Film crust is admirable for fast straight running, because at any moment the pace can be regulated by a swift turn. There is nothing finer than a run down the glaciers in the early morning on film crust.
We have seen that the cold nights of winter produce marble crust, which is hard and slippery. Marble crust is the kind of surface which needs crampons or a chip with the axe if it is climbed on foot.
Further, marble crust, and all forms of crust produced by winter winds, yield a very unsatisfactory surface, too slippery to give good purchase.
We have seen that the milder nights of April produce perforated crust which is much rougher and gives good ski-ing, while the even warmer nights of May and June produce film crust in which Christianias and Stemming turns are easy and safe. In fact film crust gives such a wonderful grip that even Telemarks are possible.
From this we deduce the following law:
Provided the night’s frost is sufficient to produce a solid unbreakable crust, the crust will give the best ski-ing when the frost has been least severe.
In other words, “The milder the frost the better the crust.”
This rule is important. It often happens in spring that one has to start down before the sun has produced a surface melting. Again and again the first thousand feet or so of descent have been spoiled because the crust was too hard and slippery. But the lower one ran down the better the ski-ing. The hard marble crust of winter gave way to the perforated or film crust of spring. In April, in the High Alps, winter conditions often prevail at high altitudes to give way to spring conditions lower down. You start a run on marble crust and end on perforated or film crust. The lower the altitude the milder the night frost, and hence the better the crust.
But of course the hard crust of dawn is soon changed under the influence of the sun. At first the sun produces a slight surface softening which is, however, quite sufficient to make a great deal of difference. Even marble crust, if superficially softened, yields excellent running. Gradually the melting process becomes more pronounced. Sun-touched crust with a slight surface moistening—quite sufficient to make Christianias and Stemming turns a joy—yields to crust covered by an inch or two of melted snow. Such crust is known as Telemark crust—not because Christianias are not easy, but because Telemarks are very simple, whereas Telemarks are difficult, if not impossible, on hard crust.
Telemark crust is a normal stage in the passage of hard crust to melted snow. It is the practical certainty of finding Telemark crust at certain hours in spring that makes spring ski-ing in fine weather such a delight.[21]