Large Nivation Hollows near Knob Lake, Quebec

A specialized type of hollow formed on slopes where snow persists into the summer is common in the highlands of the Labrador lake plateau in the vicinity of Knob Lake, Quebec. The hollows originated primarily by nivation, that is, the sapping and erosive action of freeze-thaw processes acting on saturated ground under or along the edge of snow patches. Particularly large, well-developed examples in the shape of deep, semicircular basins with backwalls at the angle of repose for coarse materials and gently sloping floors are situated in glacial drift on the northern sides of bedrock hills in one part of this highland area. Many of these hollows were enlarged and modified by collapse and sliding of the backwalls. Hence two stages of development are apparent: (1) formation of the simple nivation hollow by nivation erosive processes, whereby the size of the hollow was limited by the depth of drift and the competence of the materials composing the backwall, and (2) complex forms, evolved when collapse of the backwalls modified the slope and outline of the simple hollows. Early disappearance of snow and ice from most of the basins during the first half of the summer and the generally dense cover of vegetation indicate that the hollows are at present stabilized and that they formed during a colder climate or under heavier precipitation, or both, than is established today.