. Discovery. Science. Fig. I.—the new pit, showing method of removing top gravel i.' 60 FEET OF OXFORD CLAY. shells, stones, which are all objectionable in varying degrees according to their quantities. (2) Must not be too plastic or " foul " ; such a clay would shrink or crack on burning. The appearance of lime would indicate such a clay. (3) Must not be too " lean," it must be plastic enough to hold together during the process of drying ; the appearance of grains of sand would indicate such a claj'. The Oxford clay, called " Knotts " in the distric
. Discovery. Science. Fig. I.—the new pit, showing method of removing top gravel i.' 60 FEET OF OXFORD CLAY. shells, stones, which are all objectionable in varying degrees according to their quantities. (2) Must not be too plastic or " foul " ; such a clay would shrink or crack on burning. The appearance of lime would indicate such a clay. (3) Must not be too " lean," it must be plastic enough to hold together during the process of drying ; the appearance of grains of sand would indicate such a claj'. The Oxford clay, called " Knotts " in the district under consideration, approximates very nearly to the above ideal conditions. The clay is dug in large open pits some 60 to 80 feet in depth, upon which there lies superimposed a stratum of some 5 to 12 feet of gravel or of clay containing numerous glacial stones and boulders. Fig. i shows a common method of removing the top deposit, which is loaded into tip-wagons and deposited into a neighbouring pit from which all the clay has already been dug. The clay itself is won by means of steps (Fig. 2) on which the men work with crowbar, pick and shovel ; and the clay is sufficiently hard to stand, winter or summer, without subsiding or falling ; and although some of the quarries are upwards of 80 feet deep, the men work on the ledges with perfect safety. Fig. 2 also shows the steep in- clined cutting made in the maiden clay to act as a shoot for the material, with the tip-wagon at the bottom ready to receive the spoil. The floor of thr quarry is at (or near) the stratum of Kelaways Rock, which forms a comparatively hard and dry workini; floor. It is not, however, everywhere in the district that such good conditions prevail; locally the top gravel-deposit or " callow " varies remarkably, and the local geology has to be known in detail in order to determine the best sites for sinking new pits ; in one quarry it was found that a glacier or a river had scooped out more than
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