In Paleozoic and early Mesozoic times (541 to 250 million years ago),
much of what is now continental Alaska were ocean basins. These ocean
basins were later squeezed and uplifted in response to tectonic plate
Mid-late Mesozoic was the time of the most dramatic changes in Alaska
geography and many of the motions initiated then persist today.
Widespread intrusion of granitic bodies into existing rocks also
occurred during the Mesozoic, presumably also a result of the dynamics
of lithospheric plate collision.
During the early Tertiary period (60 to 30 million years ago)
extensive volcanic activity occurred, particularly in the Wrangell
Mountains, the Alaska Peninsula, and the Aleutian Islands. During this
warm period, large peat swamps formed and subsequently produced the
coal deposits in the Cook Inlet region and on the northern side of the
Alaska Range. Throughout the late Tertiary (30 to 2 million years ago)
volcanic activity remained widespread.
During the Pleistocene ice age (2.5 million to 12 thousand years ago),
broad snowfields formed in the Brooks, Alaska, and Coast Ranges and
filled the trough between the Alaska Range and the coast ranges. Great
glaciers from the snowfields carved the mountains and valleys of
Alaska into their present shape and provided the sediments that now
cover valley floors and basins.