trees in the current forests on Cortes are 40 to 45 meters tall, and 60 centimetres in diameter. Old growth stems in the same area are 60 meters tall, and 120 to 150 centimetres in diameter.

Prior to settlement and logging, the Cortes Island landscape was likely dominated by Douglas-fir, red cedar and hemlock old growth forests. Now, old growth forests are rare. No old forest patches large enough to have an “interior area” exist on good growing sites at this time. The remaining old growth features generally consist of 1 to 20 very large trees on good growing sites, and patches of smaller old trees on rocky terrain. The old forest structures on rocky terrain are very ecologically valuable, and provide a range of habitats, but they are not the ecological equivalent of the giant old Douglas-fir trees which once dominated the Cortes landscape.

A moderate amount of logging has occurred in most second growth stands on private land on Cortes Island in the last two decades, but most of the island is still forested. Little recent logging has occurred on Crown land.

Source: Community Forest Agreement Proposal: Draft September 2002

Geology and Landforms

Cortes Island has a dendritic coastline, broken by many bays and several major inlets. The majority of the island is relatively low lying, with elevations less than 200m above sea level. Cliff Peak, an outlier of the massive coast mountains, rises from the northern peninsula of Cortes to an elevation of 420m.

Cortes Island has a complex geology. The island bedrock is largely composed of massive granitic intrusive rocks, as found in the Coast Mountains. These are cut by faults and fractures, and by dykes of fine-grained intrusive rock. However, unlike the towering cliffs of the Coast Mountains, the bedrock terrain on Cortes is largely a gently rolling, relatively level plain with subdued relief. Low rock hills, narrow rock walled valleys, and small elevated plateaus or terraces are common.

Most if not all of the island was overridden by glacial ice during the last ice age. The moving ice scraped away existing soil deposits, smoothed bedrock outcrops, and enhanced the naturally striated nature of the faulted and jointed bedrock terrain. The ice also deposited till materials as mantles on bedrock hills, as valley fill between hills, and as elevated rolling plateaus in several locations.

The weight of the glaciers depressed the crustal plate on which Cortes Island rests, while the growth of the glaciers reduced worldwide sea level by converting water to ice. As the glaciers melted, sea levels rose and the crustal plate rebounded simultaneously. At times, Cortes was largely submerged by the rising seas, creating marine beach deposits on sites which are now elevated hills, but further rebound caused the island to rise from the sea to its present level.

This complex geological history has produced a complex soil landscape. Elevated rock hills tend to be bare of soil. The valleys between hills often contain deep glacial or marine deposits. Soil parent materials are extremely variable, ranging from fine marine clay and silt to coarse glacial outwash gravels, with a wide range of types between these two extremes. Perched water tables and wetlands are common above shallowly buried bedrock outcrops, above impermeable clay/silt layers in the soil profile, and in small valley bottoms.

The key feature of the central portion of Cortes Island is variability. Small patches (1 to 5 hectares) of deep well-drained soil with excellent forest growth potential are nestled between barren rock outcrops, stunted forests on shallow rocky soil, and perched wetlands. A complex landscape pattern of these basic components repeats across the central portion of the island.

The terrain of the Sutil Peninsula, the long southern arm of Cortes Island, and nearby Marina Island, is very different from the rocky central area discussed above. Bedrock in these areas is overlain by many tens of meters of surficial deposits, laid down in marine environments or by glacial outwash during glacial retreat. The land in this area is flat and quite uniform, but elevated above sea level. Vegetation growth is uniform and luxuriant throughout the area, but many sites are susceptible to summer drought due to deep, porous soils.

The long North Peninsula of Cortes Island also has a different terrain expression. This area is occupied by Cliff Peak, a typical, steep-sided, granite Coast Range mountain peak.

Source: Community Forest Agreement Proposal: Draft September 2002