Geology of County Dublin

 

The rocks that occur in County Dublin are all, with the exception of the latest Pleistocene glacial tills, of Palaeozoic age having been formed between 590 and 350 million years ago during the Cambrian to Carboniferous periods

Geological periods in the Palaeozoic Era:   

Permian                        248 – 286 million years

Carboniferous               286 – 360        …..

Devonian                      360 – 408        …..

Silurian . . . . .              408 – 438        …..

Ordovician                   438 – 505        …..

Cambrian                     505 – 590        …..

 

During the Cambrian, Ordovician and Silurian periods the area, which was later to become the present-day British Isles, was affected by the slow closure of a major ocean by means of the collision of two continents. This resulted in shallowing of the ocean, increased volcanic activity during the Ordovician, intrusion of granite masses such as those in Leinster and final closure of the ocean during the Silurian, the elevation of north-east to south-west trending mountains, and large scale regional metamorphism.

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The island of Lambay is partially composed of lavas that were extruded from an active volcano into a shallow warm water sea.  In the area around Balbriggan in the north of the county, a folded and faulted succession of Ordovician and Silurian mudstones and sandstones is poorly exposed beneath glacial deposits.

 

The Lower Carboniferous was a time when a shallow sea migrated northwards over the Devonian continent at a rate of 3 cm per year.   Across the British Isles, shallowing of the seas at the end of the Lower Carboniferous produced swampy environments in which the coal measures were deposited.  Subsequently an arid terrestrial environment during the Permian and Triassic prevailed. 

 

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Extensive movement of the Earth's crust took place during the Hercynian orogeny, or mountain building event, (c. 290 million years ago) that caused the Lower Carboniferous limestones at Loughshinny to be folded into spectacular erect chevron folds.

 

Pleistocene - Holocene: 2,000,000 years ago to the present day

In the last 2 million years, towards the end of the Tertiary and into the Quaternary period, fluctuations of temperature resulted in an ice age (Mitchell and Ryan 1997).  At least eight episodes of ice expansion, known as glacials, occurred during this time and these were separated by warmer periods known as interglacials.  In Ireland evidence for several, but not all, of these glacials and interglacials may be seen. The Dublin region has been most strongly affected by two glacials: the Munsterian (300,000 - 132,000 years ago) and the Midlandian (120,000 - 10,000 years ago) with the effects of the latter being most strongly imprinted on the landscape (Coxon 1992).

During the Midlandian, ice from three sources was present in the region.  A glacier moved from the north down the Irish Sea from Scotland; ice sheets covered most of the Irish midlands and moved across Dublin from the north-east, and a mountain ice cap was perched on the Dublin and Wicklow Mountains.  ….

 

The ice sheets from the Midlands blanketed much of Dublin under a layer of limestone-rich till (or boulder clay). Till is a poorly sorted mixture of rock types which range in size from pulverised rockflour to large boulders. The Irish Sea Glacier also produced till which contains a characteristic blue granite from Ailsa Craig in the Clyde Estuary. The presence of this material in tills along the east coast demonstrates the direction of the flow of the Irish Sea Glacier from north to south (Hoare 1975).

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The extinct Giant Irish Deer Megaloceros giganteus roamed Ireland about 12,500 - 11,000 years ago.  Although this animal lived in mainland Europe as well as in the British Isles, its fossilised remains have been found in large numbers in Ireland, particularly at Ballybetagh and at Howth, preserved beneath peat.  A cold spell which occurred about 11,000 years ago caused its demise.  Soon afterwards a warmer period, the Holocene, began which continues to the present day.  Many plants and animals returned to Ireland, it is thought from the south via France and Britain, and with the development of soils the familiar Dublin flora began to emerge.

 

Note: Literature relating to the geology of Co. Dublin is dispersed throughout several books which are contained in the references and in serial publications including the Irish Journal of Earth Sciences, Bulletin of the Geological Survey of Ireland, Proceedings of the Royal Irish Academy, Scientific Proceedings of the Royal Dublin Society and the Journal of the Royal Geological Society of Ireland and its predecessor the Journal of the Geological Society of Dublin.  Additional valuable information may be obtained from the maps of the Geological Survey of Ireland, particularly from McConnell and Philcox (1995) which includes the southern half of the county.The northern half will be included in the 1:100,000 map Sheet 13,  Meath - Louth.

 

 

Soils

 

The soils that now cover Dublin have developed in the last 11,000 years on top of parent material, which as we have seen earlier, may be granite, quartzite, shale and sandstone, limestone, or alkaline boulder clay. Parent material is one of several important factors in soil formation; others include climate, topography, time, and human influence. Modification of soils to enhance agricultural yield is widespread, and the constant expansion of urban areas is diminishing the open areas in Co. Dublin. It is estimated that some 20% of the county is now covered by urban development.

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Grey Brown Podzolics: in a crescent surrounding Dublin City, extending from Skerries in the north to Maynooth and Clondalkin in the west and to Bray in the south, Grey Brown Podzolic soils are found developed on glacial tills of Irish Sea origin on land below 60 metres. They are moderately well drained loamy textured soil types, and support good pasture land, suitable for vegetable growing in the north of the county. Due to extensive agriculture, plant communities are poorly defined.

 

Gleys: these soils occur in association with the preceding type, but on land over 60 metres. They are best developed in the northernmost areas of the county on Silurian shales and sandstones overlain by Midlandian glacial tills, but also occur in the east of the county. They are generally poorly draining soils due to high concentrations of clay minerals or the presence of a high water table, and have a poor agricultural potential unless drained. As with the Grey Brown Podzolic soils, plant communities are not well defined.

 

Adapted from Flora of County Dublin,   Dublin Naturalists Field Club.