The core of Glenlyon and northeastern Carmacks map areas is underlain by a northwest-trending belt of metasedimentary, metavolcanic and (meta)plutonic rocks of the Yukon-Tanana Terrane. It includes two successions of Carboniferous arc volcanic rocks, associated plutonic suites of Mississippian age, Devonian-Mississippian metaclastic rocks, and their basement complex. To the southwest, Yukon-Tanana Terrane is juxtaposed with the Semenof block a belt of mafic metavolcanic rocks of uncertain terrane affinity along the Needlerock and Big Salmon faults. To the northeast, the Tummel fault zone delineates the contact between Yukon-Tanana and Cassiar terranes. The narrow belt of chert, argillite and greenstone which occurs within the Tummel fault zone probably correlates with the Slide Mountain Terrane. The area is intruded by Early Jurassic and Cretaceous plutons and is dissected by a series of late faults, which results in approximately 56 km of dextral offset of the Yukon-Tanana Terrane.