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3 Forces that Shift the Shore

Archaeologists reconstruct the geological history of the coastline to find the best place to dig.
June 14, 2018

Coasts have been a desirable home for humans for tens of thousands of years. So when archaeologists want to investigate past inhabitants in lands that border oceans, they seek out old coastlines. One problem. Most coastlines don’t sit still. Sea levels change dramatically over time. Today’s shorelines can be shifted surprising distances from those of the past, which may lie underwater, far up a slope, or even buried under layers of accumulated soil.

On the coast of British Columbia, Hakai archaeologists are untangling the many factors influencing sea level, and studying how that geological change affected the human landscape.

“In the case of where [sea level] is falling, the village you were born in was a coastal village, but by the time you die it is well inland,” says archaeologist Quentin Mackie.

Today anthropogenic climate change is the most well known sea-level-swelling culprit, but many natural processes cause slower changes, too. In this video, archaeologist Daryl Fedje explains the three main factors that affect historical sea levels on the British Columbia coast.

3 Forces that Shift the Shore

The shores of Quadra Island, British Columbia—at the northern end of the Salish Sea—are a place where sea levels fluctuated wildly over the past 15,000 years.

In a new paper—published in the journal Quaternary Science Reviews—researchers from the Hakai Institute, University of Victoria, Geological Survey of Canada, and University of California Irvine studied all these factors and reconstructed the shoreline history of the region around Quadra Island.

Quadra Island historical sea level map
Sea level changes in the northern Salish Sea over the last 14,300 years. Base map prepared by Keith Holmes

Luckily, seafloors have distinct sediment and fossils compared to forest floors, lake bottoms, or other habitats. So by digging down into the layers of sediment along the coast, the scientists can tell whether it was submerged or not at a given time. 

“The very first order of business is to collect these [sediment] cores so that we can establish where the marine shoreline would have been at different points in time,” says archaeologist and study co-author Nicole Smith, who added that obtaining these cores requires a true team effort.

The respective debris of marine and terrestrial life is preserved in this sediment and can be associated with a date, says Smith. The preserved skeletons of diatoms, for example, can tell us if the sediment was deposited in fresh or salt water. “From those sediment cores, we’re able to piece together the sea level history for this area,” says Smith.

Finding the ancient shoreline is just the beginning, says Fedje, the study’s lead author. The people who lived along these ancient shores had to deal with the sea level changes, and the researchers are hoping to learn how.

“We’re looking at people adapting to a very rapid change in climates and sea level, which is something we’re starting to see happen in our world now,” says Fedje. The causes of sea level change may be different than today, but future adaptations can be enriched by our knowledge of the past.