Pinkham Notch was originally a riverine, V-shaped valley until the Laurentide Ice Sheet shaped it into its current form, a U-shaped valley. This shaping occurred during the Wisconsinian Ice Age, 25-50,000 years ago. The geology of the region became greatly altered by this event; much of the weaker rock was stripped from the region, leaving only highly-resistant mica schist. As the glaciers retreated, a layer of glacial till was deposited, including several glacial erratics. A notable glacial erratic in the area is Mount Washington's Glen Boulder.