To log this EarthCache you need to answer the following questions:
- Is Gabbro an igneous, sedimentary or metamorphic rock?
- As you look back down the valley there is a green stile: how many steps has it on the near side?
- Does a mafic rock contain more or less silica than a felsic rock?
- Optional, but please include in your log a photo of you or a geocaching item with Slieve Foye in the background
The Parking Waypoint here is at the Long Woman’s Grave court cairn. Legend has it that the site marks the grave of a Spanish noblewoman who dropped dead with shock when she was brought here to view the large estate of her new husband – she hadn’t imagined that it would be mountain and bog.
A short walk from there will get you to GZ. From here, there is a good view of the contrasting landscape either side of the fault that runs through the Windy Gap. The western side of the valley is composed primarily of granite covered by glacial sediment and the terraces on the side of the mountain are glacial moraines that mark the edge of an ice sheet.
The scenery on the EarthCache side of the road is dominated by the dark gabbro crags of Slieve Foye. There is a noticeable change in both the slope of the hillside and the scenery at the geological boundary, with the gabbro generally forming a much more rugged landscape than the granite.
You can examine the gabbro in the large boulder here that has come off the mountainside. The term gabbro was used originally by Italian geologist Tozzetti in 1768 and brought into geological terminology by a German geologist Leopold von Buch. The name comes from the town Gabbro, a village in Tuscany (Italy)
Gabbro is coarse-grained, mafic intrusive igneous rock formed from the slow cooling magma deep beneath the Earth's surface. In talking about rocks ‘mafic’ means it contains relatively less silica than a ‘felsic’ rock. An intrusive rock is formed from magma forced into older rocks at depths within the Earth’s crust, which then slowly solidifies below the Earth’s surface. Here at Slieve Foye it was later exposed by erosion.
Igneous rocks are composed of interlocking crystals. A crystal is a solid whose atoms are arranged in a "highly ordered" repeating pattern. It can be difficult to see the crystaliine structure without a magnifying glass but here a piece of the top of the boulder on the North side has broken off and allows for a clearer examination of the rock structure.
Note the large dark rectangular crystals of pyroxene. Pyroxenes are the most significant and abundant group of rock-forming ferromagnesian silicates. They are found in almost every variety of igneous rock. The name pyroxene is derived from the Greek pyro, meaning "fire", and xenos, meaning "stranger".
Note also the slightly rusty coloured spots that indicate weathered olivine. Olivine is a magnesium iron silicate and being primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, however it weathers quickly on the surface. Olivine is named for its typically olive-green colour but the reddish colour in some samples here is caused by the oxidation of iron.
Congratulations to medic143 and freakycat on being First To Find four hours after publication