Note the image from Larsen and Lipman.  The image is looking northeast from the point here.  It shows the Rio Grande graben eastern flank in the valley and the east-northeast face of the La Garita caldera.  The Fish Canyon Tuff at the top of the caldera has columnar jointing.  The Watson Park Tuff also has columnar jointing.  The Fish Canyon Tuff covered almost 11,000 sq miles - most of Colorado and 100 m thick. It erupted about 26.8 MA.  Stop 1.6 in Larsen and Lipman.