The geology of Armidale provides a quite interesting picture. From the above cross section (Figure 3), it can be seen that for the area encompassing the city of Armidale, there is a basement of quartzites, probably of a Carboniferous age. After this, there is a layer of tertiary 'gravels, etc.'. Another map groups this deposit into a broad category, encompassing 'Conglomerate, sandstone, shale, greybilly' (Binns, et. al., 1967). From samples collected above Kirkwood Street, these Tertiary formations can be seen, at least for the immediate city area, to be sedimentary, which are somewhat silicated, likely the result of groundwater movement under the later Tertiary basalt cap (Hutton, Twindal & Milnes 1980:35), which has now eroded from the survey area. These rocks appear to be relatively hard, demonstrating hardness above 5, but below 7 on Mohs' scale.

Water action seems to have cut down through these three layers, creating the valley in which Armidale is situated. The valley partly filled with recent alluvial deposits from this process. These alluvial deposits are unconsolidated, and allow water to flow with relative ease. The geology suggests that this changes radically at the depth at which the quartzite bedrock is reached. Quartizites have a quite low porosity and permeability (Murin, 1998), which therefore makes the stone unsuitable to the movement of large amounts of ground water. It would seem, only considering these two rock types, that most of the subsurface water would be found in the alluvium of the valley. The Tertiary sediments also need to be considered. These seem to be sandstone, which is usually quite porous, however, there is silication of the stone matrix, which may result in a lower flow rate than unsilicated sandstones. It seems reasonable to suggest that the permeability and porosity of the three geologic units can be ranked, in descending order as alluvium, Tertiary sediments, quartzite bedrock.

It thus seems possible to propose a tentative model for the water table for the survey area. It seems reasonable to suggest that groundwater penetrates only a shallow distance into the quartzite bedrock, or, if deeply, in amounts too negligible to be considered suitable for wells. Above this, in the alluvium, there is a saturated zone, which may parallel the surface topography to some extent (McGeary, Plummer & Carlson, 2001:426). A water table line can Figure 4: implied saturated zone, cross section along Dangar St. only be guessed at for the Tertiary sediment formation, it may experience an increase in depth relative to topography as height increases. It seems likely that, to some extent, the water table of the Tertiary sediments reflects the local topography. Certainly it is true to state that this zone experiences a low flow rate relative to the alluvium, which would require a deeper penetration of the water table to maintain constant supply.

Next, the limits of this Tertiary formation needs to be established. From a combination of a detailed sketch map (Voisey, nd.) and a 1:125000 geological map (Voisey et. Al. 1961). As can be seen from the map, this formation occupies quite a large proportion of the southern extremity of the city, and a not insignificant section of the survey area. These areas, as would be expected, are associated with topographical rises.

Figure 3: Geological Cross-Section, approx.. 1.5 km NW of Armidale (Voisey, n.d.)

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Figure 4: implied saturated zone, cross section along Dangar St.