The edifice of Mount Fee is the remains of a volcanic feature that has been significantly eroded by glacial ice. It likely represents a dissected stratovolcano (also known as a composite volcano) that was larger in area and higher in elevation than its current form. Stratovolcanoes can reach heights of and consist of alternating layers of lava flows, volcanic ash, cinders, blocks and bombs. During the glacial periods, much of the volcano's original outer cone of pyroclastic material was eroded away by moving layers of ice and rock. The removal of the ejected volcanic material has exposed the dacite lava that forms the narrow north-south trending ridge of Mount Fee. The Black Tusk, a pinnacle of dark volcanic rock to the southeast, is also interpreted to be the remains of a deeply eroded volcano that was once covered with pyroclastic material. The present day edifice of Mount Fee contains several lava spines that reach heights of to above the main ridge.