Question by amy e: how old are the rockie mountains?
Answer by Cristi@n
The Rocky Mountains are commonly defined to stretch from the Liard River in British Columbia, down to the Rio Grande in New Mexico. The mountains can also be considered to run all the way to Alaska or Mexico, but usually those mountains are considered to be part of the entire American cordillera, rather than part of the Rockies.
The younger ranges of the Rocky Mountains uplifted during the late Cretaceous period (140 million-65 million years ago), although some portions of the southern mountains date from uplifts during the Precambrian (3,980 million-600 million years ago). The mountains’ geology is a complex of igneous and metamorphic rock; younger sedimentary rock occurs along the margins of the southern Rocky Mountains, and volcanic rock from the Tertiary (65 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) (Peterson 1986; Knight 1994).
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). Recent 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 (Pierce 1979). Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation (Knight 1994). 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 (Grove 1990).
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