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.