Batholith

Armstrong and others (1977) called the northern part of the hourglass the "Bitterroot" lobe and the southern part the "Atlanta" lobe. He also proposed that most of the southern lobe was emplaced 75 to 100 million years ago (Late Cretaceous); whereas the northern lobe was emplaced 70 to 80 million years ago. Armstrong (1977) further noted that older plutons of Jurassic age occur on the northwest side of the Bitterroot lobe and many Eocene plutons have intruded the eastern side of the Atlanta lobe of the batholith. On the western side of the batholith, there are more mafic plutons (quartz diorites or tonalites) than to the east.
Radiometric dates and field relationships, where plutons of the batholith cut older rocks, restrict the age of the Idaho Batholith to an interval between 180 million years ago (Late Triassic) to 45 million years ago (Eocene); however, the dominant interval of emplacement was Early to Middle Cretaceous. There is a general west-to-east decrease in age for plutons of the batholith.