The Sugar Loaf Earthcache EarthCache

Geology of Wicklow

Sedementary Stage

Around 500 million years ago, the seabed of the Iapetus Ocean, the predecessor of the Atlantic Ocean, built up huge volumes of silt, mud and calciferous deposit from existing sea life, in addition to vast quantities of materials deposited through volcanic action. As these deposits stratified, the lower levels were subjected to enormous pressures and the result was limestone, mudstone and sandstone. Sedimentary rocks are typically soft and prone to erosion. These sedimentary rocks were some of the foundations from which Ireland would grow.

Metamorphic Stage

Around 420 million years ago, the tectonic plates of Europe and North America collided beneath the Iapetus Ocean with tremendous force and the bedrock was buckled and forced upwards forming landmasses in the area that we now know as Wicklow. The tremendous geological forces of pressure and heat exerted upon the limestone, sandstone and mudstone changed, or metamorphosised, the sedimentary rock in to new rock types – schist, slate, and most especialy quartzite. These are metamorphic rock types and are typically much more resistant to erosion than sedementary rock types.

Green, red and white quartzite is commonly found around Bray Head, the Little Sugar Loaf, the Great Sugar Loaf, the Sugar Lump and Carrigoona (where Kyle Cache 1 is located).

Igneous Stage

A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of intrusive, or plutonic, igneous rock (meaning that its materials do not reach the surface at the time of formation) that is solidified from cooling magma rising from deep in the Earth's crust. A batholith is formed when many plutons converge to form a huge expanse of granitic rock – the Leinster batholith extends from Dún Laoghaire in Co. Dublin to New Ross in Co. Wexford and is the largest in Ireland and Britain. 

The molten layer of the earth’s crust, magma, was also forced upwards at the time of the collision of the two tectonic plates and pooled underneath the overlying metamorphic crust of quartzite, much as fluid pools underneath a blister. The rock formed by this pooling cooled slowly underneath the surface, forming plutonic granite, an igneous rock type. In Wicklow, the cooling process was slow due to the covering of metamorphic rock above and this allowed the formation of large flaky silica mica crystals typical of Wicklow granite. The covering layers of quartzite, schists and slates were weathered over the millenia through natural weathering and glacial erosion, revealing rolling granite domes typical of Wicklow - visible to the southwest of the Great Sugar Loaf.

The Sugar Loaf Mountains

Atkins (2010) concludes that the Sugar Loaf mountains are comprised of highly erosion-resistant quartzite, which results in the pale colour and craggy appearance of the mountains. The dramatic conical shape of the Great Sugar Loaf derives its name from a resemblance to conical cakes of sugar sold through the late 19th century.

The Sugar Loaf quartzites and surrounding sandstones are part of the same geological formation as Bray Head, and date back to the Cambrian period, 542 to 488 million years ago. The most recent period of glaciation in Ireland lasted from about 28,000 years before present (BP) to about 14,500 BP.

In northeast Wicklow, three separate ice sheets met to shape the landscape. The main Midland ice sheet flowed south and east from central Ireland into the foothills of the Wicklow Mountains. There it met the ice flowing out from the Wicklow Mountains ice cap. Ice from the Irish Sea basin flowing south from Scotland dominated the east coast of Ireland. These three ice masses each influenced the landscape and geology of the Sugar Loaf mountains area in ways that can be deduced by the presence of rocks and debris transported long distances to the Sugar Loafs and by the orientation of striations carved into rock by glacial scouring.

These clues include granite boulders deposited by the Wicklow Mountain and limestone glacial till and flints from the north of Ireland deposited by the Irish Sea glacier. As the depth of the ice was approximately 725-735 m thick at its maximum, it is likely that the Sugar Loaf Mountains themselves were completely covered by ice at the last glacial maximum. However, their sides were probably sculpted at a later stage, when the summits were exposed by melting glaciers. This sculpting and the nature of quartzite weathering and fracturing result in the conical and craggy summits of the Great and Little Sugar Loafs.

Reference: PDF file of the Atkins Report (2010)

Logging Requirements

To log this cache, please email the answers to the following questions via my geocaching profile:

1. Did  volcanic or glacial forces sculpt the iconic shape of the Great Sugar Loaf ?
2. What rock colour, apart from white quartz crystals, is visible in the quartzite at the listed coordinates? (The answer is not grey)
3. Find a  rock a little higher up the path; try to break it against the exposed bedrock. Is the result crumbly, or does it break in to shards? Speculate and draw a conclusion as to whether this is sedementary, metamorphic or igneous rock. 

Please send your answers before proceeding to log a find on this Earthcache; in the event of difficulty with the answers provided I will contact you.

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