Summer visitors to Denali will quickly se we have a mild climate (at least in the summertime) I spite of being in a sub arctic environment, but was it always like this? How can we find out what the environment was like thousands, millions, or even billions of years ago? The answer is ROCKS! What can the rocks tell us about the environmental conditions here in ancestral Alaska, or even before Alaska was here?
The oldest, pre-Alaska picture is available right in Nenana Canyon, where perhaps you are staying in one of the hotels. The Nenana River cuts the basement rocks of Denali Park, exposing metamorphic types such as schists, gneiss, greenstones and marble. Before being metamorphosed, these rocks were seashore sediments (limestones, sandstones, shales and conglomerates) that had been deposited along the rugged mountainous coastline (complete with erupting volcanoes) of ancestral North American 1-2 billion years ago.
Although having gone through over a billion years of burial, heat and tectonic pressure, these rocks still display beach pebble textures in some locations in the canyon. You can see a good example of these rocks near the Save River where the park check station is located. Just ask for "Savage Rock," but don't expect to recognize the beach!
Perhaps the youngest environmental story of Denali is available just as you drive into the park past the railroad station and continue up the hill. The south end of the railroad bridge (crossing Hines Creek cuts through a terminal moraine (glacier ice limit) of the Riley Creek glaciation. This deposit of gravel represents a time of about 10,000 years ago, when glacier ice filled the valleys. About 20,000 years ago the ice actually extended over the tops of the ridges and covered the entire park. House-sized blocks of granite from 40 miles away were transported on these great sheets of ice and deposited on the ridge above Park Headquarters. Just ask at the Visitor Center or Park Headquarters for the "erratics", but be consistent about it!
For a really hot time try a visit to the Polychrome pass area, where volcanic eruptions poured lava and volcanic ask over 3000 feet thick all over the countryside about 56 million years ago. The dark rocks are basalt, which can flow easily like pancake syrup; the light rocks called rhyolite occasionally burst out of the ground like bubblegum, and sometimes blew the area to smithereens! A large batch of this lava (magma) never erupted at the surface, but cooled at depth for perhaps several million years to become the future Mount McKinley. Oh course, for Mount McKinley to "rise" up to 20,320 feet in elevation required several other environments and millions of years that we don't have time or space for here. Just ask for "awesome"!