http://skaftafell.org/en/jokulfell-the-oldest-tillite-in-iceland/ Tillite is the term used for solidified gravel and rock produced below and in front of glaciers. This rock is evidence that glacier ice has covered the country. Finding such rock provides geologists with evidence to define cold periods referred to as glacials. Sofar, rock formations that provide evidence of glaciations, have not been mapped except locally in Iceland.
Still, it so happens that one of the oldest tillites that has been found in Iceland is located at the foothill og Jokulfell in Morsardalur valley. Its age is 4 million years based on radiometric dating of the lava flow next on top of the tillite, unit JF7, that was dated as 4.01 +/-0.08 million years (Johann Helgason, 2007; Johann Helgason og Robert Duncan, 1999; Thome, 1968)