<P> Periods of glaciation occurred from the Pleistocene Epoch (1.8 million--70,000 years ago) to the Holocene Epoch (fewer than 11,000 years ago). The ice ages left their mark on the Rockies, forming extensive glacial landforms, such as U-shaped valleys and cirques . Recent glacial episodes included the Bull Lake Glaciation that began about 150,000 years ago and the Pinedale Glaciation that probably remained at full glaciation until 15,000--20,000 years ago . Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation . The little ice age was a period of glacial advance that lasted a few centuries from about 1550 to 1860 . For example, the Agassiz and Jackson Glaciers in Glacier National Park reached their most forward positions about 1860 during the Little Ice Age . </P> <P> All of the geological processes, above, have left a complex set of rocks exposed at the surface . For example, in the Rockies of Colorado, there is extensive granite and gneiss dating back to the Ancestral Rockies . In the central Canadian Rockies, the main ranges are composed of the Precambrian mudstones, while the front ranges are composed of the Paleozoic limestones and dolomites . Volcanic rock from the Cenozoic (66 million--1.8 million years ago) occurs in the San Juan Mountains and in other areas . Millennia of severe erosion in the Wyoming Basin transformed intermountain basins into a relatively flat terrain . The Tetons and other north - central ranges contain folded and faulted rocks of Paleozoic and Mesozoic age draped above cores of Proterozoic and Archean igneous and metamorphic rocks ranging in age from 1.2 billion (e.g., Tetons) to more than 3.3 billion years (Beartooth Mountains). </P>

Which mountain range is older the rockies or the appalachians