As you tour Lamar Valley and the northern section of Yellowstone you will notice Douglas fir trees scattered out in the open plateau. Look closely and it’s hard not to notice a rock at the base of each of these trees. The granitic rocks are strewn about the valley floor which leads one to wonder where they came from.
The boulders scattered around the meadows, tell you that glaciers passed this way recently. Geologically recently, at least. With all the focus on Yellowstone’s volcanic geology many visitor don’t realize this area at one time was covered with glacial ice up to 4000 feet thick. The last glaciers retreated about 13,000 years ago, and they left behind the rocks or glacial erratics.
The boulders look out of place because they are. They were carried along with soil and other debris by glacial ice from the Beartooth Mountains miles away to the north. When the ice receded or melted, boulders dropped out and were deposited where you see them today. They are possibly 3-4 billion years old, and yet they are recent immigrants. They’ve only been in their new locales less than 20,000 years, having migrated here during the last ice age.
Though they are foreign here, many of the boulders are large and thus serve as nursery rocks for young native seedling trees. Out in the open meadows it is hard for Douglas fir seedlings to germinate because they prefer to grow up initially in shade. The glacial boulders provide that and more.
Douglas fir seeds can be carried by wind and if they are fortunate to fall beside a large protective rock, they have a better chance of germinating and surviving those first and most vulnerable years. The glacial boulders provide shade, moisture, shelter from wind, and even absorb and radiate heat so ice and snow melt off quicker around their bases. The seedlings get a shot at becoming trees and in this way, the rocks from afar help support native life in their new homes. Now many of the trees overshadow their glacially deposited nursery rocks.
Look for rock-tree partners on the Northern Range of the park and also check out the huge Glacial Boulder near Inspiration Point at Canyon and the exhibit on glaciers in the Canyon Visitor Education Center.