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The geomorphology and permafrost conditions of Garry Island, N.W.T. Kerfoot, Denis Edward

Abstract

Garry Island, approximately 11 kilometres (7 miles) long and 0.8 to 3.2 kilometres (0.5 to 3.2 miles) wide, is located at about latitude 69° 28'N and longitude 135° 42'W in the southern part of the Beaufort Sea. The stratigraphy consists mainly of unconsolidated sands, silts, clays and stony clays which have been intensively deformed by the thrusting action of glacier-ice moving from the south. The deformed sediments are locally overlain by undisturbed sands and gravels containing marine fossils dated at >42,000 years. The absence of any evidence of glacial till on top of the sands suggests that Garry Island lay beyond the northwestern limits of the Laurentide ice sheet during the late-Wisconsin glaciation. Elevated strand-lines, which may be of great antiquity and occur at approximately 7.5 metre (25 feet) intervals to an altitude of almost 46 metres (150 feet), indicate the extent of Pleistocene fluctuations of sea level and the drowning of a pre-existing topography. The development of tundra polygons, in small flats behind sandspits or bars built across the drowned valleys in association with the former sea levels, has imparted a distinctive, stepped longitudinal profile to the stream courses. The tundra vegetation of Garry Island is classified into ten major habitats which are primarily related to drainage conditions and type of geomorphic activity. The island is underlain by permafrost and the thickness of the active layer is greatest, and ground temperatures in this layer are highest, beneath unvegetated surfaces and where the substrate is composed predominantly of mineral soil. Stratigraphic, geomorphic and historic evidence indicates considerable recession of the coastline in recent times. Current rates of retreat, reaching maxima of 10.5 metres (35 feet) per annum, are primarily related to the composition of the permafrost, being greatest in areas of fine-grained sediments, containing high ice contents, with a southerly exposure. Thermal erosion of the permafrost is the dominant process influencing cliff retreat and the primary role of wave action, on a short term basis, is in the removal of thawed debris from the base of the cliffs. Observations of three highly active mudslumps, created by the exposure of segregated ground ice, show that the rate of headwall recession is strongly correlated with ambient air temperatures. Maximum recession occurs where the ice content is high and the slumped debris is frequently removed from the base of the scarp. The cyclic development of a gully system on the ice face is described. The longevity of mudslump activity is prolonged where strong mudflows carry the thawed material away from the foot of the headwall, thus preventing the progressive burial of the scarp face. Mudflow velocities reveal a rhythmic pulsation related to periodic blocking of their channels. Mud levees, bordering the mudflows, result from the progressive bleeding of moisture from, and subsequent stagnation of, the mud rather than as residual features pushed aside by the advancing mudflow. Patterned ground on Garry Island is primarily restricted to non-sorted types. Angular intersections of thermal contraction cracks, representing the incipient stages of tundra polygons, exhibit a preferred tendency toward slightly-oriented, orthogonal systems. The initial micro-relief of earth hummocks is believed to originate through the accentuation of a miniature desiccation/frost crack pattern. Following the establishment of a vegetation cover, their subsequent growth involves further differential frost action and solifluction. Statistical tests show that the height, size and shape of earth hummocks are closely related to their position on the slope profile.

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