ES 331/767 Lecture

James S. Aber

Table of Contents
Introduction Pleistocene
Holocene Cucharas vicinity


The southern Rocky Mountains comprise several ranges and intervening basins in Colorado and northern New Mexico. Tectonic deformation of the Rocky Mountains took place during the Larimide Orogeny, beginning in the latest Cretaceous and continuing into the early Tertiary. This episode of crustal compression was completed in the Eocene in the southern Rocky Mountains. A shift to crustal stretching took place during the Oligocene and Miocene in connection with the Rio Grande Rift system. Volcanic eruptions and shallow intrusions were widespread, particularly in the San Juan Mountains, San Luis Valley, and Raton Basin. Since the Miocene, the southern Rocky Mountains and Colorado Plateau have undergone substantial vertical uplift, of at least a mile (1.6 km), and deep erosion by streams and glaciers has carved the modern topography.

General shaded-relief map of Colorado showing major physiographic features. Prominent mountain ranges glaciated during the Quaternary: 1 - Front Range, 2 - Sawatch Range, 3 - San Juan Mountains, and 4 - Sangre de Cristo Range. Map adapted from Colorado Geological Survey.

Approximately 30 ranges in the southern Rocky Mountains supported alpine glaciers during the Pleistocene and Holocene (Richmond 1986). Glaciation took place mostly in the form of cirque and valley glaciers, but in a few cases ice caps developed. The most extensive glaciation was in the San Juan Mountains of southwestern Colorado, where a complex of ice caps and valley glaciers covered about 5000 km². Other prominent glaciated mountains include the Front Range, Sawatch Range and Sangre de Cristo Range.

Winter aerial view looking northward along the crest of the Sangre de Cristo Range, southern Colorado. Present elevation and relief of the mountains are a consequence of late Cenozoic crustal uplift and deep erosion. Typical alpine landforms include glaciated valleys, cirques, arÍtes and horns. Image courtesy of J. Zupancic ©.

Pleistocene glaciations

Each range in the Rocky Mountains has its own unique history of glaciation and stratigraphy of glacial deposits. However to avoid confusion, stratigraphic terms from the Wind River Range in Wyoming are applied throughout (Richmond 1986). For the southern Rocky Mountains, stratigraphy is relatively well known for some ranges, whereas many other ranges have been studied only in a reconnaissance mode. The following summary is based primarily on the Front Range, Sawatch Range and San Juan Mountains in Colorado, plus the Sangre de Cristo Range in southern Colorado and northern New Mexico--see Table I.

Table I. Composite stratigraphy of Pleistocene glaciations in the southern Rocky Mountains, Colorado and northern New Mexico. Based on Richmond (1965, 1986) and Armour et al. (2002).
Glaciation Age Range* Oxygen-isotope
pre-Bull Lake 500,000-700,000
14 and 16
pre-Bull Lake 300,000 to 500,000
8 and 12
Bull Lake 130,000 to 300,000
6 (and 8)
Pinedale 12,000-23,000
Late Wisconsin

* Based on various radiometric and relative dating methods.

Two or more pre-Bull Lake glaciations are generally recognized. Pre-Bull Lake tills are preserved as isolated, strongly weathered and eroded patches that lack morainic landforms. The two most common tills are ascribed to oxygen-isotope stages 8 and 12 with an age range of 300,000 to 500,000 years ago. Additional older tills may correspond to oxygen-isotope stages 14 or 16 in the age range 500,000 to 700,000 years ago.

The Bull Lake glaciation is widely recognized in the southern Rocky Mountains. In most cases, two tills are preserved. End moraines are broad, subdued, and lack undrained depressions. Soil is well developed and most surface boulders are weathered. Bull Lake moraines extend farther downvalley than moraines of younger glaciations in most situations. Bull Lake glaciations took place mainly during oxygen-isotope stage 6, 130,000 to 300,000 years ago, which corresponds to the Illinoian glaciation of the mid-continent region. Some Bull Lake tills may relate to oxygen-isotope stage 8.

The youngest, extensive glaciation in the southern Rocky Mountains is the Pinedale. This glaciation is marked by multiple well-preserved moraines, which have experienced minimal erosion, weathering, or soil development. Undrained depressions, kettle holes, bogs, ponds and lakes are common on or between Pinedale moraines. The first Pinedale glacier advance is also the most extensive, dated approximately 23,000±1000 radiocarbon years ago. Many slightly less extensive ice advances took place during the next several millennia, and the Pinedale terminated before 12,000 radiocarbon years ago.

Roadside exposure of till in a Pinedale moraine, Taos Canyon, northern New Mexico. Photo © J.S. Aber

During the Pinedale interval, permafrost was widespread in high-altitude terrain of the Rocky Mountains in Colorado and to a lesser extent in New Mexico (French and Millar 2014). All the Pinedale glacier advances and permafrost relate to the Late Wisconsin stade of glaciation in the mid-continent region, corresponding to oxygen-isotope stage 2.

Holocene glaciations

The early and late Holcene were times of relatively cool climate, minor glaciations, and periglacial activity throughout the southern Rocky Mountains. Stratigraphic terminology has not been standardized for the Holocene, so each range has its own names for glacier advances, moraines, and deposits. In many cases, local glacial episodes remain unnamed. Holocene glaciers were confined to valley heads and cirques, particularly in northern Colorado. In southern Colorado and northern New Mexico, rock glaciers and other periglacial phenomena were active during cold periods, but glaciers were scarce. Once again, details vary from range to range, but broad similarities are found across the region--see Table II.

Table II. Composite stratigraphy of Holocene glaciations and periglacial episodes in the southern Rocky Mountains. Based on Richmond (1986) and Armour et al. (2002), but some glacial episodes remain controversial or uncertain.
Periglacial Episode
Age Range* Period
Grenadier, Temple Lake
Ptarmigan, Triple Lakes
9000 to 10,000 Younger Dryas
Earliest Holocene
Minimal glaciation or
periglacial phenomena
8000 to 5000 Altithermal
Early to mid-Holocene
Rock glaciers
2500 to 5000 Early Neoglacial
Late Holocene
Arapaho, Gannett Peak
Rock glaciers
120 to 400 Little Ice Age
Late Neoglacial

* Based on various radiometric and relative dating methods.

The latest Pleistocene and earliest Holocene span an interval of minor glaciation that represents the final cold phase following general retreat of Pinedale glaciers. Several named and unnamed moraines and tills come from this interval--Ptarmigan, Temple Lake, Triple Lakes, and Grenadier, although dating and correlation of some of these are debatable. This period, from about 9000 to 10,000 radiocarbon years ago, corresponds to a global event known as the Younger Dryas. It was a time of sharply colder climate and re-expansion of glaciers and ice sheets, particularly in Scandinavia.

The early Holocene cold phase was followed by relatively warm and dry conditions throughout the western United States during the early and middle Holocene, about 5000 to 8000 radiocarbon years ago. Known as the altithermal or hypsithermal, this phase witnessed little in the way of glacial or periglacial activity. The Great Sand Dunes and other dune fields were active in the Rocky Mountains and Great Plains during this interval. The mid-Holocene climatic optimum was a worldwide episode of warm conditions, which included, for example, the Atlantic Period in northern Europe.

See San Luis Valley (Great Sand Dunes).

The late Holocene was a time once again of cooler, wetter climate in the southern Rocky Mountains. Small glaciers reformed and advanced, and periglacial phenomena were also active again. This Neoglacial period began around 5000 years ago and continued until modern times. Two main phases are recognized. The first is marked by Audubon moraines and rock glaciers between 2500 and 5000 years ago. The second took place within the last several centuries, 120 to 400 years ago. The latter represents the Little Ice Age, which reached its culmination in the Rocky Mountains about A.D. 1850 based on historical record (Richmond 1965). The Neoglacial period was global in extent, and in many places the Little Ice Age advances were the most significant glaciations since the Younger Dryas.

Cucharas vicinity, Culebra Range

The upper Cucharas Creek valley heads on the slopes of Trinchera Peak in the Culebra Range, Sangre de Cristo Mountains of southern Colorado. Glaciation of the this vicinity was documented by Ray (1940), who recognized moraines of several advances and ages. Glacial history of the region was reviewed by Richmond (1965, 1986), and till deposits and rock glaciers were mapped by Wallace and Lindsey (1996).

During the Pleistocene, the Culebra Range supported many alpine glaciers. On the eastern side of Trinchera Peak, three glacial valleys descend toward Cucharas Creek valley--see Cucharas map. Glaciers within these three valleys joined to form a single glacier that flowed down to a maximum lower limit about 10,000 feet in elevation during the maximum Pinedale glaciation. During retreat of the glacier, the ice limit stabilized around 10,400 feet elevation at a point where Cucharas Creek valley is constricted by resistant bedrock. A substantial recessional moraine accumulated at the junction of the three tributary glaciers at this position. Composed of lateral and end segments, the moraine complex dams several small lakes--Blue and Bear Lakes, which are the site of a National Forest campground today.

Upper Blue Lake seen from above during autumn. Blue Lake is a "kettle lake" in the moraine complex within the valley of Cucharas Creek. This moraine was constructed during a recessional phase of the Pinedale glaciation; it is dammed by a lateral moraine (*) on the far side.
Steven Veatch demonstrates till on the valley side adjacent to Blue Lake. The till consists of red and gray sandstone fragments within a finer matrix. He points to a thin bed of sandy sediment (darker color) within the till. Photos © J.S. Aber.

The moraine at Blue and Bear Lakes is composed mainly of sandy till, in which red and gray sandstone comprise the larger rock fragments, which are angular to subrounded in shape. Many bear striations, grooves, and other glacial markings. Numerous ridges, knobs, and kettle holes make for rough terrain within the moraine complex. In the higher portions of the glacial valleys, till forms smaller recessional moraines, and a large lateral moraine is present on the southern side of uppermost Cucharas Creek valley. Erosional features include oversteepened bedrock steps, basins, cirques, and glacial pavements.

Head of Cucharas Creek valley with Trinchera Peak in the background (left). Trinchera Peak is a classic alpine horn, steepened by glacial erosion on all sides. Note tents (blue dot) on end moraine in valley to lower left side. This moraine rests on a bedrock bench and is the uppermost end moraine preserved on the valley floor. Closeup view of end moraine (right) in uppermost Cucharas Creek valley.
Left: Looking downvalley with the small end moraine (*) and West Spanish Peak in the right background. This moraine may date from latest Pinedale glaciation or from the Younger Dryas ice advance. It records the last time a glacier existed at the head of the valley. Right: elk herd in the alpine zone below Trinchera Peak. Photos © J.S. Aber.

In the alpine zone (above 12,000 feet), strong frost action has produced various periglacial features. Rock glaciers, talus streams, and stone festoons and terraces occupy steeper slopes on valley sides. On the gently sloping crest of Trinchera Peak, patterned ground is well developed, and smaller patches of patterned ground occur along the ridge crest to the north. Polygonal networks are formed by loose stones in shallow trenches surrounding raised turf mounds. Such features require deep seasonal ground freezing, if not permafrost, to form. All are considered to be Neoglacial in age, but lichen cover suggests that periglacial features on steeper slopes were active more recently than were stone polygons on gentle slopes--see

Stone festoons on the crest of Culebra Range. Overview (left) showing jeep trail; note ATV at lower right for scale. Close-up (right) of stone stripes, festoons, and terraces. These features creep downslope gradually with repeated freezing and thawing.
Left: small rock glacier (foreground) with Trinchera Peak behind. Note lobate zone of rock accumulation at the base of the slope. The rock lobe has flowed out onto the flat floor of the basin below the steep slope. Right: close-up view of lichens on stones of the rock glacier.
Left: overview of patterned ground on the crest of Trinchera Peak, approx. 13,500 feet elevation. Jay Aber sits on a turf mound at the center of a stone polygon. Right: close-up view of lichens on stones of the patterned ground. Photos © J.S. Aber.

Patterned ground in the Culebra Range (Vopata et al. 2006).

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ES 331/767 © J.S. Aber (2015).