Quarrying for the GNR

An important part of the GNR operations was the quarrying of the useful sandstone rock and the shaping of blocks for the construction of the many side walls and embankments which were necessary. Between Wiseman's Ferry and Mount Manning, several large quarries have been investigated by GNR historians. Within the quarries, there are terraced levels where rock has been split. In some places, clear indications of the wedging methods which were used to extract the rock in suitable size pieces for conversion to prepared blocks. Large sections of rock were also moved from the face of the quarry by drilling and blasting, for further preparation into blocks.

Conservation work in the largest of the quarries on Devine's Hill above the buttressed wall revealed some interesting features and at least one of the pieces of equipment which the convicts had used. Wedge pits were dug into the rock about 10 inches (250mm) apart, using the pointed end of a pick, along a line suited to cracking of the block from the quarry face. Iron wedges were inserted in the pits so prepared, often with a piece of bark on each side of the wedge to aid the operation. The wedges were struck with a sledge hammer, working back and forth along the line of wedges until the rock cracked along the line. In a large quarry such as that at Devine's Hill, it would often be necessary to assist in the cracking of a block of suitable dimensions, by using wedges inserted across the vertical face of the rock along a horizontal line. The action of a vertical and a horizontal crack would assist the quarryman to produce workable blocks.

The picture shows a row of wedge pits in rock at the Devine's Hill quarry. The rock had wedge pits along the horizontal face (as in the picture) and along the vertical face as well (not visible). The block so cracked was left and it is not obvious why it was not used. During the conservation work in 1985, a small iron wedge was discovered. Ian Webb (Ref.1) notes that the wedge was 90mm by 40mm. A blacksmith had forged it by folding a strip of 40mm by 10-12mm bar in half. The folded end was forged and flattened to make the taper of the 40 mm wide wedge. Elsewhere, tools such as the head of a stone breaking sledge hammer, a pointed pick head and an iron stone chisel have been found (Ralph Hawkins, Ref.2).

Preparation of Stone Blocks

The large sections of rock at the quarry face were split into blocks of a size appropriate to the need in wall construction and for drain and culvert covers. The same technique for breaking down the large rock sections - creating wedge pits and then carefully splitting the rock to size. For buttressed walls such as those along Devine's Hill, the blocks were methodically reduced to an accurate size with the chisel end of a pick and then faced with the pointed end by continual picking of the surface. The block was thus reduced to a near rectangular item which would fit neatly in a wall without any grouting between blocks. Ian Webb comments that one large block on Devine's Hill required 273 pick strokes to complete the facing, a remarkable technique, considering that many of these walls and other structures would not have been seen. This facing technique can be seen in many of the sandstone walls of buildings in Sydney itself and in many beautiful cathedrals and other buildings in Britain.

References

1. Webb, Ian; Blood Sweat and Irons, p.49-50, Dharug & Lower Hawkesbury Historical Society, 1999.
2. Hawkins, Ralph; The Tools that Built the Road, Exploring the Great North Road Forum Papers p.75-6, 1998.