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 conclusionthat 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 degrees to the north-east, the depth of the furrows (only 7 feet apart) was carefully measured, and was found to be about 4.5 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 2.5 to 3.5 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 2.5 inches, and in the lower part 4.5 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 from8 degrees to 10 degrees, 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 2.5 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.