The range of the slope of permanent stability obviously depends upon the nature of the chalk, whether it is denuded or covered or mere loose-jointed strata, the effect ground and surface waters may have upon it, and also the position of the beds, and whether a cutting or embankment is on the side or the base of a hill, and consequently at the place where it is likely to be in a wet condition.
The Needle-rocks in the Isle of Wight and Beachy Head may be mentioned as familiar examples, showing that firm and comparatively pure chalk will stand practically perpendicular, even when much exposed, if pure and free from faults and homogeneous in texture; and in blocks with beds inclined away from a cutting it will permanently stand
Almost vertically TO ½ to 1, at a great height,
a slow regular crumbling of the surface or falling down of small fragments, which seldom produce serious movement in such material, being the only deleterious effect of weather influences.
As the chalk becomes broken and less evenly bedded,
Loose, friable chalk in surface beds will often not permanently stand at a less slope than from
according to the depth and degree of exposure. The most usual slopes being
Impure wet chalk and marly chalk will require a slope not less than
Much depends upon its freedom from faults, crevices, and pot-holes, as they hold water, and the surrounding soil may fall away, for water quickly passes in quantity through the fissures and crevices which are generally numerous in the upper chalk, especially at the bottom of a bed of flints which in consequence of their impermeability lessen the upward flow; but flint beds in soft chalk are an advantage, as they act as drains.
The affinity chalk has for water, which has been considered a reason for the absence of important rivers in that formation, as water does not flow away freely upon it, causes it to be readily affected by rain and disintegrated from the effects of the expansive and contracting action of frost and thaw; hence draining and covering the surface may be important, but care must be taken not to interfere with natural springs. This property of chalk, viz., its affinity for water, although a disturbing cause in earthwork in that formation is of value for covering or filling in open trenches, counterforts, or drains in _other_ soils, as the chalk attracts water, and therefore dries the surface of other earths.