7 Geology and Hydrogeology
7.2. Bedrock
The northern
and central sections of the area of study are underlain by
sedimentary Carboniferous limestone with Silurian slates and
shales at the east (Kilcullen to Castledermot). Granites (igneous
rocks) and surrounding metamorphic deposits (Leinster batholith)
occurs along the eastern part of the study area. The central
Castlecomer plateau area is composed of sedimentary Carboniferous
sandstones, siltstones and shales.
Sedimentary
rocks
The main geological
formations occurring in the study area are described, starting
with the oldest sequence, in Table 7.1.
Some of the
Carboniferous limestone formations identified may be associated
with Karst features (Ballyadams and Clogrenan Formations,
designated Rck-3 on the geotechnical constraints mapping).
The dolomitised Limestone rocks may also prove hard to excavate
(the boundary of dolomitisation which runs from Carlow to
Thomastown can be identified on the geological bedrock maps).
The Quartzitic sandstone occurring in the Castlecomer plateau
could also prove hard to excavate.
Granite Complex
A granite complex
known as the Tullow granite extends in a broad zone to the
east of Gorebridge-Carlow (Rck-5). The granites are igneous
rocks and the resulting heat and deformation during their
formation has created the surrounding metamorphic rocks. The
Tullow granite is the largest granite pluton igneous body
in the Leinster Batholith and is mostly of medium-grained
texture but some coarse and fine veins cut the mass. Outcrops
of granite were observed to the east and south-east of Carlow
Town: Kernanstown, Chapelstown and Tinryland (Reference 8),
see Rck-5 on the geotechnical constraints maps.
The granites
can be very hard and difficult to excavate, although along
the joints, the granite is often deeply weathered to sand
with solid blocks in-between. This deep weathering can also
result in a highly or completely decomposed granite structure.
To the east and north-east of Carlow, geophysics surveys (Reference
8) indicated the presence of weathered granite in some locations
ranging in thickness from 3.5m to 7.8m.
|
