Currently we are in the Holocene Epoch of the Quaternary Period of the Cenozoic Era (see Figure 1 on Geology page). The Holocene began approximately 10,000 years before present (YBP) when global climatic conditions warmed, resulting in the last of the continental glaciers retreating from the North American and Eurasian continents. Thus, the start of the Holocene coincides with the end of the Ice Age.
Originally, the Pleistocene was intended to coincide with the Ice Age. Scientists used the position in a stratigraphic column where fossils changed to cold tolerant species to mark the start of the Pleistocene, or the start of the Ice Age (Andersen & Borns, 1997). The date for the change to cold tolerant species, however, varies with location, making it difficult to pin an exact date for the start of this epoch. Today, scientists have agreed that the start of the Pleistocene is defined by a stratigraphic section in Italy, and it began approximately 1.7 million years before present (Andersen & Borns, 1997). Evidence suggests however, that mid-latitude glaciations began approximately 2.5 million years before present, in the Pliocene Epoch. Thus the Ice Age spans the Pleistocene and the latter part of the Pliocene Epoch.
The Pleistocene is subdivided into the Early, Middle and Late Pleistocene. The Late Pleistocene began approximately 130,000 YBP with a warm period, the Sangamon Interglacial. Cooling began approximately 115,000 YBP, and this marks the start of the Wisconsin Glaciation (Dawson, 1992). Not only did numerous glacials and interglacials occur during the Pleistocene, but more than one glacial advance and retreat occurred during the Wisconsin Glaciation. Determination of the sequence and timing of these glacial events requires examination of various types of information, including the stratigraphic sequencing of deposits, soils, ash layers, tree ring analysis and the relative position of landforms on the landscape.
Pre-Wisconsin Glacial Advances
Soils provide evidence for glacials and interglacials prior to the Wisconsin glaciation in the Glacier National Park region. Karlstrom (1988) examined buried (old) and exposed (young) soil samples from several ridges in Glacier National Park, including Saint Mary Ridge, which is considered an example of a lateral moraine. On the Saint Mary Ridge, Karlstrom found at least five soils separated by five tills. Examination of all the soil samples showed that pre-Wisconsin soils were morphologically and mineralogically different from modern soils: among other things, the depth of leaching was greater, the degree of oxidation was greater and clay mineral alteration was greater in the older soils (Karlstrom, 1988). Karlstrom (1988) concluded that these buried soils represented interglacials that were warmer and wetter than at present. Magnetic polarity of materials in some of the buried soils indicate they were deposited during a time period when the magnetic poles were reversed from today, and this provides an approximate time bracket for these samples (Karlstrom, 1988). In this case, soil analysis provides a means for approximating the number of glacials and interglacials (which is when soils form), the climatic conditions during the interglacial, and in some cases, approximate dates for these events.
Events During the Wisconsin Glaciation
During the Wisconsin Glaciation, two extensive ice sheets occurred in North America: the Laurentide Ice Sheet to the east of the park which covered much of Canada and the northern U.S. east of the Rocky Mountains, and the Cordilleran Ice Sheet to the west of the park which formed from valley glaciers throughout the Canadian Rocky Mountains (Carrara, et al., 1986). The alpine glaciers existing throughout Glacier National Park flowed together forming large valley glaciers. These glaciers merged to form piedmont glaciers where the landscape flattens at the edge of the mountains (Carrara, et al., 1986). These piedmont glaciers interacted with the Laurentide and Cordilleran Ice Sheets.
According to Andersen & Borns (1997), glaciers in the western portion of North America were at their maximum during the last major advance approximately 14,000 to 15,000 YBP. Carrara (1989), however, states that late Wisconsin valley glaciers reached their maximum extent approximately 22,000 to 23,000 YBP in the Colorado Rockies. At this time, the Saint Mary Glacier on the eastern side of the park extended beyond the mountains and into Canada (Carrara, 1989). The till from the Saint Mary glacier is covered by till from the Laurentide Ice Sheet (Carrara, 1989). The implication is that the valley glaciers in Glacier National Park extended to areas not covered by the Laurentide Ice Sheet 22,000 to 23,000 YBP, retreated, and the till left by these valley glaciers was subsequently covered by a readvance of the Laurentide Ice Sheet. Thus the timing of events in the mountains and on the rest of the continent are not synchronous, and in this case, the stratigraphic position of various tills allows us to estimate the sequence of events, assuming that younger material overlies older material.
Although the stratigraphic position of tills indicates that at 14,000 to 15,000 YBP the Saint Mary Glacier was no longer at its maximum, it does not reveal the exact position of the glacier at that time. Analysis of ash layers and tree rings provides a means for estimating the timing of deglaciation in the Glacier National Park region.
Three volcanic ashes are found in the Glacier National Park region, including Mount Mazama ash, Glacier Peak G ash and Mount St. Helens Jy ash (Carrara, 1989). The Mazama ash originated from Mount Mazama, now Crater Lake, Oregon, and is dated at 6845 YBP (Carrara, 1989). Glacier Peak is located in Washington, and only one of several ashes from this volcano is found in the park, the G ash which is dated at 11,200 YBP (Carrara, 1989). Mount St. Helens, also in Washington, is fairly active producing much ash. Only the Jy ash from Mount St. Helens is found in the park. The Jy ash is dated at 11,400 YBP and stratigraphically, is found below the Glacier peak G ash (Carrara, 1989). Carrara (1989) provides details for distinguishing these three ashes from each other.
These three ashes have been preserved in bogs, lake sediments, soils and exposures throughout the park and the surrounding region. In the Lake McDonald region, on the west side of the park, the Glacier Peak G ash has been found at several sites, indicating this area was deglaciated before 11,200 YBP (Carrara, 1989). In the region of Marias Pass, which is located at the southern edge of the park, lake sediments contained the Glacier Peak G ash underlain by the Mount St. Helens Jy ash (Carrara et al., 1986). The presence of these ashes indicates this region of the Continental Divide was ice free before about 11,400 YBP (Carrara, et al., 1986). In the Lower Saint Mary Lake region, one bog contained the Glacier Peak G ash and the Mazama ash, which indicates the Saint Mary glacier no longer covered this area approximately 11,200 YBP. In all three locations, the lowermost ash layers were often underlain by fine-grained sediments, most likely glacial outwash sediments deposited after glaciers retreated but prior to ash deposition, indicating the areas were deglaciated prior to the ash date by several hundred to several thousand years (Carrara, 1989). This information still does not tell us where the glaciers were, but rather where they were not.
End Moraines and Holocene Glacial Activity
Many of the existing glaciers and snowfields within the park are fronted by moraines. These moraines generally lie at altitudes between 1900-2400 meters (6234-7874 feet) and within 1-2 km (3281-6562 feet) of their cirque headwall (Carrara & McGimsey, 1988). In some instances there are two groups of moraines, the older group and the younger group.
The moraines of the older group are generally less than 10 meters (33 feet) high, are well vegetated, and are found only a slight distance downvalley from the younger moraines (Carrara & McGimsey, 1988). Moraines of the older group are found in approximately 25 locations throughout the park (Carrara & McGimsey, 1988). Mount Mazama ash has been found in the soil of some of these moraines; since the ash overlies the moraines, the moraines must have formed more than 6845 YBP (Carrara & McGimsey, 1988). Because there is no organic material in the moraines for radiocarbon dating, the actual age of the moraines is unknown (Carrara, 1989). It is also unknown whether these moraines are the product of a separate advance or the product of the last stillstand of late Wisconsin glaciers (Carrara, 1989).
Moraines of the younger group are found at over 150 sites throughout the park (Carrara & McGimsey, 1988). These moraines are sharp-crested ridges up to 50 meters (164 feet) in height, lack soils or ash and are generally devoid of vegetation (Carrara & McGimsey, 1988). Tree ring studies of trees located just beyond the outermost of the younger moraines at Agassiz and Jackson Glaciers indicate retreat from this position occurred in the mid-19th century (Carrara & McGimsey, 1981). Neoglaciation refers to the regrowth of glaciers after their minimum extent in the early Holocene (Benn & Evans, 1998). This includes the Little Ice Age, a cool period lasting from the late 16th to early 20th centuries, that caused glaciers to advance worldwide at high latitudes and altitudes (Benn & Evans, 1998). Thus, the moraines of the younger group are most likely a result of glacial advance culminating during the Little Ice Age. Since older moraines are not found in many locations with younger moraines, the glacial advance that produced the younger moraines most likely overran the older moraines (Carrara & McGimsey, 1988). This indicates the cool period producing the younger moraines was the most severe since the end of the Wisconsin Glaciation (Carrara & McGimsey, 1988).
At some sites, such as Sperry Glacier, Carrara & McGimsey (1988) found several moraine crests belonging to the younger group. This indicates more than one advance or pause occurred during the last several hundred years.
Modified from Carrara & McGimsey (1988) with information from Andersen & Borns (1997) and Carrara (1989).
Created March 15, 1999
by Karen A. Lemke (firstname.lastname@example.org). Last
updated March 15, 1999 by KAL.
The URL for this page is http:/www.uwsp.edu/geo/faculty/lemke/gnp_vft/general_information/glacial_info.html