Cirques, moraines, and trimlines: The legacy of our shrinking glaciers
The low snowpack this year will almost certainly result in further recession of our local glaciers. Although the meltback during a single year may not be apparent, anyone who has been around these parts for 20 or 30 years can remember when areas of now barren rock were formerly covered by ice. But the best record is preserved in the landforms produced by the glaciers during their various stages of growth and decline.
An alpine glacier is like a slow-moving conveyor belt carrying rock debris plucked from the headwall and dumping it as moraine at the terminus. In the process it excavates an amphitheatre-shaped depression, or cirque. When a glacier is in equilibrium with the climate, neither advancing or retreating, the debris piles up around the snout as a terminal moraine. In receding glaciers much of the debris is redeposited by meltwater streams and, as the ice melts, a zone of bare, scoured rock is exposed between the edge of the receding ice and the trimlines which define the margins of the ice during its maximum extent.
Most of the cirques on our local mountains are much too large to have been formed by the glaciers now occupying them. They were formed by huge late Pleistocene glaciers when the Cordilleran Ice Sheet covered most of B.C. Debris excavated from these large cirques was carried away by valley glaciers and dumped many kilometres away.
Disappearance of the Cordilleran Ice Sheet about 10,000 years ago was followed by a period of fluctuating climate. A prolonged warm period 500 to 1,000 years ago gave way in the mid-1600s to a drop in global temperatures. Commonly referred to as the "Little Ice Age" this period saw the rapid expansion of alpine glaciers which reoccupied cirques abandoned by the late Pleistocene ice. Tree-ring studies in the Whistler area indicate that this second generation of glaciers continued to expand until about 1860, when the present recession began. It was during this time that the prominent trimlines and terminal moraines surrounding our present glaciers were formed.
When the first trail was cut through the Sea-to-Sky corridor by CPR surveyors in 1877, the glaciers must still have been close to their maximum, filling the now empty space between the existing ice and the trimlines high up on the adjacent valley walls. Horstman and Blackcomb glaciers would have been at least double their present volume. A glacier would cover the north side of Little Whistler down to its terminal moraine, now the Camel Humps. And two lobes of ice would fill the Couloir on Blackcomb, depositing between them a rocky S-shaped moraine half way down the present ski slope. Even viewed in the context of our own short history the rate of glacier recession on our local mountains is cause for concern.
Written by: Jack Souther