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About 1,664 million years ago, a massive igneous intrusion occurred, today preserved as Boulder Creek Granodiorite and related pegmatites. The Boulder Creek batholith was the heat source for regional metamorphism. In a separate widespread intrusive episode, the granite of Hagues Peak (about 1,480 million years ago), mafic dikes (about 1,430 million years ago), and Silver Plume Granite and minor pegmatite (about 1,420 million years ago) were emplaced into the older biotite gneisses and schists throughout the Front Range. Deformation around these younger intrusions was localized but significant. For example, the near horizontal layering of banded schists and sheets of gray granite exposed in cirques east of the Continental Divide formed as Silver Plume Granite intruded into the older rocks and spread out laterally. Metamorphic effects related to the intrusions of Silver Plume Granite were relatively minor because the biotite gneisses and schists had already been recrystallized at high temperature and pressure.
Scientists do not know much about the Precambrian mountain ranges in which these rocks formed because they were worn away by erosion at the end of Precambrian time. The convoluted gneisses and schists, however, do tell us that Precambrian tectonic activity occurred repeatedly. Major faults produced during Precambrian events were later reactivated and play important roles in localizing mountain building in both Pennsylvanian and Laramide times.
Paleozoic and Mesozoic Eras
Late Paleozoic mountain building (Pennsylvanian Period, about 300 million years ago) formed the Ancestral Rocky Mountains, which were elevated along some of the faults in the crystalline (Precambrian) basement. In general, the Paleozoic structures are not well preserved or are covered by later strata. Furthermore, in many areas where they are exposed, the possibility that later deformation reactivated and rotated these earlier structures cannot be completely ruled out (Kluth, 1997). Interpretation of the Ancestral Rocky Mountains, therefore, is based on thickness, facies, and contact information within the sedimentary rocks deposited near the uplift during the orogeny (Kluth, 1997). The position of mountain uplift during Pennsylvanian time is coincident with the present- day Wet Mountains, Front Range, Medicine Bows, North Park Basin, Gore Range, and Park Range (Sonnenberg and Bolyard, 1997). As such, they extended from Boulder to Steamboat and were longer than the Front Range of today.
The area remained above sea level well into the Jurassic Period (about 150 million years ago). During this time, sediment deposition occurred in rivers, swamps, and dunes. These deposits are preserved outside of the park. By Cretaceous time, however, seas once again spread across the land. Evidence of the Cretaceous Interior Seaway is preserved in Rocky Mountain National Park in the form of Pierre Shale. The summits of Howard Mountain and Mount Cirrus in the Never Summer Mountains consist of this marine shale that was once mud at the bottom of the Late Cretaceous sea.
By the end of the Cretaceous Period and beginning of Tertiary Period (70 million years ago), the land was once again rising from the sea during a time of mountain building known as the Laramide orogeny. The events to follow consisted of a long series of repeated uplifts, periods of volcanism, and episodes of erosion (Richmond, 1974).
In addition to deformation expressed in the mountain uplifts, substantial Laramide deformation occurred in the bordering basins (Tweto, 1980a). These basins subsided concurrently with the uplift of the adjoining mountains. Each basin, e.g., the Denver, Raton, San Juan, Piceance, and Sand Wash, received Laramide orogenic sediments that constitute the principal record of events in the uplifts (Tweto, 1980b). Prior to uplift, thousands of feet of Paleozoic and Mesozoic sedimentary rocks overlay Precambrian crystalline rocks. Erosion, during and after the Laramide orogeny, removed the sedimentary rocks and some of the crystalline rocks from the range. The sedimentary rocks survive, however, in the flanking basins where they are commonly covered by Cenozoic rocks (Bradley, 1987).
Widespread volcanism affected most of the Southern Rocky Mountains in Oligocene time (Steven, 1975). The lava flows at Mount Richthofen mark an episode occurred 29.5 million years ago. Near Specimen Mountain, mudflows, lava flows, and obsidian are indicators of more recent activity (27.5 million years ago). In the Troublesome Formation (lower Miocene and upper Oligocene), lava flows in the lower units and ash beds in the upper units record volcanic activity 23 to 29 million years ago (Braddock and Cole, 1990). Volcanic ash is preserved as rhyolite welded tuff (late Oligocene) near Iceberg Lake along Trail Ridge Road. Volcanism was accompanied by igneous intrusion (Steven, 1975). The granite of Mount Cumulus stock (28.5 million years ago) is evidence of this.
A period of erosion continued after widespread Oligocene volcanism and eventually removed many of the volcanoes that formed in the Front Range area. Widespread fluvial deposition of volcanic and Precambrian clasts started in middle Miocene (14 to 11.5 million years ago); beds in the upper Troublesome Formation are evidence of this (Steven and others, 1997).
Broad uplift, block faulting, and erosion characterize Neogene (Miocene and Pliocene) time. Considerable scientific debate continues concerning the origin of the topographic difference between the mountains and flanking lowlands that formed during this time. Does the escarpment indicate Neogene faulting along the mountain front or is it caused by differential erosion of the rocks on opposite sides of the Laramide- age faults? Some evidence shows that nearly all of the modern relief along the eastern escarpment of the Front Range is due to differential erosion (Leonard and Langford, 1993). Other evidence, from the White River Plateau and Elkhead Mountains, suggests that there was nearly 2,000 feet (610 m) of offset during Neogene time (Tweto, 1980a). The debate continues, and new concepts and evidence are fueling the dialogue (e.g., Pederson and others, 2002).
The elevation of present- day mountains attests to Neogene events, as does a deposit in the park: a diamicton�unsorted, unstratified deposit�near Meeker Campground. This deposit is evidence of the Tertiary tectonic history in the area. The diamicton was deposited on a surface developed during Tertiary time that was later uplifted and deeply dissected (Wahlstrom, 1947; Madole, 1982).
The diamicton also played a role in interpreting the Quaternary glacial history of the area. The diamicton resembles till, but investigators have interpreted it as a fan deposit of Neogene or Quaternary age (Braddock and Cole, 1990), thereby having an alluvial origin, not a glacial one. This distinction is important because of the implications it has on Cenozoic climatic history. With respect to the location of this diamicton, if it were of glacial origin, early Pleistocene (i.e., pre- Bull Lake) glaciers must have been much larger than glaciers during later Bull Lake and Pinedale advances. A glacial interpretation of this diamicton goes against the overall pattern of glaciation documented thus far in the Front Range and neighboring ranges; that is, the extent of early and late Pleistocene glaciation was not notably different (Madole, 1976b).
The most recent regional tectonic event that influenced the Front Range was general uplift of the entire Southern Rocky Mountains, western Great Plains, and Colorado Plateau during the late Miocene and Pliocene, and possibly Quaternary time. Fault movement diminished after Pliocene time but continued on a minor scale through the Pleistocene and into the Holocene (Kirkham and Rodgers, 1981; Widmann and others, 1998). Uplift rejuvenated erosion and caused the long recognized �canyon cycle� (Lee, 1923) that cut deep canyons in the mountains. Canyon cutting probably continues today (Tweto, 1980a).
Uplift of nearly a mile (1.6 km) and worldwide cooling during the last few million years caused a profound transformation of climates. At times, winter snow at high elevations no longer completely melted during the summer but piled up in favorable locations, ultimately forming large valley glaciers. Although several glaciations occurred during Pleistocene time, geologists distinguish deposits of only three ages within or in close proximity to the boundaries of Rocky Mountain National Park. These glaciations are referred to as pre- Bull Lake, Bull Lake, and Pinedale. Investigators have identified deposits of Bull Lake and Pinedale glaciations in the park.
The glacial chronology of Rocky Mountain National Park is a work in progress. The beginning of the Bull Lake glaciation is based on correlations to marine isotope stage 6, which began about 180,000 years ago (Imbrie and others, 1984). Researchers estimated the timing using sea- surface temperature recorded in the oxygen- isotope compositions of the shells of foraminifera. Using oxygen isotopes, investigators date the end of the Bull Lake glaciation at 127,000 years ago (Bassinot and others, 1994). An interglacial (warm) period followed the Bull Lake glaciation, during which time Bull Lake glaciers completely melted away. Another glaciation followed for which there are good estimates of the timing of events. Using radiocarbon ages of organic-rich sediments at several high- elevation sites (Nelson and others, 1979; Madole, 1976b, 1980, 1986), investigators have estimated the timing of Pinedale glacial advances and retreats. Pinedale glaciation began about 30,000 years ago and was at its maximum between 23,500 and 21,000 years ago. Final deglaciation occurred between 15,000 and 12,000 years ago. Prior to 10,000 years ago, all Pinedale glaciers disappeared from the area. Pinedale glaciation was the last true valley glaciation to affect the Front Range.
Another glacial advance that occurred in late Pleistocene time is referred to as the Satanta Peak advance. This cirque- glacier advance was the most extensive to occur after the large valley glaciers disappeared. Researchers have documented this advance at Sky Pond in the park (Menounos and Reasoner, 1997). The Satanta Peak advance began prior to about 14,400 years ago and ended about 10,000 years ago (Rich Madole, written communication, 2003). The deposition of the Satanta Peak moraine in Loch Vale appears to be coeval with the European Younger Dryas event (Menounos and Reasoner, 1997).
Ice accumulation during Holocene time was not extensive enough to be considered an ice age. Some investigators have applied the term �Neoglaciation� to ice accumulation during the Holocene, but Neoglaciation may be a misnomer for the Front Range (Benedict, 1981). Post- Pleistocene ice accumulation is limited to within cirques. Investigators have identified many deposits in Rocky Mountain National Park, and deposits record four intervals of ice accumulation in the vicinity of the park. From oldest to youngest, they are referred to as Ptarmigan (7,250�6,380 BP), Triple Lakes (5,200�3,000 BP), Audubon (2,400�950 BP), and Arapaho Peak (350�100 BP) (Benedict, 1985). The Ptarmigan advance represents a brief reversal of the warming trend that had begun about 10,000 years ago (Benedict, 1985). The Triple Lakes accumulation was the most extensive. Arapaho Peak deposits are the Front Range equivalent of Little Ice Age moraines (Davis, 1988). Like modern glaciers, Audubon and Arapaho Peak glaciers depended on wind- drifted snow for their existence (Benedict, 1985).
Braddock and Cole (1990) identified 34 snow banks and ice masses in the vicinity of Rocky Mountain National Park. These include the snow and ice bodies shown on 1:24,000- scale topographic maps published between 1957 and 1962. Fourteen of the ice masses have been named: Rowe Glacier (between Rowe Peak and Hagues Peak), Sprague Glacier (at Irene Lake in Spruce Canyon), Tyndall Glacier (at the head of Tyndall Creek), Andrews Glacier (east of Andrews Pass), Taylor Glacier (at the head of Icy Brook), Chiefs Head Peak Glacier (above Frozen Lake), Mills Glacier (on the east side of Longs Peak), Moomaw Glacier (south of The Cleaver), and the six St. Vrain Glaciers (outside of the park at the head of Middle St. Vrain Creek).
All but one of these snow banks or ice masses are on the east side of the Continental Divide; the exception is the mass above Murphy Lake (near Snowdrift Peak). All but one occur at the heads of cirques: north- , northeast- , or east- facing; the exception is the large snowbank northeast of Rowe Mountain, which is in a northeastfacing gully. Some of the ice masses, such as Andrews Glacier, are actively moving and can be considered actual glaciers; others are stagnant. Winter winds�sometimes at velocities that exceed 200 miles (320 km) per hour� blow snow over the Continental Divide where it drifts into the Pleistocene- age cirques and nourishes some small glaciers. The process of snow deposition gives these �wind- drift glaciers� their name. It is not known whether, over a long period of time, these masses are growing or shrinking (Braddock and Cole, 1990).
The periglacial features that exist today are relicts of previous climatic conditions. Most of these features are not active today (Rich Madole, written communication, 2003). Features include patterned ground and other periglacial deposits on ridgetops and upland surfaces, tongue- shaped and lobate rock glaciers, and colluvium (e.g., talus and scree), alluvium (e.g., deposits at the margin of lateral moraines).
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