and domes that are interbedded with flows of alkali basalt and hawaiite. Formation of the shield was followed by collapse of a small central caldera in which the intracaldera assemblage was deposited. This assemblage consists of a thick sequence of tuffs that are overlain by a ponded single cooling unit of trachyte more than in thickness. The postcaldera assemblage comprises alkali basalt and hawaiite flows that issued from flank vents and from a few small vents within the central caldera. These are the youngest lavas of the Ilgachuz Range. The Itcha Range is the easternmost and youngest of the three AVB shield volcanoes, having formed between 3.8 and 0.8 million years ago. With a diameter of , the Itcha Range is also the smallest AVB shield. Its structure is unique among the AVB shields in that it consists of small coalescing volcanic units rather than a stratiform volcanic pile. The basal unit comprises a series of aphyric trachyte flows and domes with minor flow-banded rhyolite, rhyolitic tuffs and thin hawaiite flows. This unit is overlain by a sequence of alkali-feldspar porphyritic trachytes which occur as pyroclastic deposits as well as small lava flows and domes. These trachytes are overlain and intruded by alkali-feldspar porphyritic quartz-trachytic and trachytic plugs, lava flows, pyroclastic flows, debris flows and dikes in the middle of the shield. The uppermost shield-forming unit consists of megacrystic trachyte flows that are distributed throughout the western part of the Itcha Range. Formation of the shield was followed by the eruption of basanites, alkali basalts and hawaiites from small cinder cones, tuff rings and fissures in the eastern half of the shield. Volcanic fields The Satah Mountain volcanic field is a linear region of cones and domes extending south from the Itcha Range to just north of Chantslar Lake. Volcanism along the entire length of the volcanic field 2.5 to 1.4 million years ago was characterized by the eruption of generally small volumes of magma that erupted at various times and at various locations. The eruption rates were high enough to build a north–south trending ridge upon which most of the volcanoes are located. This elongated feature, known as Satah Ridge, rises above the surrounding plateau surface with an average topographic prominence of . The alignment of the Satah Mountain volcanic field corresponds with the orientation of two sets of normal faults in the Itcha Range, suggesting that volcanism there