End Moraine of Grinnell Glacier

November 08, 2016

Grinnellboots (1)

Photographer: Rod Benson
Summary Author: Rod Benson

This photo taken near Grinnell Glacier in Glacier National Park, Montana, shows a ridge of rock material beyond my boots known as an end moraine. It consists of rocks of various sizes (till), ranging from silt to boulders - an obvious sign that they were deposited here by a glacier. Rocks that had been stuck to the bottom of Grinnell Glacier (or had fallen onto it, etc.) were carried by the glacier and then dropped as the ice melted at the terminus of the glacier. A significant moraine such as this indicates that the climate must have been stable for several decades, causing the terminus of the glacier to be stationary. If it had been advancing due to a cooling climate or receding due to warming, the rock material wouldn't have built up as it did here. The span of stable climate that allowed this moraine to form happened during the Little Ice Age, a cold period that lasted for centuries, ending in the mid-1800s. Since then the Grinnell Glacier has been receding due to a warming climate, causing the glacier to melt away from its moraine.

Contrary to popular belief, the remnant glaciers in Glacier National Park are not left-overs from the last ice age, which ended roughly 10,000 years ago. There's little doubt that ice age glaciers were indeed responsible for carving the majestic peaks and valleys of today's park, but glaciologists believe those glaciers completely melted away during an especially warm period 9,000 to 5,000 years ago -- known as the altithermal or Holocene Climate Maximum. On the other hand, glaciers present in the park today formed during the Little Ice Age (not a true ice age). These Little Ice Age glaciers formed in cirques that were carved by glaciers during the last real ice ages. The photo at the top of this page shows the end moraine that marks the farthest advance of the Grinnell Glacier during the Little Ice Age. Photo taken on July 28, 2016.

Photo Details: Camera Model: Canon PowerShot ELPH 360 HS; Lens: 4.5-54.0 mm; Focal Length: 4.5mm; Aperture: ƒ/10.0; Exposure Time: 0.0063 s (1/160); ISO equiv: 160; Software: QuickTime 7.6.6.