 Although underlain by much older Triassic age formations that protrude to form what would once have been islands—such as Athelney, Brent Knoll, Burrow Mump and Glastonbury Tor, which is composed of Blue Lias, the lowland landscape was formed only during the last 10,000 years, following the end of the last ice age.As the sea level changed following the Pliocene era, vegetation was laid down which was later converted into peat.The peak of the peat formation took place in swamp conditions around 6,000 years ago, although in some areas it continued into medieval times.