Skarðsheiði and neighbouring Hafnarfjall are the remains of an extinct, 4-5 million years old late pliocene central volcano south of Borgarfjörður.
The "sandwich" structure of tertiary basalts, andesite, numerous rhyolithic layers and gabbro intrusions makes it highly interesting for geologists.

de.wikipedia.org/wiki/Skar%C3%B0shei%C3%B0i

Skessuhorn, part of an old collapsed caldera on Skarðsheiði


Tungukollur, a prominent peak on Hafnarfjall