The Merkjá River has several beautiful waterfalls, but the most outstanding is Gluggafoss or Window Falls. (also known as Merkjárfoss) The upper half of the cliff is palagonite or tuff rock and the lower ledge is basalt. The river has formed tunnels and grooves through the soft rock and a series of ‘windows’ in the tunnels, thereby earning the name ‘Gluggafoss’. At the very top of the falls, the river passes under a stone arch. As the rock is rather soft, the waterfall has changed over time. Around 1947 the upper half of the waterfall could hardly be seen, as the water flowed into a vertical tunnel behind the cliff. It was only visible through three different openings or ‘windows’ one above the other. The water came out through the bottom ‘window’, forming a beautiful arch, except when the water rose in the river, forcing it through all three windows. Further changes occurred when Hekla erupted in 1947, causing a 20 cm thick layer of volcanic ash to be carried downstream by the river. The vertical tunnel formation nearly disappeared as it filled with ash. It has taken nearly 50 years for the falls to return to its former glory.