Kostal Cone is made of fragmented and solidified lava called cinder and its summit contains a bowl-shaped crater. Kostal's cinders were ejected by lava fountain eruptions and accumulated around the volcano's vent in the shape of a cone when they fell back around its surroundings. Lava flows from Kostal's 400 BP eruption are basaltic in composition and forms a lava bed. It is an example of volcanic activity that has occurred in the Wells Gray-Clearwater volcanic field since the last glacial period; others include the Dragon's Tongue lava flow from Dragon Cone just north of Kostal Cone and the Flourmill Volcanoes north of Mahood Lake.