Shield volcanoes undergo at least two stages of volcanic activity. The initial shield stage is the most productive volcanically and features repeated eruptions of large volumes of predominately fluid peralkaline felsic magmas that become progressively more evolved. During this stage, a small summit caldera may form, as is the case for the Ilgachuz Range. After the shield stage has been completed, the post-shield stage succeeds. This stage of activity is characterized by small volumes of more mafic lavas expressed as small cinder cones and capping flows. Dissection of the shield by stream erosion is also apparent, resulting in the creation of deeply incised radial valleys.