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January, 1937

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The Great Whin Sill of the North of England is probably the largest single igneous intrusive sheet or sill in this country. It is known to traverse the carboniferous rocks of Durham and Northumberland from the High Force in upper Teesdale to the Fame Islands, a distance of quite 80 miles, and must underlie the greater part of Durham and a considerable portion of South Northumberland. Its greatest development — in some places it is 100 ft. and more — occurs at either end of its range. It varies in texture from compact to coarsely crystalline, the most common variety being that in which the individual constituents are just recognisable by the naked eye. On the whole the rock of the Whin Sill is remarkably uniform in general aspect.

Geological formation

At Cawfields the "whinstone," as it is commonly termed, is described as a medium grey crystalline rock composed of sub-ophitic augite, laths of plagioclase (labradorite-andesine) felspar, large plates and crystals of hypersthene (slightly pleochroic) and interstitial quartz with a little alkali-felspar. There is a fair amount of magnetite in rather large patches and a little brown hornblende. The rock is doleritic in texture and in the opinion of Professor E. J. Garwood, the geological age is Permian, as it is found to be intersected by later tertiary dykes such as the Cockfield-Armathwaite Dyke.

The following is a typical analysis of this coarse grey dolerite :–

Silica	49.61	per cent.
Oxides of Iron	16.90	, ,
Oxides of Aluminium	11.89	, ,
Oxides of Titanium	2.51	, ,
Oxides of Calcium	9.91	, ,
Oxides of Manganese	1.49	, ,
Oxides of Magnesium	6.33	, ,
Calcium Carbonate	Trace	, ,
Alkalis, etc.	1.36	, ,

The National Physical Laboratory report on the Cawfields dolerite reveals that it is classed as "Very Good" quality as regards the determination of crushing strength, and resistance to impact, attrition and absorption of water. The specific gravity is 2.96. It will readily be seen that the rock is very suitable for broken stone aggregates (tar or bitumen-coated or sprayed), chippings and for the manufacture of setts.

Before passing on to a description of the actual operations at Cawfields, some reference should be made to the Great Wall of the Emperor Hadrian. The line of the Wall originally passed along the crest of what is now the quarry face. Of course all traces of the wall in the immediate vicinity have long since disappeared due to the attentions of bygone generations who appreciated the ready-dressed stones of the Wall when their houses, outbuildings, yards and roads required repairing. For many centuries after the end of the Roman occupation of Britain, the Wall remained more or less intact and it was not until the days of Border warfare and afterwards when the land was settled, that in places the Wall was despoiled. Life and tenure of property and stock on the Border Marches were uncertain to say the least, and the Wall provided a ready means of building stout watch towers (peels) and farmhouses.

Looking in an easterly direction from the quarry, the Wall can be seen stretching for miles along the top of the high ridge it is in this direction that it is to be seen in its greatest state of preservation. Looking North on a clear day, an uninterrupted view of miles of Bewcastle fells can be seen looking South there is the beautiful valley of the river South Tyne lying at one's feet. Truly the Romans had an eye for country and if, as recent observations tend to show, the Wall was built from Wallsend-on-Tyne on the East Coast to Solway Firth on the West Coast — a distance of roughly 70 miles — in about four years, it was an achievement in engineering skill and organisation of which we ~y might well be proud. Bede of Jarrow informs us that the Wall was 28 to 30 ft. in height and about 8 ft. in width and no great stretch of imagination is necessary to transform the now unbroken quietude to the stirring es of activity when the Wall was a permanent camp outpost of Imperial Rome.

Quarrying operations

All primary drilling is done by Ingersoll-Rand tripod-mounted machines using 1¾ in. round hollow drill steels with four-pointed cruciform bits. Secondary drilling to reduce large pieces of rock to manageable sizes is done by Climax and Ingersoll-Rand jackhammers using 1 in. hexagon hollow drill steels with chisel bits. These latter bits are all hand sharpened; the cruciform bits used for primary drilling are sharpened by means of a Climax drill sharpener and furnace. There Is very little overburden to remove and this, together with any soil that has found its way into fissures, is transferred to a spoil heap.

Crushing and screening operations

The filling roads are situated about 15 yd. apart and the rock is loaded at the face by hand into Hudson 30 cwt. capacity side-tipping wagons which are trammed by hand to a Pooley 4-ton weighbridge. Fig. 2 shows the general layout. After weighing, the wagons are hauled up an incline by an Ord and Maddison winch, the stone then being tipped into a chute feeding into two 24 by 15 in. Baxter crushers operating at a 4 in. setting. By means of a 24 in. belt conveyor, the product from these crushers is passed to a No. 3 "Kubit" impact breaker supplied by Frederick Parker Limited, and from thence by another conveyor belt to a Broadbent counterbalance screen from which the following sizes are obtained :– dust, ¼ in., 3/8 in. ½ in., ¾ in., 1¼ in. and 2¼ in. The plus 2¼ in. oversize is passed to a 20 by 12 in. Baxter secondary crusher which reduces it to 1 in. and it is then recycled to the Broadbent counterbalance screen which has a capacity of 40 tons per hour. We understand this screen gives a very good grading result.

The different sized products go direct to separate storage bins from whence they can be loaded direct into road transport wagons. If required for tarred macadam, the roadstone, either in separate sizes or graded as required, is loaded on to a conveyor belt and carried to a Thompson dryer. After leaving the dryer the stone is raised by a bucket elevator to a bin above an Ord and Maddison tar mixer capable of a throughput of 150 tons of tarred macadam per 8-hour day; this mixer can be used for ordinary tarred macadam or bitumastic (Liquaphalt) binder.

Any surplus 1¼ in. and 2 in. stone is drawn off in tubs from the storage bins to an incline where it is raised and fed to a Hadfield "Hecla" disc crusher which reduces it to in. chippings, the ½ in., in., and dust being taken out by a revolving screen. These latter sizes are mainly used as aggregate for concrete making and tar spraying on roads. Tar is supplied by the Newcastle and Gateshead Gas Co. Ltd., and The Consett Iron Co. Ltd. in road-car tanks from whence it is pumped to the storage tanks. The "Liquaphalt" bitumastic binder is supplied in barrels by Berry, Wiggins & Co. Ltd. A general view of the plant, with the quarry face in the left background, is shown in Fig. 3.

Sett making

The production of whinstone setts is another and important section of the undertaking at Cawfields. Operations are conducted on rather similar lines to those on which the Welsh sett makers work. Selected stone at the face is roughly trimmed into blocks and then sent by the "blockers" to the sett makers who split and trim them to the shape and sizes required. These sett makers work to a tolerance limit of in. and, we understand, serve a three years' apprenticeship before they are adjudged competent craftsmen. It is, of course, well known that a certain amount of controversy has always existed as to whether the North Country or Welsh sett makers are the more highly skilled. We have been privileged to watch experts of both schools at work and to make any comparison or venture an opinion would be invidious indeed. Both possess a high degree of skill and take an honest pride in their craftsmanship. In each case the stone lends itself, by reason of its homogeneity, to precise splitting and the results are marked by the same degree of excellence.

At Cawfields, setts are produced in the following sizes 4 by 6 in., and 4 by 5 in., each of an average length of 8 in., and 4 in. cubes. Other sizes can be made if required. A view of one of the sett makers at work is shown in Fig. 4.

Power plant

The main unit of power is a 132 b.h.p. Tangye fuel oil engine which drives the crushers, screens and an Ingersoll-Rand compressor supplying air at 90 lb. pressure to the drills. A view of the Tangye engine is shown in Fig. 5. The tar plant and heater are driven by a Ransomes, Sims and Jeffries portable steam engine and boiler. In passing, it should be mentioned that the tar is steam heated to 145 to 160°F. according to atmospheric conditions. The "Hecla" disc crusher is driven by a 24 h.p. Ruston-Hornsby paraffin engine.

If necessary, chippings can be loaded into tubs and hauled up a gantry to facilitate direct loading into road transport wagons. For this duty and for the transport of material to and from the stock piles, a 50 h.p. Whitcomb petrol locomotive is employed. Any material for despatch by rail is taken down an incline 2¼ miles long having an average gradient of 1 in 12, to the London & North Eastern station at Haltwhistle in 25 ton loads by this locomotive which is seen in the right foreground in Fig. 2. All track is 24 in. gauge.

A throughput of 200 tons per 8-hour day shows that the scale of operations at Cawfields is quite representative of a large number of quarries at work in this country. It is well and efficiently laid out and we understand that the rate of production of the 40 men and boys employed, compares favourably with other and larger quarries.

We desire to thank the Alston Limestone Co. Ltd., for facilities to visit the quarry and to take photographs we also wish to place on record our appreciation of the help and information afforded us by Mr. A. Corbett, the manager at Cawfields.

Drawings and Photographs accompanying the article