[From Lamplugh, Geology of IoM, 1903]

CHAPTER XIV.

AGRICULTURAL GEOLOGY AND WATER SUPPLY.

Soils, etc.

The glacial deposits are the chief factors in determining the character of the soil in the cultivated portions of the Island, as the principal tracts of solid rock occur on the mountains above the limits of cultivation. The northern plain, which constitutes the largest area of fertile land, exhibits the variability commonly found wherever drift has accumulated thickly, ranging from still clay to light sand and gravel, with peaty patches in the hollows. In this case, however, the boulder-clay rarely forms the stiffest soil, having a sufficient admixture of sand to work into loam at the surface; and the beds of laminated clay belonging to the stratified part of the drift make the heavier land. The difficulty of indicating the kind of soil under these conditions by the conventional methods of drift-mapping has been discussed in Chapter XI. (p. 430), where information will be found as to the localities where the map might seem misleading. The practice of ‘marling’the lighter tracts was at one time universal in this area, but is now practically abandoned except on the blown-Lands along the southern edge of The Ayre. As already described, the ‘ marl ‘usually consisted of boulder-clay or stratified glacial clay (p. 418), but occasionally of late-glacial or postglacial fresh-water deposits (pp. 378—81); both contain some lime (pp. 464—5), but, as Cumming remarked, the benefit to the land was probably more on account of improved texture and general renovation than from this ingredient alone.¹ Berger mentions that "150 tons of marl are computed to be necessary for an acre of land," but this was probably an extreme quantity; he adds that the cost of the operation was £6 per acre if the cartage did not exceed a mile, and that "the cost of liming, a practice chiefly used in the southern part of the isle, is nearly the same," 90 bushels of lime to the acre being allowed,² Seaweed is still extensively used for manure in parts adjacent to the coast, and in this way, as well as by ‘marling,’ many pebbles etc. have been artificially introduced into the soil,—a fact to be borne in mind by the glacial geologist.

Next in importance to the northern plain as an agricultural tract is the southern lowland, overlying the Carboniferous rocks and extending beyond them up the gentle slopes of slate ,to 600 feet or more above sea-level. The lower ground in this tract closely resembles that of the northern plain, excepting that sand-and-gravel soils are less prevalent and tend to be more loamy, and the boulder-clay soils are darker, tougher and more calcareous. On the upland slopes the drift is mainly derived from the local slates and, though mapped as ‘boulder-clay,’ gives rise to soil which is not, as a rule, .stiff, but usually a deep stony loam full of subangular fragments of slate, with patches of thinner rubbly soil where bosses of the solid rock approach the surface. This kind of land is predominant all over the hilly tracts wherever agriculture is attempted. Where the soil consists entirely of loose slaty debris, derived either from the subjacent decomposed rock-surface or from slaty drift of the gravelly type, it is termed by the Manx farmers "shilly" land; this produces better crops than its extremely stony appearance would lead one to expect, especially of oats, turnips, and potatoes; but it loses moisture rapidly, and suffers in a dry season. The patches of cold wet clay-land, difficult to drain and difficult to cultivate, occurring on flats and depressions both on the uplands and at the foot of the hills,—the result of the rain-wash and weathering of the drift—have been described in a previous chapter (p.- 453); such ‘Colby wash’ soil has usually a dark grey colour instead of the rusty brown of the mellower slaty land, and is generally full of bits of vein-quartz which have remained unaltered while the slate fragments have decomposed into mud.

On the eastern flank of the Island there is more or less cultivation on the hill-slopes from end to end, but on the western side the cultivated strip extends only as far south as Dalby. In the interior, south of the central valley the greater part of the drift-covered area is or has been tilled, but among the hills north of that valley a much smaller proportion is cultivated, and the area devoted to upland sheep-walks is constantly increasing at the expense of tillage. Macculloch as long ago as 1819 pointed out that the cultivation of elevated ground had in some places extended "further than prudence would have dictated or profit will ultimately justify ";‘ and his dictum has been fully borne out, the uplands being everywhere dotted with the ruins of deserted cottages and small farmsteads; the little fields surrounding them, omice under the plough, now producing only rough pasturage. This diminution in the proportion of arable land, as marked in the Island as in Great Britain and Ireland, is illustrated in the following statistics.² These figures also show to what an insignificant percentage the growth of wheat has shrunk, the grain crops now consisting almost entirely of oats and barley, with turnips and potatoes as the principal root crops.’

1 "Isle of Man," p. 306.

2 Trans. Geol. Soc., vol. ii., p~ 55,

3 "Western Isles," vol. ii., p. 519.

2 From the annual Parliamentary Blue books: "Agricultural Returns." In the Return for 1899, the quantity of "Mountain and Heath land used for grazing" (not included in the Table) is given as 23,110 acres, and "Woods and Plantations "as 826 acres.

1 Berger quotes (op. cit., p. 32) the following figures from J. C. Curwen’s "Report of the Agricultural Society in the Isle of Man" (Workington, 1810)

"Acres.

100,400 of mountain.
69,045 for grazing.
30,158 in oats.
15,079 under barley.
14,761 under green crops, 710 of which may be considered as potatoes.
9,047 in wheat.
7,~70 in roads, rivers, houses.

Total 245,760 acres."

But as this total greatly exceeds the actual acreage of the Island, the table is not of much use for comparison.

Water Supply.

The water supply of the Island is practically all obtained from the surface-streams or from shallow wells, there being no available deep source. The slate-rocks are waterless, except for a slight percolation along the lodes (see description of mines, pp. ~32-3, etc.) and along joints near the surface in the quartz-veined grits; consequently there are scarcely any true rock-springs, though water oozes out abundantly from the base of the rubbly local drift and shaken rock on the hill-slopes and in the valleys. The Carboniferous Limestone of the south is too thinly bedded and intercalated with shale to afford a good supply, and few wells have been sunk in it (pp. 207, 471); some deep quarries in this rock W. of Ballasalla are unwatered by wind and steam power, the discharge being of good quality, and estimated to average about twenty-six million gallons per annum²

All the towns possess organised water-systems supplied from reservoirs near the heads of the streams, Douglas thus utilising the Glass and the Groudle Rivers; Castletown, the Silverburn; Port St. Mary and Port Erin, the Colby River and a stream E. of Flesh-wick; Peel, a stream from the Slieau Whuallian ridge and from Glen Rushen; and Ramsey, the head of Ballure Glen. The country districts depend either upon the streams or upon small shallow wells, often very inadequately protected from contamination; the latter, on the low ground, are generally in water-logged beds of drift not far below the surface; and in the hilly districts are usually little basins excavated in the slate in a position to catch the percolation between the solid rock and the overlying rubble or drift. The conditions of supply on The Ayre and other parts of the northern plain have previously been discussed (pp. 285 & 417), and particulars of a few wells in other districts are given in Chap. XI.

2 We are indebted to T. Moore, Esq., for this information,