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The Geology of Ancient Alaska

by Bill Jones

    At one time, some 200-400 million years ago, ancestral low mountains existed in what is now the Beaufort Sea of the Arctic Ocean. These were the only lands within present Alaska. These low mountains of the arctic shelf extended across present Canada to near the Hudson Bay. Across present Alaska these mountainous islands were the only separation between the Arctic and Pacific oceans. The Pacific Ocean then extended from somewhere near the present location of Prince Rupert in Canada to the arctic islands. For hundreds of miles southward of the arctic islands the Pacific was shallow over North America's western shelf.

    Then some 150 million years ago, a drifting tectonic plate arrived from Asia. The pressure of this moving mass pushed up the Arctic basin and formed the Brooks Mountains. Thus it is not uncommon to find sea shells today at 3-5000 feet elevation in the Brooks. Underlying the seashells and old beach sand is the ancient bedrock of the arctic shelf. These limestone and shale layers were both pushed upward and sometimes overridden by the drifting tectonic plate to make up the present core of the Brooks Range. During that time of plate collision, 150 million years ago, the layers of the Arctic bedrock and the Asian plate tumbled and turned so that now there are mountains of the Brooks that have some older rock layers lying on top of younger layers whose ages are more than 100 million years apart!

    Succeeding plate movements spaced millions of years apart formed the White Mountains to the south and then later the Alaska Range. Thus Alaska was built from the north to the south by successive collisions of drifting Asian tectonic plates. During these plate collisions in the arctic, the Brooks was for scores of thousands of years a rim of violent active volcanoes. Eventually the pressures of the plates reduced and became more static, but the Brooks still has a few "smokers" today.

    Underlying the Brooks and the White Mountain ridges are relatively thin covering crusts over the earth's mantel and in some areas old cracks or vents allow melting snow water to reach super heated rock below causing hot springs to bubble to the surface. Some of these thermal springs have substantial flow and their waters flow under the ice of the rivers throughout the winters.

Relaxing at Chena Hot Springs     Three hot springs areas near the Yukon River are at Kokrines, Chena Hot Springs (seen to the right), and Manley Hot Springs, and there are many others along the 700 mile wide Brooks Mountain Range.

    Later as the White Mountains formed, its northern slopes merged with the southern slopes of the Brooks to form the Yukon Valley. The Yukon River, however, would not exist until many million years later.

    As the drifting plate continued to pressure from the south the Alaska Range formed and built ever higher and higher and still another mountain ridge developed along the coast. Each of Alaska's main valleys represent different time lines several million years apart in their formation.

    These tectonic plate movements were not isolated to Alaska. Each episode extended along the Pacific coast, forming the mountain ranges and increasing the size of North America by lands drifting from Asia. So, one might say that western North America and almost all of Alaska consists of borrowed lands from Asia.

    The Asian tectonic plate continues to press its force northward in Alaska and is the cause of continuing episodes of earthquakes, now mostly along the Alaska Peninsula and the upper Aleutian Islands.

The Ice Age

    In the meantime, since Alaska had been partially formed by tectonic plate collision, the ice age began. Geologists have not been able to form an exact time line for the beginning and duration of the ice age. As the earth became ever colder the summers became shorter and snow melt reduced over a long period of time, perhaps for 100,000 years. The arctic ice cap grew southward to cover all of Canada and then crept further south to somewhere around the present state of Tennessee.

Icefield in the Storm Range, Alaska     Due to the lack of snow melt the oceans of the earth were robbed of water. The depth of the western Arctic and Pacific oceans gradually reduced some 200 feet or more to expose a wide land mass connection between Siberia and Alaska. Geology has named this land mass, that does not exist today, Beringia.

    Just how long Beringia was exposed above water is contested by some geologists. Some believe it was 50,000 years and others say it was a much shorter time. Most, however, agree that the warming from the peak of the ice age released the waters from the ice cap back to the oceans sufficiently to cause the oceans to rise and cover Beringia some 23,000 years ago (plus or minus 3000 years).

    So for a relatively short time (in geologic terms), Asia and North America were connected by the Beringia land bridge. During that period, it was possible - but I think not very credible - for land mammals, even aborigine people, to walk from Siberia to Alaska.

Did humans cross Beringia to populate North America?

    Well, the science of geology has not taken a stand on this issue, but archaeology has. Back during the early 19th Century, not long after archaeology became a science, and without much if any evidence, the archaeology community formed a conclusion that humans did not exist in the Americas until Beringia was exposed, and once exposed humans migrated across Beringia from Siberia into Alaska. From there, it was opined, these migrants continued moving ever southward to populate both North and South America.

    The single thread of proof of this everlasting archaeology paradigm is the fossil evidence of the Folsom people found in the regions of Arizona and New Mexico. These fossils were carbon dated to be 14000 years old. There were no human remains to date, only such items as wood shaped by hand, charcoal from campfires where other evidence of life existed, bones of animals with butchering marks that were attributed to man's actions, etc.

    Archaeology has since held steadfast to the theory that the original humans in North and South America were hunter groups who crossed the Beringia land bridge when it was exposed by the lowered sea. But lately, younger and better educated archaeologists have begun to tear down the old paradigm with new evidence of human existence found as far to the south as Chile that date as far back as 17,000 years B.P.

    Frankly, I do not know, but the preponderance of logic tells me that aboriginal hunters could not have possibly traveled hundreds of miles across Beringia during the depth of the ice age when this bleak frigid land would have been covered all year by snow hundreds of feet in depth. Even the existence of humans in arctic Siberia during the depth of the ice age is not very credible.

Features & Maps by Bill Jones

Archaeology & Anthropology in Alaska

Photos are used courtesy of the Alaska Division of Tourism. The photo of the icefield in Alaska's Storm Range is also ©2002 by Al Clough.