On a bitterly cold Saturday, 7th Feb. 2015, Dr, Richard Unitt gave us a guided tour of the Geological Garden which is set up outside the Butler Building of the UCC campus in the old Distillery Fields off the North Mall.
The rocks, 10 of them at the moment, show us a great variety of rock types representing two billion years of Irish Geological history.We visited the rocks in more or less chronological order.
Rock number 10: He began with this rock of Psammitic Gneiss, which is more that 900 million years old, from near Ballyshannon in South Donegal. Psammitic is term used for highly metamorphosed sandstone containing mainly quartz, fieldspar and mica. The term gneiss is used for highly metamorphosed rocks that have distinct banding or layering.
The dark layers in this example are rich in red-brown garnets. The dark garnet layers show the flow-like folds that are typical of the rocks of the Slishwood Division found in parts of Donegal, Fermanagh, Sligo and Leitrim. The rocks of this division have been transported during the mountain-building events to the base of the continental crust and back to the earth's surface.
Rock number 7: Connemara Marble was formerly a lime mud sediment deposited on a shallow sea floor 650-750 million years ago, and later metamorphosed, probably during the Caledonian Orogeny. The coloured layers have been caused by the different minerals in the rock - serpentine, tremolite, phlogopite, etc.
Rock number 6: This Banded Amphibolite from Maam Cross, Co. Galway contains both metamorphic and igneous rock types. The green, banded rock represents a series of metamophosed lava and ash from volcanic eruptions from 600 -650 million tears ago. The dominant mineral in the metamorphic part of the rock is the dark green amphibole. Later, possibly during the intrusion of the Galway Granite about 400 million years ago, the banded metamorphic rock was injected by veins which crystallised to form aplite
Rock number 8: This Volcanic Breccia comes from Arklow Head, Co. Wicklow, the result of an explosive volcano from around 475 million years ago. It consists of a combination of shattered rock fragments and fine ash. The dark, angular pieces are obsidian. This part of Ireland was then situated in a volcanic island-arc created by converging plate boundaries.
Rock number 5: This Volcanic Agglomerate, also from Arklow Head, was also ejected from an explosive volcano about 475 million yeas ago. The large rounded rock fragments of rhyolite and pumice were mixed in with volcanic ash. Evidence of this volcanism during the Ordovician period can be seen today extending from Co. Waterford across to the English Lake District.
Rock number 3 Lithium Pegmatite rock was crystallised from granitic fluids around 405 m.a. The large and long bladed crystals are composed of the lithium mineral spodumene, one of the main sources of lithium. This pegmatite comes from near Tinahely, south Wicklow and others are found along the margins of the Blackstairs Mts. They also contain some grey quartz and muscovite and small crystals of minerals rich in valuable metals such as tantalum niobium, etc
Old Red Sandstone: This small sample of Old Red Sandstone was actually extracted on this site from the bedrock at the geophysics garden. Old Red Sandstone was formed during the Devonian Period around 380 million years ago as a flood deposit on the ancient continent of Laurussia. It is a fine- grained sandstone and siltstone and does not always have a very obvious cleavage. O.R.S. was also affected and folded by the Caledonian orogeny and later by the Variscan orogeny.
Rock number 2: Valentia Slate: this upright example comes form the well-known Valentia Island Slate quarry in Co. Kerry. the rocks date from the Devonian period around 385 million years ago but during the Variscan orogeny about 300 million years ago these mudstones, siltstones and sandstones were deformed causing folding. faulting and even uplift. This great pressure produced the slate we see today. Note also the faint trackways and burrows in this fine-grained rock.
Rock number 9: Liscannor flagstone: These two flagstones come from sandstone deposited around 320 millions years ago on what was then an ancient offshore river delta, It was laid down horizontally and has preserved the feeding trails (fossilised trackways) of an unknown organism which travelled on or near the surface, These two flagstones were taken from the Liscannor quarries near the cliffs of Moher in Co. Clare.
Rock number 4: Faulted Limestone: This rock records the life of an active fault where limestone has been repeatedly fractured and mineralised during earthquake events. The limestone was originally formed in a warm tropical sea about 340 million years ago and later magnesium-rich fluids converted the limestone to dolomite. This cream-coloured mineral can be seen where the fractured grey fragments are cemented together. Crystals of dolomite and aragonite can be seem in cavities. The rusty brown areas show the movement of oxidised iron and manganese-rich solutions through the fault. Did these types of faults give rise to our lead and zinc mines? This particular stone came from Castlemore Quarry, Crookstown. Co. Cork.