A B Cochetopa Dome, 11, 132' Sawtooth Mountain, 12,147' Green Mountain Cochetopa Hills--C.D. Sawtooth Mtn (NW caldera rim) Cochetopa Dome, 11,132 ft Tcdm (NE caldera rim) orth Pass (E caldera rim) 12,147 ft Razor Creek Dome N (intracaldera rhyolite flows) (northwest rim, 11,520 ft partly in clouds) (north caldera rim Green Mountain, 10,930 ft Tpff (northeast caldera rim) Archuleta Creek Cochetopa Canyon Cochetopa Van Tassel Gulch Intrcaldera tuffaceous sediments (Tpt) NE-trending tongue, Nelson Mountain Tuff

Caldera-filling tuffs and volcaniclastic sedimentary rocks SW NE Cochetopa Dome (11,132')

Sawtooth Mtn Tcdu (12,147'), NW caldera rim D Tcdm Tcdl C Tgt

Photo 19. Cochetopa Dome Rhyolite—sequence of lava flows within caldera pamphlet cover photo). A. Cochetopa Dome and north caldera walls viewed from south. Partly obscured in clouds at left of image is Sawtooth Mountain (12,147 ft), the northwest caldera rim; the north to northeast caldera rim is marked by Razor Creek Dome and Green Mountain at right. Light-colored eroding outcrops in foreground are weakly indurated tuff and volcaniclastic sedimentary fill of the caldera basin. In valley (head of Van Tassel Gulch) are buildings of the Quarter Circle Ranch, base for much of the fieldwork in the present study. B. Panorama of Cochetopa Dome and north topographic rim of caldera. The middle flow of Cochetopa Dome Rhyolite (Tcdm, 26.88 Ma) forms bold cliffs on the southeast flank of Cochetopa Dome; the timbered high point is in the upper flow (Tcdu). In foreground, intracaldera tuffaceous sediments (Tpt) underlie and interfinger with the rhyolite flows on Cochetopa Dome; this is site of geochron sample no. 03L-35, 26.83 Ma). To the right (east), Archuleta Creek cuts through caldera-margin fanglomerate (Tpff), which onlaps the intracaldera tuffaceous sediments. In far distance (from west to east) are erosional remnants of the Oligocene caldera topographic rim: Sawtooth Mountain (snow-covered), Green Mountain, and the Cochetopa Hills along the Continental Divide (C.D.). C. Sequence of the three major rhyolite flows on Cochetopa Dome (Tcdl, Tcdm, Tcdu), as viewed from east access road to Cochetopa Park (CP). In foreground are light-colored caldera-fill tuffaceous sediments (Tgt). D. Cochetopa Dome viewed from the southeast, just east of Quarter Circle Ranch road (CD). Prominent cliffs are the middle flow of Cochetopa Dome Rhyolite (Tcdm); timbered high point in center distance is the upper flow (Tcdu) at the summit of Cochetopa Dome (elev., 11, 132’). Fore- ground slopes (with cows) are underlain by tuffaceous sedimentary fill of the caldera (Tpt). E. Close-up of typical flow-laminated Cochetopa Dome Rhyolite. Mafic inclusion (light-colored andesite) is an unusual feature. West side of Cochetopa Dome (CD).