On the Rock-basins in the Granite of the Dartmoor District, Devonshire

In this Memoir the origin of Rock-basins in the Granite of Dartmoor and its vicinity is alone considered; and it is not attempted to draw therefrom any law as to the manner of the formation of similar basins elsewhere. It has been here attempted to notice all the hollows known as Rock-basins in this district; and in the table appended, particulars of these and of the chief Tors are contained. A few localities it has not been in the power of the Author to visit; and the description is in those cases taken from the observations of others: doubtless also some basins have escaped notice, but a considerable number have been examined, and the particulars noted on the same system, so as to give an opportunity of comparison. The situation of Dartmoor is too well known to need description. The geological formation is entirely of granitic rocks. From near the same point in the northerly part of the moor the Tavy nows westerly and the Teign easterly: to the north of these rivers very few rock-basins exist, and those too are in the immediate vicinity of these rivers. To the south of a line formed by the Plymouth and Dartmoor railway and the River Dart, it is believed rock-basins have not been found; but this last district has not been completely examined by the Author. The part of Dartmoor, therefore, in which the rock-basins exist is a central belt, which occupies about one-third of the area of the moor. The particulars of the various basins are set out in a tabular form; it will therefore be only needful to notice some of the most important. The only basin to the north of the Tavy with which the Author is at Hare Tor. To the south of the Tavy Fur Tor is the highest point of Dartmoor, being 2000 feet above sea-level. The rocks composing this Tor are scattered over a considerable area, are much weathered, for the most part broken and low, and forming ledges, but occasionally rising into large masses. The granite is generally porphyritic, being coarse-grained with large felspar, and has a laminated structure. On this Tor there are three basins. Between the Tavy and Wallcombe River, Roose or Roll's Tor, Staple Tor, Vixen Tor, and Pew Tor are worthy of notice: the two last mentioned are particularly described in Bray's Tamar and Tavy. Between Wallcombe River and the East Webber, Mistor is the most conspicuous and known Tor; it is situate on an insulated hill 1760 feet above sea-level. The Tor conssits of long, narrow cliffs, which rise on the eastern and western sides of the summit of the hill (that on the eastern side being the most lofty and extensive), ranging nearly from north to south. The space between these is occupied by detached masses varying in size. The granite is laminate and much weathered at the junctions of the beds and of the perpendicular planes, and there are occassinally overhanging beds. “Mistor Pan” is the only basin on this Tor; it is situate near the northerly end of the eastern range on the summit. This basin has been supposed to be artificially formed, and is described in Rowe's Dartmoor and Bray's Tamar and Tavy. The sides are nearly perpendicular, leaning slightly back. At the south-easterly side the highest bed of rock projects slightly, and under this a hole reaches through to the eastern side of the Tor about two inches above the bottom of the pan. The only other places where similar perforations have been observed are at Fur Tor and Willistone Rocks. From the northerly side, a lip or channel runs for about five inches in a northerly direction, and then irregularly; at the basin it is about five inches wide, and there reaches nearly to the bottom of the basin. The diameter from N. to S. is 36 inches, from E. to W. 35 inches, and the longest diameter from N. to S. The depth (at the lip) on the north side is 4 inches, on the easterly 6 inches, on the southerly 5 inches, and on the westerly 6 inches. The bottom is nearly level, but slopes slightly towards the centre, where it is about half an inch deeper than at the sides. The bottom was covered with smal sharp gravel, consisting chiefly of felspar and quartz, formed by the disintegration of the granite. With the exception of the perforation, and that a lip is not of very frequent occurrence, the above description is also applicable, the dimensions being altered, to nearly every rock-basin with a flat bottom; and there seems to be no reason for considering the origin of this basin different from that of the other basins. On Bell Tor, to the east of the East Webber, there is a large basin that well exhibits the atmospheric action on the granite. Hounter Tor in the same locality is one the grandest and most picturesque Tors of the Dartmoor district. No rock-basins have been observed there, or at the rocks by Bowerman's Nose or Manaton Tors. The remaining basins are mostly classed with those lying within the watershed of the Teign. At a rapid, a short distance below the bridge at Teignhead, “pot-holes” are in the course of formation: these will be noticed hereafter. About a mile and a half lower down on the left bank of the riger is the well-know “Tolmen” or Holed Stone: the perforation is about three feet in diameter, and 2 feet 8 inches in depth, and passes obliquely through the stone. This rock is not in situ , and rests on other transported blocks at a rapid where the Teign has worked its way between the rocks, causing a gradual lowering of the level of its bed; so that the surface of the Tolmen is now only covered by the water in very heavy floods. There are portions of “pot-holes” on this rock; and the origin may, with little if any hesitation, be ascribed to the same cause as that which is now forming the “pot-holes” just mentioned. About three-quarters of a mile to the south of this spot is Castor or Kestor Rock, a large insulated Tor about 1417 feet above sea-level. The summit of this Tor consists of three bosses, rising slightly above a level central platform; the rock is weathered and rounded; and the beds, with the exception of a bed of elvan, are not of fine or compact granite. The directions of the perpendicular joints are W. by S. to E. by N., N. by W. to S. by E., and N.W. by N. to S.E. by S. On this Tor, besides some hollows which have not been regarded as “basins,” there are four basins on the northerly boss, three on the central platform, and two on the southerly boss; and of these the particulars are given in the Table. One large basin on the central platform will require description. This basin was discovered by the Author on 17th Sept., 1856. The opening at the top, as marked by the line to which water stands when it is full, is oval, measuring 8 feet from N. to S. and 6 feet 8 inches from E. to W. A few inches below this level the basin is nearly circular; the diameter half way down is 50 inches; at the bottom 24 inches: the perpendicular depth is 31 inches. The bottom is flat, rounding up at the edges to the sides, which curve outwards in the form of the mouth of a trumpet, and two identations, caused by decay, run round the basin. The bottom and sides up to the lowest identation (12 inches) were covered with dead Sphagnum, which formed a compact mass, adhering so closely as to have taken a cast of the granite; between these there were not any stones, nor the small fragments of quartz and felspar usually found in rock-basins. At the bottom of the basin, above the Sphagnum, there was a thin layer of fine black peaty soil, and the remaining contents—evidently of a later date—consisted of, first, a compact mass of fine dark sand and gravel 6 inches in thickness, and above this is a brown peaty soil containing fragments of quartz and felspar, which filled the basins to the level, and the whole was covered over by a thick growth of heath. This arrangement is such as would arise from a maceration of peat exposed to the wet and storms that occur at this spot. Attention having been attracted by the opening of thsi basin, the Author has received information derived from an old “Moorman,” which leads him to suppose that the basin was filled with peat between 100 and 150 years ago to prevent accidents to sheep. This basin has not been empty of water since it was opened in September, 1856. A short distance below the junction of the North and South Teign Rivers, at Legh Bridge, on a spur projecting on the right hand of the valley, a rock-basin is found on the top of a large transported block of granite known as the Puckie stone. This basin is irregular in shape, and measures 40 inches from N. to S. and 43 inches from E. to W. The longest diameter is nearly N.W. to S.E., 58 inches; the sides are nearly upright, and from 2 to 5 inches high; the bottom is nearly flat, but rough from the projecting felspar; it was covered with fine sand composed of disintegrated granite. The rock is broken through the centre, and the fracture must have taken place since it was placed in its present position. The River Teign runs in a hollow about 80 feet below this stone; and it seems very probable that the spur on which this stone is deposited, formed part of the lower bank of a small lake extending upwards to the commencement of a narrow gorge above Gidley Park. On Middleton (or Meldon) hill, above Chagford, there are several basins. In Rushford woods, near that town, to the north of the Teign, there is a basin, called “The Punch Bowl,” on an insulated piece of rising ground known as “The Bowling Green,” and a small irregular basin is formed on the Tors at Sandsgate near the same locality. At Willistone, as before mentioned, there is a performation in the side of a basin. Near Bridford, at Hell Tor, we find several basins. A cleft passes through the centre of this Tor running from E. by N. to W. by S.: this direction, and from N.E. to S.W.,, are the directions of the perpendicular joints. The granite is laminated and coarse with the exception of a bed of very compact crystalline granite running nearly horizontally about 8 feet below the summit of the Tor. On the southern part of the Tor there are 4 perfect and 1 imperfect basins, on the northern part 5 basins, and of the last, one is the largest basin that the Author has seen in this district. This basin communicates with the northerly side of the cleft; in shape it approaches to that of a large caldron, but is not very regular in form; it measures about 12 feet from N. to S. and 11 feet froM E. to W.: the height from the centre of the bottom to the level of the top of the sides is about 5 feet; the height from a cleft (5 inches wide) at the bottom of the broken southerly side is about 7 feet; this cleft extends across the basin, and reaches 4 feet beyond its edge in a northerly direction; and on the easterly and westerly sides hollows open into this basin. Nearly adjoining, there is a large oval hollow measuring about 10 feet from N. to S. and 20 feet from E. to W.: at the N.N.E. the side is broken away, but the average height of the sides above the grass is about 24 inches: the depth to this rock below the turf in the centre is 10 inches: this hollow has not been included in the list of basins, as doubts may be entertained as to its origin. On the hill to the left of the valley that extends from Moreton Hampstead to Bovey there are two basins worthy of note; one at Ingstone, on account of its size; the other at Bullaton Rock, as being in a small transported block resting on a large rock. To the north of the Teign, the most remarkable Tors are Waattern, Wild, Belstone, and Yes (2050 feet above sea-level, the highest point of ground in Devon); on these no basins exist.