Gluggafoss or Merkjárfoss is a waterfall in the south of Iceland near Hvolsvöllur. The river Merkjá tries to find its way through a hill, split open ready to drain the melting water from the Tindfjallajokull glacier. From the hill down the Merkjá River cascades down over several steps. Total height of the Gluggafoss is appr. 120 meters.
The Merkjá River has several beautiful waterfalls, but the most outstanding is Gluggafoss which means Window Falls. I have seen several names used for the Gluggafoss, like Merkjárfoss, Merkifoss or Hervifoss.
From road 1 near Hvolsvöllur, take road 261 (Fljótshlíðarvegur) to the East. After 17 kilometers, near Fljótshlíð, the Gluggafoss will show up. Just near the crossing at road 261>250. From road 1 it is also possible to take road 250 until you reach the crossing 261>250. The waterfall is visible from the roadside.
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.
Info from Katla Geopark
A few hundred meters to the West lies the Þórðarfoss. It is said that Gunnar Hlíðarendi, one of the main characters from the great saga of Njáll, here is buried.