Download Geology of Howth

Geology of Howth
The rocks in Howth were made in three different geological periods:
1. The Cambrian Period
2. The Carboniferous period
3. The Quaternary Period
Colour Key
Cambrian Rocks
The Cambrian Period
The Cambrian Period was over 500 million years ago. At that time the Iapetus Ocean
was opening between the continents Laurentia and Gondwanaland. The Howth area
was under the sea on the continental shelf off the North West coast of Gondwanaland.
This was a sedimentary environment in which sandstones, quartzites and mudstones
were formed. The Cambrian rocks make up the higher ground on the south side of the
peninsula with the quartzites being particularly resistant to erosion.
The Carboniferous Period
A
B
Colour Key
Muddy Limestone
Fossil Rich Limestone
Cherty Limestone or ‘Calp’
Faults
The Carboniferous period happened between 300 and 350 million years ago. At that
time Ireland was close to the Equator and covered by a warm sea. This sea was full of
life. When living organisms died they fell to the sea bed and their soft tissue decayed.
Their shells and bones which were made up of the mineral calcium carbonate built up
to form sedimentary rocks known as limestones. There are different types of
limestones in the Howth and Sutton areas because the sea in which they were
deposited changed through time. Most of the limestones are rich in fossils and they all
react with acid when tested. The limestones are exposed on the northern and western
side of the peninsula. During the Armorican mountain building period earthquakes
affected the Howth area and movements occurred along fault lines which shattered the
surrounding rocks. These broken rocks are called Breccias which is the Italian word
for ‘broken’ A major fault line runs from Balscadden Bay at A on the map above to
Sutton at B on the map.
The Quaternary or Ice Age
Colour Key
Marine deposits
Till with limestone
Bare Rock
Till with quartzite
Till with granite
The Quaternary Period is the last 1.8 million years and includes the ice-age. Howth
was glaciated like the rest of Ireland. There are a variety of glacial deposits on the
peninsula but the commonest is till or boulder clay. Some till is rich in limestone,
some in quartzite and others in granite. A lot of the higher areas are just bare rock and
have covering of glacial deposits. The material in the till gives us an indication of the
source of the ice which passed over the area.
The Geological Survey of Ireland own the copyright to all the maps in these notes