EarthCache goal: Introduce visitors to the glacial geology of Denali National Park.
Background:
Glacial erratics, or erosion-resistant boulders transported by glaciers, are strewn all over Denali National Park and Preserve. The erratic at this location, however, is only about 15 feet away from an off-trail outcrop of local rock. Some erratics have been carried enormous distances, like some Scottish rocks that ended up in southeast Ireland (600 miles), or rocks in Kentucky that originated from north of Lake Huron or beyond. The largest known erratic in the world is located in Alberta, called Big Rock or the ‘Okotoks’ erratic, and weighs about 16,500 tons.
House-sized granitic erratics located further west near the Park Headquarters (see Figure 1) arrived at their locations during the Browne Glaciation which occurred about 2 million years ago. The erratics were carried by ice from the Yanert Valley about 30 miles to the east.
Erratics tell a story about where the glacier plucked them from their original bedrock. Their composition can be matched with their parent rock to determine how far the glacier moved them. The Roadside Trail erratic is totally different from the local rock. Off-trail use is discouraged, but a piece of the local rock about a foot long from the rock outcrop is right next to the erratic on the northwest side. Remember that bushwhacking is often unnecessary – gravity has done the work for you to bring pieces of the uphill outcrop down to the erratic. The local rock is a pinkish-orange schist (metamorphic rock) with shimmering bits of mica, while the erratic does not match at all – it’s granite, an unmetamorphosed igneous rock, formed beneath the earth’s surface when a large magma body slowly cooled.
How rounded or angular the erratic is can also tell you how it was transported. A rough, angular surface suggests it endured relatively little erosion – maybe it wasn’t transported near the base of the glacier. A smooth, rounded boulder indicates it was well-eroded, traveling a great distance.
All these clues are put together to form an impression of the landscape thousands of years ago which underwent major changes that continue today. Researchers are investigating and monitoring these changes in Denali National Park, and they’ve found incredible evidence for climate change, including: Denali tundra is blooming earlier and snow is melting a month early, tundra is becoming brushier and giving way to forests, the regional tree line is moving farther north, and permafrost temperature is increasing.
Figure 1: Park Headquarters erratic. Note the person for scale on the right side (Photo: NPS)
Denali glaciations:
Denali National Park and Preserve and the surrounding region experienced at least seven glacial episodes in the Pleistocene and early Holocene (see Figure 2).
• 2.6 Ma: Teklanika Glaciation
• 2 Ma: Browne Glaciation – Glacial deposits are found as far as 30 miles north of the Park Depot and own the lower Savage River.
• 150-125 ka: Bear Creek Glaciation
• 140 ka: Dry Creek (or Lignite) Glaciation – Poorly preserved.
• 70 ka: Healy (or McLeod) Glaciation – Terminal moraine near Healy.
• 25-17 ka: Riley Creek (or Wonder Lake) Glaciation – Terminal moraine at creek near Park Depot.
• 8 ka: Carlo Creek readvance – Ice stopped within 9 miles south of the park entrance.
Figure 2: Extent of major glaciations in the Nenana River valley, just east of the park entrance (Image: NPS)
How to access this EarthCache:
This cache is practically on top of the Roadside Trail. Access this trail at the Denali Visitor Center right across the street.
Please remember to practice Leave No Trace ethics to preserve this experience and environment for future visitors:
• Hiking off-trail is not required at this EarthCache and is discouraged.
• Collecting samples is not allowed in National Parks.
• Please do not use rock hammers.
To log this EarthCache: Email the answers to the following questions to me.
1. Rub the surface of the erratic. Is it smooth or rough? Is it angular or well-rounded? Do you think it traveled a relatively short distance, or relatively long distance?
2. Which glaciation do you think brought this erratic here? Why?
Sources:
Beget, J.E., 1995. Tephrochronologic correlations between Quaternary loess and glacial sequences, Alaska. Abstracts with Programs – Geol. Soc. Of Amer. vol. 27, no. 6, pp. 61.
Beget, J.E., and Keskinen, M., 1991. The Stampede Tephra, a middle Pleistocene marker bed in glacial and eolian deposits of central Alaska: Canadian Journal of Earth Sciences, v. 28, p. 991-1002.
Bemis, S.P., 2004. Neotectonic framework of the north-central Alaska Range foothills. University of Alaska, Fairbanks, unpublished M.S. thesis, p. 153.
Brease, Phil. Denali National Park and Preserve in Geology of National Parks. Harris, Ann; Tuttle, Esther; Tuttle, Sherwood. 2004 Kendall/Hunt Publishing Company: Dubuque. 6th ed.
Collier, Michael. The Geology of Denali National Park and Preserve. 1997. Alaska Natural History Assocation.
Elias, Scott A. The Ice-Age History of Alaskan National Parks. 1995. Smithsonian Institution Press: Washington D.C.
Gilbert, Wyatt G. A Geologic Guide to Mount McKinley National Park. 1979. Alaska Natural History Assocation.
Lambert, David. The Field Guide to Geology. 2007. Checkmark Books, New York.
Okotoks Erratic (Government of Alberta)
Thorson, R.M., 1986. Late Cenozoic glaciation of the northern Nenana valley, in Hamilton, T.D.; Reed, K.M.; and Thorson, R.M., eds., Glaciation in Alaska – The geologic record, Alaska Geological Survey, p. 99-122.
This EarthCache was created by the Geological Society of America summer 2010 GeoCorps intern at Denali National Park and Preserve.