Life on Mars – The Yellow (and Purple) Peril

Back in the Land Rover we drove over a ridge and dropped into Breakneck Valley.  The vegetation increases here and the geology softens into the background for the first time.  But it does so with a struggle – a number of caves in a black ash cliff strike you right in the eyeballs as you go over the ridge.  I wanted a closer look so we shut off the engine and took a walk across the valley.  Edsel asked if we were going to do this everywhere.  I retorted  – why not – the end of the road is barely 5 miles away.  It won’t take us long.

The caves varied in size and shape and it would be easier to describe them as ash that had fallen down over lava flows and frozen in situ, leaving fangs of rock hanging down over the spaces they had consumed. So many times Ascension looks like a geological text book.  As much as the biodiversity, the volcanic features should be preserved.  We did document in our mapping what others have noted as key geological features, things like the fumaroles that are long tubes where lava flows hollowed out previous deposits, or the most likely areas to find the hard black pieces of obsidian after which the hotel was named.  And the colour of the rock changes frequently , greys, oranges , reds , blacks and whites to make a geological tapestry that rarely gets covered by anything else.

ASI213 Grazing Valley.JPG

The Weird and Wonderful Geology


Passing the island’s motorcycle scrambling track where kids and machines get bright red every weekend, you rise up out of Breakneck Valley and for the first time feel the full force of the south easterly winds on the south coast.  I noticed that along the road there were a series of yellow emergency telephone stands – I wondered how often there had been the need to report a break down or report that the road had been blocked by a landslide when the NASA station had been in operation.

A long grey scree ran off the side of Green Mountain coated in low growing ginger and guava plants, and also the healthiest lichens I have ever seen.  Their growths were almost the size of the flowering vegetation and must have represented hundreds of years of growth in this pristine air.  The air is moister this side of the island and as we progressed eastwards the vegetation started to grow larger, the odd tree here or there.  But our way was impeded by, horror of horror, ferocious yellow and purple warriors.

We were in a Land Rover several hundred times larger than our foe, but these sentries set up along the road were fearless and waved their weaponry purposefully at our tyres.  Just in case, we made sure we swerved and avoided them.  We stopped and got out; still they poked their armoury at us in an aggressive fashion, as if they were saying “Come on, I’ll ‘ave you”.  They were giant yellow land crabs.  That is their official common names although, as I say, there were also purple versions of supposedly the same species.  Their bodies were 8-10 inches across, and their legs extended a similar distance out.  For all the famous animals on the island, these have been the most  understudied until recently.  One lone scientist had done some work on them, logging their presence along the NASA and NE Bay Roads which we had been able to map as a sort of imprecise transect of sightings, but it did not really tell the whole picture.  More recent work had allowed tags to be implanted and the crabs monitored more extensively.  They seemed not to travel much further than from Green Mountain down to the sea in the south and east sides of the island.  They obviously preferred the richer vegetation over here.  I assumed either the pickings were too slim down the western and northern sides, or else they did not much relish too much of an insectivorous or mammalian diet.

I’ve watched them at many locations but find land crabs a bit incongruous.  I am used to crabs swimming about in the sea, but to be at such elevations juxtaposed in amongst twigs and leaf litter or climbing trees, and to be these vivid colours, was just ostentatious.

  Over the years more studies have been made and progress has discovered that they have a few key routes off the mountain, the NE Bay Ghut being the main one, to reach the sea for the inevitable release of eggs and spawning that needs sea water to be a success.  It is odd to see that turtles spend their life at sea and come ashore once to lay eggs, while the crabs do almost the complete reverse.


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