This is, however, unusual, and south slopes soon crust up even in midwinter. The sun melts the snow by day, and the melted snow refreezes at night, producing a crust. Such a crust will, at first, be soft and breakable.
Soft breakable crust is not a bad running surface. It is quite true, i.e. does not vary in pace from one place to another, and is usually soft enough to enable the ski-runner to make Telemark swings, either stop or linked, without much difficulty. Sometimes the crust is harder, and in this case the jump-round or quersprung is the only possible manœuvre.
A peculiarly villainous form of crust, fortunately rather rare, is known by the appropriate name of Trap crust. This crust is perfectly solid at one spot, and bears the weight of the ski without breaking, only to crack a few yards lower down. The ski-runner skids with great speed over the hard crust, only to be pulled up suddenly as his ski break through the soft crust below.
Trap crust is more usual in spring than in winter, especially on slopes which change in direction, so that the ski-runner passes from snow which has been sufficiently thawed to form a hard solid crust at night to slopes which have only been superficially melted. Trap crust is only found after fairly heavy falls of snow which require considerable time to form a solid crust. As a rule, trap crust covers a foot or more of powder snow.
To return, however, to more normal winter conditions. Under normal conditions the snow on south slopes in winter is gradually transformed into crust.
This crust is at first soft and breakable, but the continuous process of melting and refreezing gradually produces a solid unbreakable crust.
Unbreakable crust, as the ski-runner soon discovers, varies greatly in quality from hard crust which is so hard that the ski slip sideways without obtaining the least grip, crust which on any slope but the most gentle is almost unskiable, to the other extreme, crust which though quite hard and unbreakable is rough enough to permit even a Telemark and to make Christianias and Stemming turns delightfully easy.
The solid crust formed in winter is usually very hard and slippery before the sun strikes it. This crust is known as Marble Crust. You find marble crust on southern slopes in winter, and at high altitudes in spring. It is common in the High Alps. Even at the very end of April I have known a slope at a height of 12,000 feet remain hard, slippery and unsoftened for some little time after the sun had struck it because a bitter north wind had lowered the temperature several degrees below freezing.
In mid-winter a slope of marble crust will yield good ski-ing after the sun has struck it, unless the air temperature is several degrees below freezing. Marble crust, touched by the sun, yields a surface which will puzzle the beginner but which is pure delight to the man who has mastered Stemming turns and Christianias. The excellent running that may be obtained on south slopes is too little exploited, for winter ski-runners are firmly convinced that good ski-ing can only be found on north slopes where powder lies.
Mid-winter may be said to last till the end of January. In February you often find perfect spring snow, Telemark crust, etc., on south slopes, and in February, as often as not, the best running is obtained on southern slopes. This type of running will be dealt with later.