Introduction
Asymmetrical topographic features related to slope aspect are frequently observed in regions having periglacial and glacial climatic regimens. Cirques are often more prevalent on cooler, north- and east-facing slopes, where firn accumulations are preserved for longer periods during the summer and physical weathering is more intense (Reference Derbyshire, Evans and DerbyshireDerbyshire and Evans, [c1976]). Valley cross-sectional asymmetry is also commonly encountered, but its relationship to aspect is less uniform. In some regions, valley slopes facing north and east are steeper; in others, the steeper slopes are south- and west-facing (Reference Kennedy and DerbyshireKennedy, [c1976]). This paper describes another form of asymmetry: the remarkably unequal erosional patterns which have developed on opposing valley sides in parts of the Nahoni Range of the northern Ogilvie Mountains. Yukon Territory, Canada. North- and north-east facing slopes are, as a rule, more intensely dissected than those directed to the south and west; indeed, many of the latter show no dissection at all (Fig. 1a and b).
(a) and (b): Drainage networks within regions studied (1:50 000 quadrangles. 116G/14, 116G/10, and 116G/15). (c): Frequency distribution of rivulet channel orientations.
Observations and discussion
During late spring and early summer, melt-water discharge rates are observed to vary according to valley side orientation. By late May or early June, all snow has melted on south- and west-facing slopes. During this period, detritus on the slopes is water-saturated. In contrast, even into early July, individual accumulations of snow are still found on north- and north-east facing slopes. The long presence of slowly melting snow on these slopes tends to result in rivulets of water discharging down-slope from each patch and embankment of snow.
To assess the relationship between valley side orientation and the degree of its dissection, a frequency distribution diagram of channel orientations for the valley-side rivulets was constructed (Fig. 1c). It is apparent that channel development is essentially a function of slope bearing and is most marked on those slopes facing the north and north-east.
The extent of slope dissection must therefore, to a great degree, be controlled by intensity of insolation and its influence on persistence of snow cover and rate of discharge of melt water provided by snow and ground ice. The lower temperatures experienced by north- and north-east-facing slopes as compared to slopes of southern and western aspect account for the observed longer persistence of snow patches and ground ice on slopes of northerly bearing. As a consequence, marked differences in rate of melt-water discharge and extent to which melt water is concentrated into distinct flow channels exist on slopes of different aspect. Warmer slopes experience rapid melting of snow and ground ice, with consequent saturation of the talus apron occupying the sides of the valley. With underlying bedrock serving as an impermeable sub-surface, detritus is rendered mobile and moves down-slope by gelifluction in a manner sufficiently uniform as to preserve generally smooth contours to the slope. In contrast, the surface cover and bedrock of the cooler slopes, sustaining a slower release of melt water, restricted to rivulets, experience erosion in narrow channels. Once formed, these channels become preferred lines of erosion.
Another factor which may contribute to the asymmetry of dissection is the predominance of carbonate rocks in the region. Melt water on cooler slopes might well contain a greater amount of dissolved CO2, thereby increasing the extent of carbonate solution on slopes of northern and north-eastern aspect.
