Over the last several million years, glaciers have repeatedly inundated northern Washington. The last glacial climatic interval was called the Fraser glaciation, which sculpted much of the topography we see today in the Puget Lowland and northern Washington. These glacial periods were interrupted by warmer nonglacial climatic periods.

The figure above shows how local glaciations correlate with the climate changes during the last 2.4 million years.

How Do Scientists Determine Past Climate?

Scientists are able to figure out what the climate was like so long ago by drilling really deep cores into thick Antarctic ice and the sea floor all over the world. After removing these cores, they study thin slices under microscopes, examining microfossils, microbes, minerals, magnetic orientation, and rock type.

Microfossils

Paleontologists look for microfossils called foraminifera to tell us more about past ocean temperatures and climate. These creatures prefer warmer oceans and create their shells out of calcite. Scientists measure the ratio of Oxygen 16 to Oxygen 18 in calcite shells, and can use these ratios to estimate past climate conditions.

Ice and Seafloor Cores

Scientists study ice cores by analyzing the light and dark rings on the cores. These rings are created by snow capturing dust and dirt as it falls to Earth. Snowfall is higher in the winter than in the summer resulting in thicker rings. In the summer, it begins to melt creating a different composition and texture before refreezing again in winter. As the ice freezes, it traps air bubbles, which scientists also study and determine prehistoric climates from using oxygen ratios just like in sediment core samples.

Photo of ice core, by Janine and Jim Eden, Flickr Creative Commons license.

Understanding Earth's Secrets, by Kevin Kurtz and Alice Feagan is a great resource for young scientists to learn about microfossils and ice core drilling. Click the image to download the pdf!

The Vashon Stade, part of the Fraser Glaciation was the latest major incursion of ice into the Puget Lowland. Ice advance as south as Tenino, WA, and was upward of 4,200 feet thick in the northern Puget Lowland.   Contours show approximate ice thickness in feet at glacial maximum during the Vashon Stade. Map derived from ice thickness contours by R. M. Thorsen (1980).

The Whens, Wheres, and Hows of the Vashon Stade

So how do geologists determine when the glacier arrived at or left a certain spot? Use the interactive graphic below to learn more.

Click on the image to open our Story Map about Washington's Ice Age Floods. This Story Map is best viewed on a desktop or laptop computer. Mobile devices will not show all of the content.

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	Ice Age Floods National Geologic Trail, Washington Section Daniel E. Coe and Ashley A. Cabibbo DNR Earth Science Week 2016
	The Cheney-Palouse Tract of Washington's Channeled Scablands Daniel E. Coe