into the chalk. But such impure superficial chalk, when dissolved, would
leave a larger supply of earthy matter to be added to the mould than in
the case of pure chalk. Besides the loss caused by percolation, some
fine earth is certainly washed down the sloping grass-covered surfaces of
our Downs. The washing-down process, however, will be checked in the
course of time; for although I do not know how thin a layer of mould
suffices to support worms, yet a limit must at last be reached; and then
their castings would cease to be ejected or would become scanty.
The following cases show that a considerable quantity of fine earth is
washed down. The thickness of the mould was measured at points 12 yards
apart across a small valley in the Chalk near Winchester. The sides
sloped gently at first; then became inclined at about 20 ; then more
gently to near the bottom, which transversely was almost level and about
50 yards across. In the bottom, the mean thickness of the mould from
five measurements was 8.3 inches; whilst on the sides of the valley,
where the inclination varied between 14 and 20 , its mean thickness was
rather less than 3.5 inches. As the turf-covered bottom of the valley
sloped at an angle of only between 2 and 3 , it is probable that most of
the 8.3-inch layer of mould had been washed down from the flanks of the
valley, and not from the upper part. But as a shepherd said that he had
seen water flowing in this valley after the sudden thawing of snow, it is
possible that some earth may have been brought down from the upper part;
or, on the other hand, that some may have been carried further down the
valley. Closely similar results, with respect to the thickness of the
mould, were obtained in a neighbouring valley.
St. Catherine s Hill, near Winchester, is 327 feet in height, and
consists of a steep cone of chalk about 1 4 of a mile in diameter. The
upper part was converted by the Romans, or, as some think, by the ancient
Britons, into an encampment, by the excavation of a deep and broad ditch
all round it. Most of the chalk removed during the work was thrown
upwards, by which a projecting bank was formed; and this effectually
prevents worm-castings (which are numerous in parts), stones, and other
objects from being washed or rolled into the ditch. The mould on the
upper and fortified part of the hill was found to be in most places only
from 21 2 to 31 2 inches in thickness; whereas it had accumulated at the foot
of the embankment above the ditch to a thickness in most places of from 8
to 91 2 inches. On the embankment itself the mould was only 1 to 11 2 inch
in thickness; and within the ditch at the bottom it varied from 21 2 to 31 2,
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