The erosion of rivers as they flow down from the mountains tends towards the production of a smooth vertical descent profile. However where either faults produced by earth movements, or rocks with different hardness occur, this smooth profile is generally interrupted by a cataract or waterfall. Along the south coast of Iceland there are many waterfalls that result from a layer of basalt rock (at around 100 metres) which is much more resistant to erosion than the underlying layers of ash and scoria. Where this difference is very pronounced, as at Skógafoss, a vertical waterfall results but if it is less obvious a sloping cataract forms, as at Stjornarfoss. Occasionally the scouring action of water at the base can undercut the softer rock and create an overhang so that it is possible to walk behind the waterfall as at Seljalandsfoss.
Basalt is a dark fine-grained rock of similar composition to the layer in the Earth’s crust called Sima, which is around 12 miles thick on average. Layers of basalt on the surface are often result of such molten Sima flowing up great fissures and cooling rapidly as it spreads out over the surface. As the layers hardens, tensile stresses build up and the layer often fractures into vertical columns – the joints are always at right angles to the surface and the columns are generally hexagonal. If the lava is still moving as the stressing occurs then the columns may twist or curve as at several Icelandic waterfalls.
Although Gullfoss is generally accepted as the most majestic waterfall in Iceland and Dettifloss the most powerful, Skógafoss must be a contender for the most beautiful with its impressive size and symmetry. To log this Earthcache you need to make several observations and take a photograph as proof of your visit.
First go to the base of the falls, as near as you can to the listing co-ordinates. Waterproofs may be useful to protect yourself from the spray! Can you see any basalt columns from this spot? If you can, are they straight or curved; if you can’t, can you give one reason why you can’t see any?
The second observation is to do with the waterfalls dimensions. Most panels and books say Skógafoss drops 62 metres so it is by no means the longest in Iceland (Glymur falls 200 metres and Morsárfoss - which only came into being only in 2007, the result of a receding glacier - drops 227m in a single fall) . Make an estimate of the width of the Skógafoss at the top ;-)
Finally, according to a noticeboard you will pass on the way to the way to the base of the falls, how many falls are there on the river? See if you can find out at what altitude is the highest. (This used to be on the original notice board)
You do not have to visit the highest waterfall in this series to log this Earthcache but, if you can, it would be great if you climbed to around N 63º 31.931 W 019º 30.662 so that you can (optionally since Feb 2013) post a photo of yourself AT THE TOP of Skógafoss - there are only 428 steps to climb (377 when I was there 
! The only exception to this requirement is if you are wheelchair bound or on a bus that is only stopping 5 minutes, in which case a picture at the listing co-ordinates would be very nice.
