The first about 6.9 million years (Mya) ago produced the northern and central hills, while the younger and highly eroded Mount Gower and Mount Lidgbird were produced about 6.3 Mya by successive basalt (an extrusive igneous rock) lava flows that once filled a large volcanic caldera (crater) and can now be seen as horizontal basalt strata on mountain cliffs (at Malabar and Mount Gower) occasionally interspersed with dikes (vertical lava intrusions).