7.
Geology and Hydrogeology
7.2
Bedrock
7.2.1
Introduction
The bedrock in the southern part of the constraints study
area changes from granites (igneous rocks) and surrounding
metamorphic deposits (Leinster batholith) along the eastern
part of the area, north of New Ross, to Ordovician slates
and shales in the south-east (Thomastown to New Ross). Carboniferous
limestone rocks underlie the section of the study area north
of Thomastown.
In the southern part of the study area, south of Ballyhale,
the bedrock is mainly composed of Devonian sandstone with
an occurrence of limestone in the southwest corner near Kilmacow.
7.2.2 Sedimentary rocks
The main geological formations occurring in the constraints
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 Formation,
designated Rck-3 on the geotechnical figures, 5A and 5B, and
the Waulsortian Formation, designated Rck-4). The dolomitised
Limestone rocks may also prove hard to excavate.
The sandstone rocks encountered, during a site visit, in the
vicinity of Thomastown, were massive and blocky and could
also therefore prove hard to excavate.
7.2.3
Granite Complex
A granite complex known as the Tullow granite extends in a
broad zone to the east of Gorebridge-Carlow (Rck-5) and along
the eastern part of the constraints study area, northeast
of Mullinavat. The granites are igneous rocks and the resulting
heat and deformation has created the surrounding metamorphic
rocks. The Tullow granite is the largest granite pluton in
the Leinster Batholith and is mostly of medium-grained texture
but some coarse and fine veins cut the mass. 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.
Table
7.1: Geological formations occurring in the constraints study
area.
|
Period
|
Formation
|
Rock
Types
|
Excavatability
|
Cutting
Stability
|
Map
Symbol
(where used)
|
|
Ordovician
|
Maulin
|
Slatey
mudstones. Mudstones altered to phyllites and schists
where they lie close to the granite
|
Generally
rippable
|
May
have instability problems along planes of weakness
|
Rck-1
|
|
Ordovician
|
Browsford
|
Schists
|
May
prove hard to excavate
|
As
above
|
Rck-1
|
|
Devonian
|
Carrigmaclea
|
Conglomerates,
sandstone and shales
|
Sandstones
may prove hard to excavate
|
Bedding
planes tend to be gentle and therefore fairly stable.
|
Rck-2
|
| Devonian |
Kiltorcan |
Interbedded
muddy limestone and shaly mudstone |
As
above |
As
above |
Rck-2 |
| Carboniferous |
Ballymartin |
Interbedded
muddy limestone and shaly mudstone |
Generally
rippable |
Fairly
stable |
|
| Carboniferous |
Ballysteen |
Muddy
limestone |
Generally
rippable |
Fairly
stable |
|
| Carboniferous |
Waulsortian
reef Limestone |
Massive,
unbedded fine-grained limestone |
May
prove hard to excavate |
Fairly
stable, except for karstified zones |
Rck-4 |
| Carboniferous |
Kilsheehan |
Limestone
occasionally cherty |
Generally
rippable. Hard in zones |
Fairly
stable |
|
| Carboniferous |
Butlersgrove |
Argillaceous
limestone |
Generally
rippable |
Fairly
stable |
|
| Carboniferous |
Ballyadams |
Shelf
Limestone, potentially Karstic |
Generally
rippable, may prove hard to excavate where the rock is
massive |
Stable
except for karstified zones |
Rck-3 |
|
