Flagstone is a type of flat stone, usually used for paving slabs, but also for making fences or roofing [1]. It may also be used for making memorials or headstones in a cemetery.

The making of flagstones was described by C B Crampton and R G Carruthers in their book Economic Geology: Caithness Flagstones (1914, Chapter XV Pages 165 – 169):

Their fitness for pavement depends on their perfectly smooth bedding planes and the facility with which the rock splits to the thickness required. Such a rock usually has widely spaced joints allowing large sizes of flags to be obtained of a regular shape.

The durability of the flagstone probably depends on its fine-grained character and cement-like properties and on the nature of its constituent minerals. The flags have been shipped to many British and foreign ports. The flagstones quarries are driven as open workings from the outcrop. At the Holborn Head Quarries where the dip is very gentle and the slope of the ground in the same direction, the working faces can be driven against the dip, and the floor of the quarry is kept dry by natural drainage. The quarries are driven as far as the expense of removal of the "tirring" (the rock overlying the band specially suitable for pavement) will allow.

No mining is resorted to, nor is blasting employed. The "tirring" is taken up layer by layer, by means of levers, until a sufficiently large surface of the flagstone pavement is exposed. The main joints in this layer are then opened by wedges and the pavement flags are lifted in layers of definite thickness by levers in the hands of experienced workmen. Each flag, as it is loosened from its bed, is taken away from the scene of operations by means of a crane and subjected to a preliminary squaring.

As the working face advances in the large quarries that have been working for long periods, a perfectly smooth surface which formed the bed of the pavement layer is left as a floor to the quarry, and where the "tirring" is easily disposed of, as at Holborn Head, this floor in its unencumbered condition may be very extensive and forms a striking demonstration of the characteristic evenness of the bedding of the Caithness Flagstones. The further shaping and finishing of the flagstones is sometimes done by hand, but cutting and polishing machinery has long been employed in Caithness.

The chief flagstone works at present are situated at Thurso Harbour. The flags are placed on long tables which can be wheeled beneath the cutting machines. In each yard six connecting rods from the machinery move backwards and forwards, for a distance of about a foot, six heavy horizontal beams of the same length as the tables. Each beam carries two knives with plane edges, and is swung on large frame supports by two iron rods attached to chains, which work over pulleys with adjustable weights. The knives lie parallel on either side of the beam along its whole length, and, travelling with it, come in contact with the flags, one knife operating each table. Two pairs of tables on wheels are supplied for each pair of knives, and, by means of cross-rails, each pair can alternately be brought beneath the knives, or drawn away, in order to have the flags that are cut removed and replaced by others. Thus the flag-cutting on one pair of tables, and on the other the removal of finished flags and adjustment of fresh flags, can be carried on at the same time.

A long sand trough with water pipes, attached to the supports on either side of the beam, continually feeds the knives with sand and water. A row of flags to be cut is adjusted to position on each table by means of short wooden levers with projecting iron points, and fixed by wooden blocks and wedges, and the tables when wheeled under the knives are steadied by wooden props. The cut is not carried completely through the flags, but after about one-half to three-quarters of an hour's cutting, according to the thickness of the stone, the machine is stopped and the tables drawn aside and replaced by others. The flags are then trimmed by knocking off the edge of stone beyond the saw cut with a hammer from below.

Sometimes the surface of the flags is polished. This is done under cover of a shed. The polishing tables move on wheels like those used for cutting, but are broader in proportion to their length, and have splashboards. The polish is obtained by means of flat concentric iron rings, slightly spaced in a horizontal plane, and given an eccentric rotation by a vertical shaft. The polished flags are used for special inside work only, such as flooring of lobbies and granaries , hearthstones , table tops, and mantelpieces . The usual stock sizes of the shaped flags are from 2 to 6 ft. in length, and 1 to 4 ft. in breadth, hut much larger sizes can be supplied. Good examples of large flags may be seen at Scrabster where each of the four walls of the office of the Holborn Head Flagstone Quarries consists of a single large flag. Flags of unusual length are sometimes obtained and are used for gateposts and even for posts for drying-greens. The thicker flags are chiefly used for paving, the thinner for damp-courses in building.