The Aplite of Bretagnes Eye EarthCache
The Aplite of Bretagnes Eye
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Bretagne Rock is a large granite outcrop in the Paarl Mountain Nature Reserve. It is one of three such outcrops in relatively close proximity to each other. Paarl Rock is the lower and more southerly of the three, while Bretagne Rock and its neighbour, Gordons Rock are higher and closer to one another.
Entry to the Paarl Mountain Reserve is limited to between 08:00 and 18:00 in Summer and between 09:00 and 17:00 in winter. There is also a fee of R40 per vehicle and R18 per person to enter. Note that there are no credit card facilities and only cash is accepted at the entrance gate to the reserve.
The granite and formation of these outcrops is thoroughly researched and described in great detail in the nearby Earthcache (GC1YZN0 - Paarl Rock) whereas this Earthcache will show you a specific and distinct feature on Bretagne Rock, called an aplite dyke.
Aplite is an igneous rock of mostly granitic affinity. Granitic aplites are light-coloured and fine-grained rocks with the same mineralogy as granite and are most commonly observed as dykes or veins (Whitten and Brooks, 1972) within a granite host rock. These dykes and veins are generally less than one metre in thickness.
At the listed co-ordinates is the start of an aplite dyke, which extends towards Reference Points 1 and 2, and beyond. It is a remarkably linear feature on the massive granite exposure that you cannot miss. Bear in mind that granite is an igneous rock and it forms from cooling of magma below the earths surface. It generally exhibits fairly large/coarse mineral grains indicating that the magma cooled relatively slowly, allowing the crystals time to grow. During the cooling process, the newly formed granite most likely cracked or split through contraction and permitted fresh magma to force its way into this crack or joint. The fresh magma cooled much more rapidly than the older magma, resulting in a much finer grained aplite dyke forming.
In order to qualify to log this Earthcache, please send your answers to the following questions to the CO by email or using the message board within a reasonable time frame after logging your find.
QUESTIONS
1. What is the thickness of the aplite dyke (in centimetres) at Reference Point 1 and at Reference Point 2?
3. Describe the colour and texture differences between the aplite and the granite (you will need to find places where the two rock types are freshly exposed and not stained or covered with lichen).
3. Describe what happens to the aplite dyke in the vicinity of Reference Point 2.
4. Although optional, it would be great if you could post a picture of yourself anywhere on Bretagne Rock.
Additional Hints
(Decrypt)
Cyrnfr erzrzore gb fraq lbhe nafjref!