Penedo do Lexim Earthcache
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Penedo do Lexim (Lexim’s Rock) is one of the extinct volcanic chimneys of the so called Lisbon’s volcanic complex. It is nowadays classified as of public interest. To do this cache don't look for a container, but bring a camera with you (read description below).
Taking a look around Lexim’s Rock, and despite the low-mountain area that surrounds us, the calm and colourful scenic view we admire (the green fields, Cheleiros village down in the valley, the main road some distance away, the little farms all over the place, the silent power plant windmills) may lead us to think we are facing a landscape that has always been like that. That, if any change would have happened, those would only been small and neglectable. But remember you are only on top one of the many volcanoes that actually shaped this landscape over 70 Million years ago. If by any chance you would have passed by then, you would have found a very different scenic view, that would have made you remember that the ground in which you step daily is part of a ever-changing globe – only at a timescale incomparably larger than our memory can even understand.
Measuring 230 metres high on its peak, this hill originated on the end of the Mesozoic period, around 72 Million years ago, in a strong volcanic activity period (‘Volcanic chart’ figure). What we can see nowadays is part of the remaining volcanic chimney, namely the solidified magma material, stronger, that survived the natural erosion process (and the former stone quarry that operated in this place…) of the last 72 Million years (‘Erosion chart’ figure). Studies indicated that this chimney originally was 30 metres in diameter and 2000 metres deep.
Basalt shows its presence here, and you should observe hexagonal basalt prisms (resulting from the magma cooling process) and several crystals.
To log this cache you need to:
- measure the prismatic forms in the fallen basaltic column in this location (cache's coordinates) (note: You don't actually need to measure every single one of them, they are quite similar);
- find why does lava assumes a prismatic form when it cools down (you will not find information in the location, you will need to chase it somewhere else);
- mail me both answers;
Please include a picture of yourself and some of the prismatic basaltic forms on the cache's coordinates in your log. You may also admire more of those at the stone quarry area (N38º 53.549 W009º18.670).
VERY IMPORTANT NOTICE:
- Park the car and go directly to the mentioned coordinates, following the trails (the path is pretty obvious). Do not approach the hill's peak (N38º53.529 W009º18.677) nor South Face, due to the archaelogical sensitivity and importance of discoveries made on that area. If you stay by the mentioned path and given coordinates, you can still admire this ancient volcano without endangering any archaelogical feature.
Information and charts extracted from Ciência Viva
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Geocaching in Portugal
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