The Antrim Lava Group (Palaeogene) is subdivided into three formations: the Lower Basalt Formation, the Interbasaltic Formation and the Upper Basalt Formation. By far the best exposures of the Lower Basalt Formation in the Ballyclare area are to be found at Craig Hill Quarry [J290 920]. The two flows seen here display a number of unusual features. The upper flow is the principal one worked. It is at least 18m thick and generally massive and coarse-grained. A strong platy jointing with steeply dipping joint-planes is developed on parts of the north and south faces of the eastern section of the quarry. On the north face joints sometimes curve round to horizontal, but are thought to be cooling joints rather than part of a magmatic roll. On the south face of the quarry the joints pass upwards into blocky basalt and are, therefore, not due to faulting. The top of the lower flow appears at the foot of the face due to a fault downthrowing to the west. The upper flow is markedly finer grained for at least 1m above the base, but is not sharply demarcated from the highly vesicular basalt beneath.
The upper 4m of the lower flow is highly vesicular with an irregular upper surface. A strong banding is due to variations in the size and shape of the vesicles. Sometimes the banding dips steeply. Generally the vesicles are rounded or almond-shaped and contain chiefly chabazite, brown or clear calcite and some fibrous zeolite minerals, probably thomsonite. A few vesicles contain levyne with an overgrowth of calcite. An undulating grey-brown band of tuffisite occurs parallel to and about 1m from the top of the lower flow. The band is up to 2cm thick and can be traced for at least 200m. In places it is merely rotten basalt and zeolite but often it is finely banded with scattered crystals of olivine and glass fragments. Below the upper 4m of vesicular basalt are at least 8m of fairly coarse basalt with vesicular bands and patches up to 1m thick.
