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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