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interior valleys in the north. Water quality from boreholes in the formation is variable,
although often of moderate quality with a TDS content of up to about 1 500 mg r 1 •
Poorer quality water is usually associated with low yielding boreholes in the drier areas,
but very poor quality water has been obtained in some boreholes with a good yield, even
in high rainfall areas. The most favourable places for boreholes in the formation are low­
lying sites on faults and major joints, particularly in the coastal zone (where these are
associated with deep weathering of the formation), as noted in some localities along the
margin of the outcrop of the overlying unconsolidated Berea Formation.
(d) Pietermaritzburg Shale and Vryheid formations
In the shales of the Pietermaritzburg Shale Formation, as well as in the shales and
sandstones of the Vryheid Formation (which also contain coal horizons), moderate yields
of up to about 7 m 3 h- 1 are usually obtained from (80 - 100 m deep), 150 mm diameter
boreholes drilled in the formations. Occasionally, higher yields of 10m 3 h- 1 and more are
obtained in the formation itself, although the better yields are often associated with
intrusive sheets of Karoo dolerite within the formations. The groundwater quality in the
formations is usually moderate to poor with TDS values of up to about 1 500 mg r 1 . A
characteristic ofthe groundwater occurring in shales of both formations - particularly those
of the Pietermaritzburg Shale Formation - is the relatively high iron content which can be
as much as 40 mg r 1 • The higher iron content is usually found in boreholes with low
yields. In the Vryheid Formation, good quality groundwater is not always associated with
areas of high rainfall. Accordingly, in the Gingindlovu-Mtunzini area which has a mean
annual rainfall of more than 1 000 mm, poor quality saline groundwater is frequently
encountered in boreholes drilled in the formation. Favourable locations for the siting of
boreholes in these formations are on valley intersections and valley lines, preferably where
there is an indication that a Karoo dolerite intrusion is likely to be penetrated at depth in
the borehole.
(e) Volksrust and Estcourt formations and various formations of the Adelaide and
Tarkastad subgroups (Beaufort Group) and the Molteno, Elliot and Clarens formations
The rocks of these formations mainly comprise shales and mudstones with a lesser amount
of fine grained sandstones intercalated therein, especially in the Adelaide Subgroup. The
Molteno and Clarens formations consist primarily of sandstones, the former medium to