Glacial Retreat

According to experts from the National Park Service (NPS), Alaska's glaciers are getting "smaller and thinner." Along with many other glaciers around the world about 90% of Alaska's are retreating.
 People have been recording this retreat for about a hundred years through photography. Using a technique called "repeat photography" where one photographs a subject in the same location as a previous photograph (or photographs) we can see the changes throughout the years to these locations, in this case: glaciers. The National Snow and Ice Data Center (NSIDC) has an archive of many repeat photography sets of glaciers. (I'm currently waiting for them to send me the download to add to this post.)
 Joel Cusick, a NPS specialist, in his quest to find out if leave nutrients from plants could be studied to get an age of a landscape, mapped out the retreat of Exit Glacier. He looked at all the old pictures of Exit Glacier that he could find and placed them in chronological order. Doing so he had a rough estimate of the glacier's retreat through time. Unfortunately the photographs most useful, those shot overhead from an aircraft, date back to only about 1950. Cusick then turned to dendrochronology, measuring time by counting tree rings.
 As a glacier recedes it leaves behind a mound of rocks that have been pushed to the tip of its terminus. These rock formations are called moraines. Eventually these moraines get overgrown with vegetation and trees will sprout. By coring these trees you can count their rings to find out how old they are. This method gives you the age of the tree but not the age of the moraine it is growing from. The period of time between the formation of a moraine (glacial retreat) and the sprouting of a tree is called the ecesis. During his research Cusick and his team determined the ecesis interval for Spruce to be 25 years and Cottonwood to be 5 years in this area. With this information Cusick could accurately map out Exit Glacier's retreat.

The Retreat of Exit Glacier. nps.gov

"During the retreat of Exit Glacier from its Little Ice Age maximum in 1815 until recent times, the glacier has left a series of more than 11 moraines and retreated more than 1.25 miles (2 km). The glacier had an average retreat of roughly 6/10 of a mile each century or one kilometer each century." - Susan Huse, NPS.

-Aimee LaFleur

Exit Glacier's Retreat
NSIDC
Glacier Q&A with the NPS