Geology is the science that deals with the solid crust of the earth; in other words, with the rocks. By rocks, however, the geologist means loose sand and soft clay as well as the hardest granite. Rocks are divided into two great classes—igneous and sedimentary. Igneous rocks have resulted from the cooling and solidifying of molten matter, whether gushing forth as lava from a volcano, or, like granite, forced into and between rocks. Sometimes pre-existing rocks waste away under the influence of natural agents as frost and rain. When the waste is carried by running water and deposited in a lake or the sea in the form of muddy sediment, one kind of sedimentary rock may be formed—often termed aqueous. Other sedimentary rocks are accumulations of blown sand : others are, like stalactites, of chemical origin : others, as coal and coral, originate in the decay of vegetable or animal life. Though, in their origin, sedimentary rocks occur in layers or strata, they may receive disturbing shocks and be tilted up at various angles or even bent into folds. Again, heat, or pressure, or both combined, may so transform rocks that their original character is completely lost. Such rocks, of which marble is an example, are called metamorphic.

The general contour of Forfarshire is an index to its geology. The trend of its rocks is from south-west to north-east. In the north we have in the Braes of Angus the oldest formation, the Silurian rocks consisting of mica schist, gneiss, hardened grit, clay slate, etc. On the borders of Aberdeenshire at the heads of the valleys the Silurian strata are pierced by granitic masses. The line of the Grampians is marked by a great “fault” which extends from the Clyde to Stonehaven.

These older rocks are succeeded as we cross the county to the south-east by the Lower Old Red Sandstone formation, which forms the basis of the whole of the Lowland region, and consists of conglomerates, sandstone, marly shale, cornstone and limestone, flagstones, tilestones, and shales. Erosion has been at work everywhere, but the rocks which have had most power to resist its ravages, form such hills and ridges as the Sidlaws, Turin Hill, and other heights that run eastwards to Brechin and Montrose. The dip of the rocks on the Braes of Angus on the one hand and that of the northern side of the Sidlaws on the other, show that Strathmore in the language of the geologist lies in a great synclinal curve or trough, which is succeeded by the anticlinal curve or saddleback of the Sidlaws. The axis of the one curve runs up Strathmore from Alyth to Stracathro, while the other does not follow the crest of the Sidlaws, but may be traced from Montrose to Friockheim, Letham, and Tealing, and thence into Perthshire by way of the hills behind Inchture.

The Sidlaws, which are geologically a continuation of the Ochils, are volcanic in origin, and consequently sheets of lavas and ashes have been interbedded with the sedimentary strata of the Old Red Sandstone. Their escarpments are tough igneous rock. In this southern district of the county there has evidently been at sundry periods a very considerable amount of volcanic activity, the chief foci being at Tealing, the Law Hill and Balgay Hill (Dundee), Rossie Hill, and other heights. Innumerable veins and irregular dykes and sheets of igneous rock have burst through the Old Red Sandstone. Hence we find porphyrite at Ninewells, bedded trap-rocks at Kinpurney (Kilpurnie), Charleston, Broughty Ferry, the Laws, Pan-mure Hill, Arbirlot, etc., and great sheets of porphyrite stretching from Letham to Montrose. Many of these igneous rocks are tufaceous in texture and not durable when exposed, but other samples, as at Craigie, are of a more basaltic character and make excellent road metal.

It has been surmised that a wide inland sea, which has been named “Lake Caledonia,” once washed the base of the Grampians; that coarser gravels and shingles brought down by streams from the Highlands were deposited near its northern edge; and that finer sediments formed closely-grained sandstones, shales, and flagstones in the deeper parts farther south. These lower series of sandstones are greyish blue and brown with shades of purple. Through time the “lake” contracted, and the salts of iron to which the colouring of the stone is due became relatively more abundant. Thus a second series of rocks was deposited, which took a deeper hue of red.

It is in the lower series, and particularly in a thin band of shale some three feet thick which may be traced to the south of the Sidlaws from Balruddery Den to Tealing and from Duntrune by Carmyllie to Leysmill, that fossil remains of fish have been so abundantly found. These also occur in a similar deposit that runs from Turin Hill through Farnell into the Mearns. The fossil fauna, which is much more abundant and interesting than the flora, comprises both crustaceans and fishes. There are three genera of crustaceans found—Pterygotus, Stylonurus, and Eurypterus—while the fishes represented in these rocks, the Cephalaspidae and Acanthodidae, kinds now extinct, have the tail fin principally developed on the under side, as in sharks. These fossils have rendered Forfarshire an important field of research to the palaeontologist. In Pleistocene clays at Carcary, Drylees near Montrose, and Barry, there occur the only other fossil deposits of the county.

The red sandy marls of the Tannadice district form the highest beds in the county and these are succeeded east and west by deposits of clay and lime that sometimes contain shells.

Forfarshire appears to have been covered during the Glacial Period by a vast ice-sheet perhaps 1500 feet thick that moved down from the Highland glens, crossed Strathmore in a south-easterly direction, surmounted the Sidlaws, which deflected it still more towards the east, and descended to the sea. Its course is marked by striae or scratchings on rocks and stones. Two important results followed. Boulder clay accumulated under the ice and remained to form the soil. Vast deposits of sand and gravel kames, as for instance between Lindertis and Glamis, were left in its wake, and broadly scattered throughout the southern part of the county from the seashore to the top of such heights as Lundie (1000 ft.) and Craigowl (1500 ft.) are boulders of Silurian and granitic rock carried down by the ice and left, when the ice melted, at spots remote from their place of origin. When the general sheet of ice had disappeared, there must for ages have been glaciers in the Forfarshire glens. An interesting relic of these is seen at Glenairn (South Esk), where a terminal moraine 200 feet high and half a mile broad runs across the valley. Above this there must, to judge by the deposits, have been a lake; but in course of time the river burst through the barrier and drained the accumulated waters into Strathmore.

It used to be supposed that the glens of the county had been formed by dislocations of the earth’s crust, but it is now believed that they have been carved out by the natural agencies of running water and frost.

The soils of Forfarshire are either primary or secondary. The first is produced by the disintegration of native rocks, and the second by the materials brought from a distance by ice or by running water. The colour varies from red to brown and black. In upland districts and on gravelly bottoms the soil is thin, while the sandstone rocks have a covering of tenacious clay. Trap rock soil is friable and fertile. Secondary soils are sometimes too sandy, at other times too stiff. Often primary and secondary soils are mixed. Accumulations of water in hollows produce mosses and bogs. If not naturally very fertile, the soil of the county by such farming operations as draining and manuring has been rendered as productive as any in Scotland.