A Land Sculpted by Ice
In the high, frozen regions of the Alaska Range, snow and ice are the main forms of precipitation. In the past, most of the snow and ice remained behind; very little melting occurred. Snow and ice accumulated and got deeper and deeper, year after year, until the mass of ice that formed was so thick it compressed under its own weight. Gravity caused this ice to flow through stream valleys as glaciers. Glaciers are rivers of ice. The glaciers we see today in the park are increasingly small remnants of their former selves, but all around us we can see evidence of how they dominated the landscape. During past ice ages, most recently about 10,000 years ago, glaciers covered the Alaska Range and much of Alaska in ice. All of south-central Alaska has been buried in ice numerous times, and the shape of the land in this area comes from the carving forces of glaciers and the debris they leave behind.
Glaciers are often fed by more snow and ice precipitating and accumulating at higher elevations. If ice builds up at its source, a glacier may flow at rates ranging from several feet per year to several feet per day. As a glacier flows downhill, it grinds away at its beds with tremendous force. It picks up rocks from its bed, grinding some to a fine powder called silt and by plucking up larger chunks. When the glacial ice melts, the silt is carried along in the meltwater to be deposited downstream as outwash. Streams flowing from melting glaciers are often milky-colored. The silt in the water is called glacial flour, and the silty water is described as glacial milk. The larger chunks get left behind as erratics or in unsorted deposits forming ridges or hills called moraines. Erratics are rocks that are foreign to the surrounding terrain. They differ from the types of rock found where they are deposited.
The rocks embedded in glacial ice grind away at bedrock, forming the jagged ridges and deep U-shaped valleys found in the range. Large blocks of ice can be stranded in the moraines left behind by retreating glaciers. When they finally melt, a water-filled depression known as a kettle lake develops. The carving action of ice forms many of the elongated lakes in the upper Susitna Valley to the south of Denali, and examination of a map reveals that they are all oriented in the direction that the ice was moving