THE DENUDATION OF THE LAND—_continued_.
Denudation aided by recently ejected castings flowing down inclined
grass-covered surfaces—The amount of earth which annually flows
downwards—The effect of tropical rain on worm castings—The finest
particles of earth washed completely away from castings—The
disintegration of dried castings into pellets, and their rolling down
inclined surfaces—The formation of little ledges on hill-sides, in part
due to the accumulation of disintegrated castings—Castings blown to
leeward over level land—An attempt to estimate the amount thus blown—The
degradation of ancient encampments and tumuli—The preservation of the
crowns and furrows on land anciently ploughed—The formation and amount of
mould over the Chalk formation.
WE are now prepared to consider the more direct part which worms take in
the denudation of the land. When reflecting on sub-aerial denudation, it
formerly appeared to me, as it has to others, that a nearly level or very
gently inclined surface, covered with turf, could suffer no loss during
even a long lapse of time. It may, however, be urged that at long
intervals, debacles of rain or water-spouts would remove all the mould
from a very gentle slope; but when examining the steep, turf-covered
slopes in Glen Roy, I was struck with the fact how rarely any such event
could have happened since the Glacial period, as was plain from the
well-preserved state of the three successive “roads” or lake-margins.
But the difficulty in believing that earth in any appreciable quantity
can be removed from a gently inclined surface, covered with vegetation
and matted with roots, is removed through the agency of worms. For the
many castings which are thrown up during rain, and those thrown up some
little time before heavy rain, flow for a short distance down an inclined
surface. Moreover much of the finest levigated earth is washed
completely away from the castings. During dry weather castings often
disintegrate into small rounded pellets, and these from their weight
often roll down any slope. This is more especially apt to occur when
they are started by the wind, and probably when started by the touch of
an animal, however small. We shall also see that a strong wind blows all
the castings, even on a level field, to leeward, whilst they are soft;
and in like manner the pellets when they are dry. If the wind blows in
nearly the direction of an inclined surface, the flowing down of the
castings is much aided.
The observations on which these several statements are founded must now
be given in some detail. Castings when first ejected are viscid and
soft; during rain, at which time worms apparently prefer to eject them,
they are still softer; so that I have sometimes thought that worms must
swallow much water at such times. However this may be, rain, even when
not very heavy, if long continued, renders recently-ejected castings
semi-fluid; and on level ground they then spread out into thin, circular,
flat discs, exactly as would so much honey or very soft mortar, with all
traces of their vermiform structure lost. This latter fact was sometimes
made evident, when a worm had subsequently bored through a flat circular
disc of this kind, and heaped up a fresh vermiform mass in the centre.
These flat subsided discs have been repeatedly seen by me after heavy
rain, in many places on land of all kinds.
_On the flowing of wet castings_, _and the rolling of dry disintegrated
castings down inclined surfaces_.—When castings are ejected on an
inclined surface during or shortly before heavy rain, they cannot fail to
flow a little down the slope. Thus, on some steep slopes in Knole Park,
which were covered with coarse grass and had apparently existed in this
state from time immemorial, I found (Oct. 22, 1872) after several wet
days that almost all the many castings were considerably elongated in the
line of the slope; and that they now consisted of smooth, only slightly
conical masses. Whenever the mouths of the burrows could be found from
which the earth had been ejected, there was more earth below than above
them. After some heavy storms of rain (Jan. 25, 1872) two rather steeply
inclined fields near Down, which had formerly been ploughed and were now
rather sparsely clothed with poor grass, were visited, and many castings
extended down the slopes for a length of 5 inches, which was twice or
thrice the usual diameter of the castings thrown up on the level parts of
these same fields. On some fine grassy slopes in Holwood Park, inclined
at angles between 8° and 11° 30′ with the horizon, where the surface
apparently had never been disturbed by the hand of man, castings abounded
in extraordinary numbers: and a space 16 inches in length transversely to
the slope and 6 inches in the line of the slope, was completely coated,
between the blades of grass, with a uniform sheet of confluent and
subsided castings. Here also in many places the castings had flowed down
the slope, and now formed smooth narrow patches of earth, 6, 7, and 7½
inches in length. Some of these consisted of two castings, one above the
other, which had become so completely confluent that they could hardly be
distinguished. On my lawn, clothed with very fine grass, most of the
castings are black, but some are yellowish from earth having been brought
up from a greater depth than usual, and the flowing-down of these yellow
castings after heavy rain, could be clearly seen where the slope was 5°;
and where it was less than 1° some evidence of their flowing down could
still be detected. On another occasion, after rain which was never
heavy, but which lasted for 18 hours, all the castings on this same
gently inclined lawn had lost their vermiform structure; and they had
flowed, so that fully two-thirds of the ejected earth lay below the
mouths of the burrows.
These observations led me to make others with more care. Eight castings
were found on my lawn, where the grass-blades are fine and close
together, and three others on a field with coarse grass. The
inclination of the surface at the eleven places where these castings
were collected varied between 4° 30′ and 17° 30′; the mean of the
eleven inclinations being 9° 26′. The length of the castings in the
direction of the slope was first measured with as much accuracy as
their irregularities would permit. It was found possible to make these
measurements within about ⅛ of an inch, but one of the castings was too
irregular to admit of measurement. The average length in the direction
of the slope of the remaining ten castings was 2.03 inches. The
castings were then divided with a knife into two parts along a
horizontal line passing through the mouth of the burrow, which was
discovered by slicing off the turf; and all the ejected earth was
separately collected, namely, the part above the hole and the part
below. Afterwards these two parts were weighed. In every case there was
much more earth below than above; the mean weight of that above being
103 grains, and of that below 205 grains; so that the latter was very
nearly double the former. As on level ground castings are commonly
thrown up almost equally round the mouths of the burrows, this
difference in weight indicates the amount of ejected earth which had
flowed down the slope. But very many more observations would be
requisite to arrive at any general result; for the nature of the
vegetation and other accidental circumstances, such as the heaviness of
the rain, the direction and force of the wind, &c., appear to be more
important in determining the quantity of the earth which flows down a
slope than its angle. Thus with four castings on my lawn (included in
the above eleven) where the mean slope was 7° 19′, the difference in
the amount of earth above and below the burrows was greater than with
three other castings on the same lawn where the mean slope was 12° 5′.
We may, however, take the above eleven cases, which are accurate as far
as they go, and calculate the weight of the ejected earth which annually
flows down a slope having a mean inclination of 9° 26′. This was done by
my son George. It has been shown that almost exactly two-thirds of the
ejected earth is found below the mouth of the burrow and one-third above
it. Now if the two-thirds which is below the hole be divided into two
equal parts, the upper half of this two-thirds exactly counterbalances
the one-third which is above the hole, so that as far as regards the
one-third above and the upper half of the two-thirds below, there is no
flow of earth down the hill-side. The earth constituting the lower half
of the two-thirds is, however, displaced through distances which are
different for every part of it, but which may be represented by the
distance between the middle point of the lower half of the two-thirds and
the hole. So that the average distance of displacement is a half of the
whole length of the worm-casting. Now the average length of ten out of
the above eleven castings was 2.03 inches, and half of this we may take
as being 1 inch. It may therefore be concluded that one-third of the
whole earth brought to the surface was in these cases carried down the
slope through 1 inch. {244}
It was shown in the third chapter that on Leith Hill Common, dry earth
weighing at least 7.453 lbs. was brought up by worms to the surface on a
square yard in the course of a year. If a square yard be drawn on a
hillside with two of its sides horizontal, then it is clear that only
1/36 part of the earth brought up on that square yard would be near
enough to its lower side to cross it, supposing the displacement of the
earth to be through one inch. But it appears that only ⅓ of the earth
brought up can be considered to flow downwards; hence ⅓ of 1/36 or 1/108
of 7.453 lbs. will cross the lower side of our square yard in a year.
Now 1/108 of 7.453 lbs. is 1.1 oz. Therefore 1.1 oz. of dry earth will
annually cross each linear yard running horizontally along a slope having
the above inclination; or very nearly 7 lbs. will annually cross a
horizontal line, 100 yards in length, on a hill-side having this
inclination.
A more accurate, though still very rough, calculation can be made of the
bulk of earth, which in its natural damp state annually flows down the
same slope over a yard-line drawn horizontally across it. From the
several cases given in the third chapter, it is known that the castings
annually brought to the surface on a square yard, if uniformly spread out
would form a layer 0.2 of an inch in thickness: it therefore follows by a
calculation similar to the one already given, that ⅓ of 0.2 × 36, or 2.4
cubic inches of damp earth will annually cross a horizontal line one yard
in length on a hillside with the above inclination. This bulk of damp
castings was found to weigh 1.85 oz. Therefore 11.56 lbs. of damp earth,
instead of 7 lbs. of dry earth as by the former calculation, would
annually cross a line 100 yards in length on our inclined surface.
In these calculations it has been assumed that the castings flow a short
distance downwards during the whole year, but this occurs only with those
ejected during or shortly before rain; so that the above results are thus
far exaggerated. On the other hand, during rain much of the finest earth
is washed to a considerable distance from the castings, even where the
slope is an extremely gentle one, and is thus wholly lost as far as the
above calculations are concerned. Castings ejected during dry weather
and which have set hard, lose in the same manner a considerable quantity
of fine earth. Dried castings, moreover, are apt to disintegrate into
little pellets, which often roll or are blown down any inclined surface.
Therefore the above result, namely, that 24 cubic inches of earth
(weighing 1.85 oz. whilst damp) annually crosses a yard-line of the
specified kind, is probably not much if at all exaggerated.
This amount is small; but we should bear in mind how many branching
valleys intersect most countries, the whole length of which must be very
great; and that earth is steadily travelling down both turf-covered sides
of each valley. For every 100 yards in length in a valley with sides
sloping as in the foregoing cases, 480 cubic inches of damp earth,
weighing above 23 pounds, will annually reach the bottom. Here a thick
bed of alluvium will accumulate, ready to be washed away in the course of
centuries, as the stream in the middle meanders from side to side.
If it could be shown that worms generally excavate their burrows at right
angles to an inclined surface, and this would be their shortest course
for bringing up earth from beneath, then as the old burrows collapsed
from the weight of the superincumbent soil, the collapsing would
inevitably cause the whole bed of vegetable mould to sink or slide slowly
down the inclined surface. But to ascertain the direction of many
burrows was found too difficult and troublesome. A straight piece of
wire was, however, pushed into twenty-five burrows on several sloping
fields, and in eight cases the burrows were nearly at right angles to the
slope; whilst in the remaining cases they were indifferently directed at
various angles, either upwards or downwards with respect to the slope.
In countries where the rain is very heavy, as in the tropics, the
castings appear, as might have been expected, to be washed down in a
greater degree than in England. Mr. Scott informs me that near Calcutta
the tall columnar castings (previously described), the diameter of which
is usually between 1 and 1½ inch, subside on a level surface, after heavy
rain, into almost circular, thin, flat discs, between 3 and 4 and
sometimes 5 inches in diameter. Three fresh castings, which had been
ejected in the Botanic Gardens “on a slightly inclined, grass-covered,
artificial bank of loamy clay,” were carefully measured, and had a mean
height of 2.17, and a mean diameter of 1.43 inches; these after heavy
rain, formed elongated patches of earth, with a mean length in the
direction of the slope of 5.83 inches. As the earth had spread very
little up the slope, a large part, judging from the original diameter of
these castings, must have flowed bodily downwards about 4 inches.
Moreover some of the finest earth of which they were composed must have
been washed completely away to a still greater distance. In drier sites
near Calcutta, a species of worm ejects its castings, not in vermiform
masses, but in little pellets of varying sizes: these are very numerous
in some places, and Mr. Scott says that they “are washed away by every
shower.”
I was led to believe that a considerable quantity of fine earth is washed
quite away from castings during rain, from the surfaces of old ones being
often studded with coarse particles. Accordingly a little fine
precipitated chalk, moistened with saliva or gum-water, so as to be
slightly viscid and of the same consistence as a fresh casting, was
placed on the summits of several castings and gently mixed with them.
These castings were then watered through a very fine rose, the drops from
which were closer together than those of rain, but not nearly so large as
those in a thunderstorm; nor did they strike the ground with nearly so
much force as drops during heavy rain. A casting thus treated subsided
with surprising slowness, owing as I suppose to its viscidity. It did
not flow bodily down the grass-covered surface of the lawn, which was
here inclined at an angle of 16° 20′; nevertheless many particles of the
chalk were found three inches below the casting. The experiment was
repeated on three other castings on different parts of the lawn, which
sloped at 2° 30′, 3° and 6°; and particles of chalk could be seen between
4 and 5 inches below the casting; and after the surface had become dry,
particles were found in two cases at a distance of 5 and 6 inches.
Several other castings with precipitated chalk placed on their summits
were left to the natural action of the rain. In one case, after rain
which was not heavy, the casting was longitudinally streaked with white.
In two other cases the surface of the ground was rendered somewhat white
for a distance of one inch from the casting; and some soil collected at a
distance of 2½ inches, where the slope was 7°, effervesced slightly when
placed in acid. After one or two weeks, the chalk was wholly or almost
wholly washed away from all the castings on which it had been placed, and
these had recovered their natural colour.
It may be here remarked that after very heavy rain shallow pools may be
seen on level or nearly level fields, where the soil is not very porous,
and the water in them is often slightly muddy; when such little pools
have dried, the leaves and blades of grass at their bottoms are generally
coated with a thin layer of mud. This mud I believe is derived in large
part from recently ejected castings.
Dr. King informs me that the majority of the before described gigantic
castings, which he found on a fully exposed, bare, gravelly knoll on the
Nilgiri Mountains in India, had been more or less weathered by the
previous north-east monsoon; and most of them presented a subsided
appearance. The worms here eject their castings only during the rainy
season; and at the time of Dr. King’s visit no rain had fallen for 110
days. He carefully examined the ground between the place where these
huge castings lay, and a little watercourse at the base of the knoll, and
nowhere was there any accumulation of fine earth, such as would
necessarily have been left by the disintegration of the castings if they
had not been wholly removed. He therefore has no hesitation in asserting
that the whole of these huge castings are annually washed during the two
monsoons (when about 100 inches of rain fall) into the little
water-course, and thence into the plains lying below at a depth of 3000
or 4000 feet.
Castings ejected before or during dry weather become hard, sometimes
surprisingly hard, from the particles of earth having been cemented
together by the intestinal secretions. Frost seems to be less effective
in their disintegration than might have been expected. Nevertheless they
readily disintegrate into small pellets, after being alternately
moistened with rain and again dried. Those which have flowed during rain
down a slope, disintegrate in the same manner. Such pellets often roll a
little down any sloping surface; their descent being sometimes much aided
by the wind. The whole bottom of a broad dry ditch in my grounds, where
there were very few fresh castings, was completely covered with these
pellets or disintegrated castings, which had rolled down the steep sides,
inclined at an angle of 27°.
Near Nice, in places where the great cylindrical castings, previously
described, abound, the soil consists of very fine arenaceo-calcareous
loam; and Dr. King informs me that these castings are extremely liable to
crumble during dry weather into small fragments, which are soon acted on
by rain, and then sink down so as to be no longer distinguishable from
the surrounding soil. He sent me a mass of such disintegrated castings,
collected on the top of a bank, where none could have rolled down from
above. They must have been ejected within the previous five or six
months, but they now consisted of more or less rounded fragments of all
sizes, from ¾ of an inch in diameter to minute grains and mere dust. Dr.
King witnessed the crumbling process whilst drying some perfect castings,
which he afterwards sent me. Mr. Scott also remarks on the crumbling of
the castings near Calcutta and on the mountains of Sikkim during the hot
and dry season.
When the castings near Nice had been ejected on an inclined surface, the
disintegrated fragments rolled downwards, without losing their
distinctive shape; and in some places could “be collected in basketfuls.”
Dr. King observed a striking instance of this fact on the Corniche road,
where a drain, about 2½ feet wide and 9 inches deep, had been made to
catch the surface drainage from the adjoining hill-side. The bottom of
this drain was covered for a distance of several hundred yards, to a
depth of from 1½ to 3 inches, by a layer of broken castings, still
retaining their characteristic shape. Nearly all these innumerable
fragments had rolled down from above, for extremely few castings had been
ejected in the drain itself. The hill-side was steep, but varied much in
inclination, which Dr. King estimated at from 30° to 60° with the
horizon. He climbed up the slope, and “found every here and there little
embankments, formed by fragments of the castings that had been arrested
in their downward progress by irregularities of the surface, by stones,
twigs, &c. One little group of plants of _Anemone hortensis_ had acted
in this manner, and quite a small bank of soil had collected round it.
Much of this soil had crumbled down, but a great deal of it still
retained the form of castings.” Dr. King dug up this plant, and was
struck with the thickness of the soil which must have recently
accumulated over the crown of the rhizoma, as shown by the length of the
bleached petioles, in comparison with those of other plants of the same
kind, where there had been no such accumulation. The earth thus
accumulated had no doubt been secured (as I have everywhere seen) by the
smaller roots of the plants. After describing this and other analogous
cases, Dr. King concludes: “I can have no doubt that worms help greatly
in the process of denudation.”
_Ledges of earth on steep hill-sides_.—Little horizontal ledges, one
above another, have been observed on steep grassy slopes in many parts of
the world. The formation has been attributed to animals travelling
repeatedly along the slope in the same horizontal lines while grazing,
and that they do thus move and use the ledges is certain; but Professor
Henslow (a most careful observer) told Sir J. Hooker that he was
convinced that this was not the sole cause of their formation. Sir J.
Hooker saw such ledges on the Himalayan and Atlas ranges, where there
were no domesticated animals and not many wild ones; but these latter
would, it is probable, use the ledges at night while grazing like our
domesticated animals. A friend observed for me the ledges on the Alps of
Switzerland, and states that they ran at 3 or 4 ft. one above the other,
and were about a foot in breadth. They had been deeply pitted by the
feet of grazing cows. Similar ledges were observed by the same friend on
our Chalk downs, and on an old talus of chalk-fragments (thrown out of a
quarry) which had become clothed with turf.
My son Francis examined a Chalk escarpment near Lewes; and here on a part
which was very steep, sloping at 40° with the horizon, about 30 flat
ledges extended horizontally for more than 100 yards, at an average
distance of about 20 inches, one beneath the other. They were from 9 to
10 inches in breadth. When viewed from a distance they presented a
striking appearance, owing to their parallelism; but when examined
closely, they were seen to be somewhat sinuous, and one often ran into
another, giving the appearance of the ledge having forked into two. They
are formed of light-coloured earth, which on the outside, where thickest,
was in one case 9 inches, and in another case between 6 and 7 inches in
thickness. Above the ledges, the thickness of the earth over the chalk
was in the former case 4 and in the latter only 3 inches. The grass grew
more vigorously on the outer edges of the ledges than on any other part
of the slope, and here formed a tufted fringe. Their middle part was
bare, but whether this had been caused by the trampling of sheep, which
sometimes frequent the ledges, my son could not ascertain. Nor could he
feel sure how much of the earth on the middle and bare parts, consisted
of disintegrated worm-castings which had rolled down from above; but he
felt convinced that some had thus originated; and it was manifest that
the ledges with their grass-fringed edges would arrest any small object
rolling down from above.
At one end or side of the bank bearing these ledges, the surface
consisted in parts of bare chalk, and here the ledges were very
irregular. At the other end of the bank, the slope suddenly became less
steep, and here the ledges ceased rather abruptly; but little embankments
only a foot or two in length were still present. The slope became
steeper lower down the hill, and the regular ledges then reappeared.
Another of my sons observed, on the inland side of Beachy Head, where the
surface sloped at about 25°, many short little embankments like those
just mentioned. They extended horizontally and were from a few inches to
two or three feet in length. They supported tufts of grass growing
vigorously. The average thickness of the mould of which they were
formed, taken from nine measurements, was 4.5 inches; while that of the
mould above and beneath them was on an average only 3.2 inches, and on
each side, on the same level, 3.1 inches. On the upper parts of the
slope, these embankments showed no signs of having been trampled on by
sheep, but in the lower parts such signs were fairly plain. No long
continuous ledges had here been formed.
If the little embankments above the Corniche road, which Dr. King saw in
the act of formation by the accumulation of disintegrated and rolled
worm-castings, were to become confluent along horizontal lines, ledges
would be formed. Each embankment would tend to extend laterally by the
lateral extension of the arrested castings; and animals grazing on a
steep slope would almost certainly make use of every prominence at nearly
the same level, and would indent the turf between them; and such
intermediate indentations would again arrest the castings. An irregular
ledge when once formed would also tend to become more regular and
horizontal by some of the castings rolling laterally from the higher to
the lower parts, which would thus be raised. Any projection beneath a
ledge would not afterwards receive disintegrated matter from above, and
would tend to be obliterated by rain and other atmospheric agencies.
There is some analogy between the formation, as here supposed, of these
ledges, and that of the ripples of wind-drifted sand as described by
Lyell. {259}
The steep, grass-covered sides of a mountainous valley in Westmoreland,
called Grisedale, was marked in many places with innumerable lines of
miniature cliffs, with almost horizontal, little ledges at their bases.
Their formation was in no way connected with the action of worms, for
castings could not anywhere be seen (and their absence is an inexplicable
fact), although the turf lay in many places over a considerable thickness
of boulder-clay and moraine rubbish. Nor, as far as I could judge, was
the formation of these little cliffs at all closely connected with the
trampling of cows or sheep. It appeared as if the whole superficial,
somewhat argillaceous earth, while partially held together by the roots
of the grasses, had slided a little way down the mountain sides; and in
thus sliding, had yielded and cracked in horizontal lines, transversely
to the slope.
_Castings blown to leeward by the wind_.—We have seen that moist castings
flow, and that disintegrated castings roll down any inclined surface; and
we shall now see that castings, recently ejected on level grass-covered
surfaces, are blown during gales of wind accompanied by rain to leeward.
This has been observed by me many times on many fields during several
successive years. After such gales, the castings present a gently
inclined and smooth, or sometimes furrowed, surface to windward, while
they are steeply inclined or precipitous to leeward, so that they
resemble on a miniature scale glacier-ground hillocks of rock. They are
often cavernous on the leeward side, from the upper part having curled
over the lower part. During one unusually heavy south-west gale with
torrents of rain, many castings were wholly blown to leeward, so that the
mouths of the burrows were left naked and exposed on the windward side.
Recent castings naturally flow down an inclined surface, but on a grassy
field, which sloped between 10° and 15°, several were found after a heavy
gale blown up the slope. This likewise occurred on another occasion on a
part of my lawn where the slope was somewhat less. On a third occasion,
the castings on the steep, grass-covered sides of a valley, down which a
gale had blown, were directed obliquely instead of straight down the
slope; and this was obviously due to the combined action of the wind and
gravity. Four castings on my lawn, where the downward inclination was 0°
45′, 1°, 3° and 3° 30′ (mean 2° 45′) towards the north-east, after a
heavy south-west gale with rain, were divided across the mouths of the
burrows and weighed in the manner formerly described. The mean weight of
the earth below the mouths of burrows and to leeward, was to that above
the mouths and on the windward side as 2¾ to 1; whereas we have seen that
with several castings which had flowed down slopes having a mean
inclination of 9° 26′, and with three castings where the inclination was
above 12°; the proportional weight of the earth below to that above the
burrows was as only 2 to 1. These several cases show how efficiently
gales of wind accompanied by rain act in displacing recently ejected
castings. We may therefore conclude that even a moderately strong wind
will produce some slight effect on them.
Dry and indurated castings, after their disintegration into small
fragments or pellets, are sometimes, probably often, blown by a strong
wind to leeward. This was observed on four occasions, but I did not
sufficiently attend to this point. One old casting on a gently sloping
bank was blown quite away by a strong south-west wind. Dr. King believes
that the wind removes the greater part of the old crumbling castings near
Nice. Several old castings on my lawn were marked with pins and
protected from any disturbance. They were examined after an interval of
10 weeks, during which time the weather had been alternately dry and
rainy. Some, which were of a yellowish colour had been washed almost
completely away, as could be seen by the colour of the surrounding
ground. Others had completely disappeared, and these no doubt had been
blown away. Lastly, others still remained and would long remain, as
blades of grass had grown through them. On poor pasture-land, which has
never been rolled and has not been much trampled on by animals, the whole
surface is sometimes dotted with little pimples, through and on which
grass grows; and these pimples consist of old worm-castings.
In all the many observed cases of soft castings blown to leeward, this
had been effected by strong winds accompanied by rain. As such winds in
England generally blow from the south and south-west, earth must on the
whole tend to travel over our fields in a north and north-east direction.
This fact is interesting, because it might be thought that none could be
removed from a level, grass-covered surface by any means. In thick and
level woods, protected from the wind, castings will never be removed as
long as the wood lasts; and mould will here tend to accumulate to the
depth at which worms can work. I tried to procure evidence as to how
much mould is blown, whilst in the state of castings, by our wet southern
gales to the north-east, over open and flat land, by looking to the level
of the surface on opposite sides of old trees and hedge-rows; but I
failed owing to the unequal growth of the roots of trees and to most
pasture-land having been formerly ploughed.
On an open plain near Stonehenge, there exist shallow circular trenches,
with a low embankment outside, surrounding level spaces 50 yards in
diameter. These rings appear very ancient, and are believed to be
contemporaneous with the Druidical stones. Castings ejected within these
circular spaces, if blown to the north-east by south-west winds would
form a layer of mould within the trench, thicker on the north-eastern
than on any other side. But the site was not favourable for the action
of worms, for the mould over the surrounding Chalk formation with flints,
was only 3.37 inches in thickness, from a mean of six observations made
at a distance of 10 yards outside the embankment. The thickness of the
mould within two of the circular trenches was measured every 5 yards all
round, on the inner sides near the bottom. My son Horace protracted
these measurements on paper; and though the curved line representing the
thickness of the mould was extremely irregular, yet in both diagrams it
could be seen to be thicker on the north-eastern side than elsewhere.
When a mean of all the measurements in both the trenches was laid down
and the line smoothed, it was obvious that the mould was thickest in the
quarter of the circle between north-west and north-east; and thinnest in
the quarter between south-east and south-west, especially at this latter
point. Besides the foregoing measurements, six others were taken near
together in one of the circular trenches, on the north-east side; and the
mould here had a mean thickness of 2.29 inches; while the mean of six
other measurements on the south-west side was only 1.46 inches. These
observations indicate that the castings had been blown by the south-west
winds from the circular enclosed space into the trench on the north-east
side; but many more measurements in other analogous cases would be
requisite for a trustworthy result.
The amount of fine earth brought to the surface under the form of
castings, and afterwards transported by the winds accompanied by rain, or
that which flows and rolls down an inclined surface, no doubt is small in
the course of a few scores of years; for otherwise all the inequalities
in our pasture fields would be smoothed within a much shorter period than
appears to be the case. But the amount which is thus transported in the
course of thousands of years cannot fail to be considerable and deserves
attention. É. de Beaumont looks at the vegetable mould which everywhere
covers the land as a fixed line, from which the amount of denudation may
be measured. {265} He ignores the continued formation of fresh mould by
the disintegration of the underlying rocks and fragments of rock; and it
is curious to find how much more philosophical were the views maintained
long ago, by Playfair, who, in 1802, wrote, “In the permanence of a coat
of vegetable mould on the surface of the earth, we have a demonstrative
proof of the continued destruction of the rocks.” {266}
_Ancient encampments and tumuli_.—É. de Beaumont adduces the present
state of many ancient encampments and tumuli and of old ploughed fields,
as evidence that the surface of the land undergoes hardly any
degradation. But it does not appear that he ever examined the thickness
of the mould over different parts of such old remains. He relies chiefly
on indirect, but apparently trustworthy, evidence that the slopes of the
old embankments are the same as they originally were; and it is obvious
that he could know nothing about their original heights. In Knole Park a
mound had been thrown up behind the rifle-targets, which appeared to have
been formed of earth originally supported by square blocks of turf. The
sides sloped, as nearly as I could estimate them, at an angle of 45° or
50° with the horizon, and they were covered, especially on the northern
side, with long coarse grass, beneath which many worm-castings were
found. These had flowed bodily downwards, and others had rolled down as
pellets. Hence it is certain that as long as a mound of this kind is
tenanted by worms, its height will be continually lowered. The fine
earth which flows or rolls down the sides of such a mound accumulates at
its base in the form of a talus. A bed, even a very thin bed, of fine
earth is eminently favourable for worms; so that a greater number of
castings would tend to be ejected on a talus thus formed than elsewhere;
and these would be partially washed away by every heavy shower and be
spread over the adjoining level ground. The final result would be the
lowering of the whole mound, whilst the inclination of the sides would
not be greatly lessened. The same result would assuredly follow with
ancient embankments and tumuli; except where they had been formed of
gravel or of nearly pure sand, as such matter is unfavourable for worms.
Many old fortifications and tumuli are believed to be at least 2000 years
old; and we should bear in mind that in many places about one inch of
mould is brought to the surface in 5 years or two inches in 10 years.
Therefore in so long a period as 2000 years, a large amount of earth will
have been repeatedly brought to the surface on most old embankments and
tumuli, especially on the talus round their bases, and much of this earth
will have been washed completely away. We may therefore conclude that
all ancient mounds, when not formed of materials unfavourable to worms,
will have been somewhat lowered in the course of centuries, although
their inclinations may not have been greatly changed.
_Fields formerly ploughed_.—From a very remote period and in many
countries, land has been ploughed, so that convex beds, called crowns or
ridges, usually about 8 feet across and separated by furrows, have been
thrown up. The furrows are directed so as to carry off the surface
water. In my attempts to ascertain how long a time these crowns and
furrows last, when ploughed land has been converted into pasture,
obstacles of many kinds were encountered. It is rarely known when a
field was last ploughed; and some fields which were thought to have been
in pasture from time immemorial were afterwards discovered to have been
ploughed only 50 or 60 years before. During the early part of the
present century, when the price of corn was very high, land of all kinds
seems to have been ploughed in Britain. There is, however, no reason to
doubt that in many cases the old crowns and furrows have been preserved
from a very ancient period. {269} That they should have been preserved
for very unequal lengths of time would naturally follow from the crowns,
when first thrown up, having differed much in height in different
districts, as is now the case with recently ploughed land.
In old pasture fields, the mould, wherever measurements were made, was
found to be from ½ to 2 inches thicker in the furrows than on the crowns;
but this would naturally follow from the finer earth having been washed
from the crowns into the furrows before the land was well clothed with
turf; and it is impossible to tell what part worms may have played in the
work. Nevertheless from what we have seen, castings would certainly tend
to flow and to be washed during heavy rain from the crowns into the
furrows. But as soon as a bed of fine earth had by any means been
accumulated in the furrows, it would be more favourable for worms than
the other parts, and a greater number of castings would be thrown up here
than elsewhere; and as the furrows on sloping land are usually directed
so as to carry off the surface water, some of the finest earth would be
washed from the castings which had been here ejected and be carried
completely away. The result would be that the furrows would be filled up
very slowly, while the crowns would be lowered perhaps still more slowly
by the flowing and rolling of the castings down their gentle inclinations
into the furrows.
Nevertheless it might be expected that old furrows, especially those on a
sloping surface, would in the course of time be filled up and disappear.
Some careful observers, however, who examined fields for me in
Gloucestershire and Staffordshire could not detect any difference in the
state of the furrows in the upper and lower parts of sloping fields,
supposed to have been long in pasture; and they came to the conclusion
that the crowns and furrows would last for an almost endless number of
centuries. On the other hand the process of obliteration seems to have
commenced in some places. Thus in a grass field in North Wales, known to
have been ploughed about 65 years ago, which sloped at an angle of 15° to
the north-east, the depth of the furrows (only 7 feet apart) was
carefully measured, and was found to be about 4½ inches in the upper part
of the slope, and only 1 inch near the base, where they could be traced
with difficulty. On another field sloping at about the same angle to the
south-west, the furrows were scarcely perceptible in the lower part;
although these same furrows when followed on to some adjoining level
ground were from 2½ to 3½ inches in depth. A third and closely similar
case was observed. In a fourth case, the mould in a furrow in the upper
part of a sloping field was 2½ inches, and in the lower part 4½ inches in
thickness.
On the Chalk Downs at about a mile distance from Stonehenge, my son
William examined a grass-covered, furrowed surface, sloping at from 8° to
10 °, which an old shepherd said had not been ploughed within the memory
of man. The depth of one furrow was measured at 16 points in a length of
68 paces, and was found to be deeper where the slope was greatest and
where less earth would naturally tend to accumulate, and at the base it
almost disappeared. The thickness of the mould in this furrow in the
upper part was 2½ inches, which increased to 5 inches, a little above the
steepest part of the slope; and at the base, in the middle of the narrow
valley, at a point which the furrow if continued would have struck, it
amounted to 7 inches. On the opposite side of the valley, there were
very faint, almost obliterated, traces of furrows. Another analogous but
not so decided a case was observed at a few miles’ distance from
Stonehenge. On the whole it appears that the crowns and furrows on land
formerly ploughed, but now covered with grass, tend slowly to disappear
when the surface is inclined; and this is probably in large part due to
the action of worms; but that the crowns and furrows last for a very long
time when the surface is nearly level.
_Formation and amount of mould over the Chalk Formation_.—Worm-castings
are often ejected in extraordinary numbers on steep, grass-covered
slopes, where the Chalk comes close to the surface, as my son William
observed near Winchester and elsewhere. If such castings are largely
washed away during heavy rains, it is difficult to understand at first
how any mould can still remain on our Downs, as there does not appear any
evident means for supplying the loss. There is, moreover, another cause
of loss, namely, in the percolation of the finer particles of earth into
the fissures in the chalk and into the chalk itself. These
considerations led me to doubt for a time whether I had not exaggerated
the amount of fine earth which flows or rolls down grass-covered slopes
under the form of castings; and I sought for additional information. In
some places, the castings on Chalk Downs consist largely of calcareous
matter, and here the supply is of course unlimited. But in other places,
for instance on a part of Teg Down near Winchester, the castings were all
black and did not effervesce with acids. The mould over the chalk was
here only from 3 to 4 inches in thickness. So again on the plain near
Stonehenge, the mould, apparently free from calcareous matter, averaged
rather less than 3½ inches in thickness. Why worms should penetrate and
bring up chalk in some places and not in others I do not know.
In many districts where the land is nearly level, a bed several feet in
thickness of red clay full of unworn flints overlies the Upper Chalk.
This overlying matter, the surface of which has been converted into
mould, consists of the undissolved residue from the chalk. It may be
well here to recall the case of the fragments of chalk buried beneath
worm-castings on one of my fields, the angles of which were so completely
rounded in the course of 29 years that the fragments now resembled
water-worn pebbles. This must have been effected by the carbonic acid in
the rain and in the ground, by the humus-acids, and by the corroding
power of living roots. Why a thick mass of residue has not been left on
the Chalk, wherever the land is nearly level, may perhaps be accounted
for by the percolation of the fine particles into the fissures, which are
often present in the chalk and are either open or are filled up with
impure chalk, or into the solid chalk itself. That such percolation
occurs can hardly be doubted. My son collected some powdered and
fragmentary chalk beneath the turf near Winchester; the former was found
by Colonel Parsons, R. E., to contain 10 per cent., and the fragments 8
per cent. of earthy matter. On the flanks of the escarpment near Abinger
in Surrey, some chalk close beneath a layer of flints, 2 inches in
thickness and covered by 8 inches of mould, yielded a residue of 3.7 per
cent. of earthy matter. On the other hand the Upper Chalk properly
contains, as I was informed by the late David Forbes who had made many
analyses, only from 1 to 2 per cent. of earthy matter; and two samples
from pits near my house contained 1.3 and 0.6 per cent. I mention these
latter cases because, from the thickness of the overlying bed of red clay
with flints, I had imagined that the underlying chalk might here be less
pure than elsewhere. The cause of the residue accumulating more in some
places than in others, may be attributed to a layer of argillaceous
matter having been left at an early period on the chalk, and this would
check the subsequent percolation of earthy matter into it.
From the facts now given we may conclude that castings ejected on our
Chalk Downs suffer some loss by the percolation of their finer matter
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 ¼ 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 2½ to 3½ 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 9½ inches. On the embankment itself the mould was only 1 to 1½ inch
in thickness; and within the ditch at the bottom it varied from 2½ to 3½,
but was in one spot 6 inches in thickness. On the north-west side of the
hill, either no embankment had ever been thrown up above the ditch, or it
had subsequently been removed; so that here there was nothing to prevent
worm-castings, earth and stones being washed into the ditch, at the
bottom of which the mould formed a layer from 11 to 22 inches in
thickness. It should however be stated that here and on other parts of
the slope, the bed of mould often contained fragments of chalk and flint
which had obviously rolled down at different times from above. The
interstices in the underlying fragmentary chalk were also filled up with
mould.
My son examined the surface of this hill to its base in a south-west
direction. Beneath the great ditch, where the slope was about 24°, the
mould was very thin, namely, from 1½ to 2½ inches; whilst near the base,
where the slope was only 3° to 4°, it increased to between 8 and 9 inches
in thickness. We may therefore conclude that on this artificially
modified hill, as well as in the natural valleys of the neighbouring
Chalk Downs, some fine earth, probably derived in large part from
worm-castings, is washed down, and accumulates in the lower parts,
notwithstanding the percolation of an unknown quantity into the
underlying chalk; a supply of fresh earthy matter being afforded by the
dissolution of the chalk through atmospheric and other agencies.