A number of highly dissected features with summit depressions are present as isolated mountains across the Coprates Rise, the Thaumasia highlands, Daedalia Planum, and Terra Sirenum and are thought to be the ancient edifices of highland volcanic activity. Despite their location and morphology, they have been stratigraphically found to cross-cut younger terrains (ranging from Noachian to early Hesperian age) and thus are probably not the remnant massifs of a putative Tharsis impact basin. These putative ancient volcanoes are morphologically distinct from the Hesperian period Tharsis shield volcanoes; they have been hypothesized to be composed of interbedded layers of lava and pyroclastic rock comparable to terrestrial stratovolcanoes. If this analogy holds, it is possible that these explosions were explosive, and possibly even phreatomagmatic (water ice/magma interaction-driven). These volcanoes are observed to approximate features observed elsewhere on Mars, including Hecates Tholus on the Elysium Rise; Tyrrhena Patera; Hadriaca Patera; and Apollinaris Patera.