A smoothly-weathered granite Tor, Dartmoor
By far the most important and striking of these volcanic formations is the great granite mass of Dartmoor, one of the most prominent features of the county, measuring 225 square miles in extent, and constituting the largest granitic area in England. Granite is a volcanic rock, formed, it has been suggested, by fusion at a great depth and under great pressure, and consisting in the main of three minerals, quartz, felspar, and mica. That of Dartmoor is, on the whole, grey and coarse-grained, but it varies a good deal in colour, fineness, and composition. Its real origin is obscure. It has been assigned by various experts to various periods, and it has been called "the sphinx of Devon geology." There can, however, be no doubt about the great disturbance which has been caused in the county by upheaval and by the intrusion of melted rock, which has bent, broken, and twisted previously-existing formations in a most extraordinary manner, the results of which are well seen in the picturesque scenery of the Start, Prawle Point, and Bolt Head. Lundy, which is twelve miles from the nearest point of Devonshire mainland, is all granite, except for a small part of its south end, which is Millstone Grit.
A long interval of time appears to have followed the laying down of the Culm measures, during which so vast an amount of shattered rock was worn away that when the beds that come next in order—the New Red Sandstones—were formed, they were, in places, deposited directly upon the Devonian, the superincumbent carboniferous or Culm strata having entirely disappeared. The New Red Sandstones occur chiefly in the east of the county, where their lower beds fill up old creeks and valleys in the carboniferous system; and they extend northwards from the coast past Exeter as far as Holcombe Regis, forming broad bands on either side of the Exe, characterised by the high fertility of the overlying soil, and with one long spur traversing the heart of the county, past Crediton and Exbourne, with isolated patches round Hatherleigh, and with another and less extended prolongation a few miles west of Tiverton. The Lower New Red consists of clays, conglomerates, red breccias and sands, in which occur many outcrops of trap, the evidence, not only of numerous eruptions, but of eruptions extending over a long period of time. These beds contain no fossils, except in fragments of older rocks. The Middle New Red, in the form of thick beds of red marl and red and white limestones, well seen on the south coast, is covered in turn by the Upper New Red, with beds of pebbles, some of which are derived from the Devonian and even from the Silurian. In this formation, near Sidmouth, have been found the remains of two remarkable reptiles, the Hyperodapedon, a strange form allied to the existing tuatera lizard of New Zealand and in England only known elsewhere in the formations of Warwickshire, and the Labyrinthodon, so named from the intricate structure of its teeth, and also called Cheirotherium, from the hand-like impressions of its feet.
Footprints of Cheirotherium, New Red Sandstone
The Rhaetic beds are not well seen in Devonshire. They occur on the coast between Lyme Regis and the mouth of the Axe, and in the estuary of that river, but are much hidden by landslips of cretaceous formations from above. One layer, consisting of black shale, with bivalve shells such as Cardium and Pecten, contains also a bone-bed, with remains of fish, such as Acrodus and Hybodus. The former is represented by its blunt teeth, and the latter, which was a huge, shark-like creature, by its long and formidable-looking fin-spines.
The Lower Lias is exposed in a narrow strip of coast from the Devonshire border to the mouth of the Axe, and to a greater extent in the valley of the river above Axminster. It has been divided on the coast into four distinct zones, each characterised by its own particular species of ammonite.
The cretaceous formations occupy a much wider area, but they also are confined to the southern part of the county. The Greensands of the Blackdown and Haldon Hills have been divided by geologists into fifteen layers, varying in thickness from a few inches to as much as thirty-five feet, some with few fossils, and some very rich in animal remains. Trigonia and Inoceramus are found in almost all the zones: other forms less widely distributed are Murex and Turritella. Chalk occurs on the south coast from the Dorset border to Sidmouth; and in isolated patches it extends inland as far as the Blackdown Hills, and also further west, in the Haldons. The Lower Chalk, well seen on the coast and to the west of Hinton, is made up of calcareous sandstones, with ammonites and pectens. The Middle beds, composed of white chalk with flints, the zone of Terebratulina gracilis, is exposed at Beer. The lower and harder layer is characterised by Rhynconella. The Upper Chalk also holds many flints, with echini; Holaster in the lower, and Micraster in the upper strata.
Last of all come the tertiary deposits, which, however, occupy only a small area in the south-east, chiefly in the valley of the Teign, from Kingsteignton to Bovey Tracy; and there are a few isolated patches, as for example near Bideford and at Plymouth. These beds consist of clays, some of them of much value, with flints from the chalk, and gravels and beds of sand derived from the wearing away of older rocks. The most interesting feature of this formation is the lignite of Bovey Tracy, on the eastern edge of Dartmoor. Lignite, otherwise known as brown coal, consists of the imperfectly fossilised remains of tropical or sub-tropical vegetation, such as the palm, cinnamon, and laurel, amongst which are found lumps of resin. By far the most abundant remains are those of a very large tree allied to the sequoia of California. It is very remarkable that in the Pleistocene clay above the lignite are found stems and twigs of Arctic birch and willow, suggestive of a far colder climate than prevailed in Tertiary times, when the trees that went to form the lignite were growing.