Breccias in the vicinity of Spor Mountain have been interpreted as landslides from the topographic wall of the Thomas caldera (Lindsey, 1982). A megabreccia in the northern Drum Mountains is also interpreted as an intracaldera landslide; it is shown in a photograph of the black glass tuff member of the Joy Tuff.
SLIDE 21 shows the breccia at Spor Mountain on the east slope of Spor
Mountain, immediately west of Eagle Rock Ridge. It consists of crudely bedded
breccia of carbonate rocks in a carbonate matrix. At Spor Mountain, carbonate
rocks of Paleozoic age form the wall of the Thomas caldera. The breccia
overlies Drum Mountains Rhyodacite and underlies Dell Tuff.
SLIDE 22 shows the breccia of Wildhorse Spring north of Spor Mountain,
at a site south of the springs for which it was named. It consists of clasts
of Paleozoic rocks and rhyodacite in a matrix of rhyodacite. The breccia
underlies and interfingers with the breccia at Spor Mountain.