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1 1 lJ 1 Igneous Mountains The area to the south and west of the Norwood Pit is the lofty peaks of the San Juan Mountains. The San Juan's are the highly eroded remnants of more than twelve (12) giant volcanoes that blasted thousands of cubic kilometers of ash and breccia over fifteen thousand (15,000) square kilometers of southwestern Colorado. Fifteen (15) huge calderas, or collapsed tops of the volcanoes, have been mapped to date. The lava associated with the San Juan Mountains is the cooler rhyolite and andesite type forming breccia, tuff, and ash flows. The basal rocks of the San Juan Mountains are Precambrian quartzites, gneiss, and schist which are metamorphosed remnants of marine material when the North American Continent was coelessing. The Precambrian material has undergone much uplift throughout the history of this area. Lying unconformably on top of the Precambrian rocks is marine limestone and sandstone of Cambrian, devonian, and mississippian rocks, then pennsylvanian arkose sediments are found, and finally late cretaceous marine sediments. Before 38 million years ago in the Eocene epoch of the Tertiary period a gigantic dome of volcanic material welled up under the entire San Juan Region containing untold square kilometers of lava and raising the land appreciably. Starting about 38 million years ago in the Oligocene epoch through the Miocene epoch, both in the Tertiary period, tremendous eruptions and mineralization ensued until about 19 million years ago. There was additional Tertiary and Quaternary period uplifts with more lava entering the dome ending about 4 million years ago. Since, then, in the Quaternary Period, approximately 2 - 5 kilometers of material have been removed from the area. Most of the material was removed by ice age glaciers that carved numerous broad 'U' shaped valleys in the area. 5