Seattle was carved by ice.
A mere 17,000 years ago, a massive glacier the height of five Space Needles covered what is now Seattle and a large part of western Washington. It carved out Puget Sound and Lake Washington as it advanced and retreated. And Seattle’s hilly neighborhoods — including Queen Anne, Capitol Hill and Beacon Hill — were etched by the glacier’s icy underbelly.
The Cordilleran Ice Sheet was big — towering 3,000 feet high in the spot where Seattle stands today. But just how much land did it cover? And was anyone around to see it back then?
Local Wonder listener Chris Lynch asked: "Were there ice-free areas of western Washington during the last ice age? If so, were they inhabited by any animal life, human or otherwise?"
To answer his question, we have to define the time frame. The last ice age was technically the Pleistocene epoch, which marked the last glacial age that began over 2.5 million years ago. But during that epoch, ice sheets advanced and retreated across the globe several times before warming temperatures caused them to melt.
“There were at least seven different advances of the Canadian ice sheet into Puget Sound during the last 2.5 million years,” said Nick Zentner, a geologist with Central Washington University. “To most people, the last ice age means the most recent of those seven advances.”
The Cordilleran Ice Sheet’s most recent encroachment on Washington state happened when an offshoot called the “Puget lobe” reached past Olympia about 16,900 years ago.
“The ice covered Puget Sound, and it covered a large part of northern Washington,” said Elizabeth Nesbitt, a paleontologist at the Burke Museum. “It dug Puget Sound — Puget Sound was a river valley before that.”
Southwest Washington was ice free, but its boundaries were probably different. Glaciers gobbled up huge amounts of the world’s water, and sea levels were considerably lower. Evidence even suggests that at that time, Britain was connected to mainland Europe.
The weight of the ice compressed the land down by hundreds of feet; the place where Pioneer Square sits today was about 275 feet lower during the glacier’s last advance. When the ice finally melted and retreated, the compressed ground rebounded to form new topography. Whidbey Island is one visible example of rebounded land, Nesbitt said.
Zentner said geologists measure the gravel, sand and rocks left behind by glaciers to track how far they advanced during global ice ages. But finding evidence of human and animal life is a little trickier. Archeologists and paleontologists have to rely on bones and artifacts left behind — hard to come by in a world that’s changed so much.
Nesbitt said there is evidence that prehistoric animals such as woolly mammoths and mastodons roamed southwest Washington when the glacier was at its maximum — and that they moved north as soon as the glacier started to retreat.
Laura Phillips, an archaeologist with the Burke Museum, said the existence of people is harder to confirm.
“Evidence left by people is extremely rare and difficult to find due to preservation issues,” Phillips said. “Plus, sea and land levels were different and quite variable right after the ice receded, and many of the archaeological sites are below water today — or have been eroded or deeply buried.”
She said the earliest signs of human life in western Washington date back roughly 13,800 years ago, when the glacier was retreating north.
Archaeologists unearthed mastodon remains near Sequim that included a spear point whittled from bone. And ancient bison bones unearthed on Orcas Island show signs of butchering, leading scientists to believe that human hunters lived in the region shortly after deglaciation.
“People likely got to the Pacific Northwest as soon as it was possible,” Phillips said.
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