Geological Survey Circular 1085 Our Changing Landscape: Indiana Dunes National Lakeshore

(Gary North)

The surface of the Earth is ever changing. During the past million years, massive ice sheets advanced and retreated repeatedly over North America. Massive sand dunes, such as those in the Middle East today, have migrated across eastern Colorado and Nebraska. Today, this area is part of the "breadbasket" of the Nation. Vegetation stabilizes the dunes. When human activities or changes in precipitation alter the vegetation, the dunes can become unstable. The "Dust Bowl," which drove thousands from their farms, was caused by severe drought in the 1920's and 1930's and by land-use and agricultural practices. A change in the temperature of the Earth's atmosphere can alter the climate significantly, including where and how much precipitation (rain and snow) will fall.

Certain gases in the atmosphere of the Earth (such as water vapor, carbon dioxide, methane, and nitrogen oxide) are called greenhouse gases. These gases behave like glass panes in a greenhouse. They permit the passage of short wavelength radiation from the sun but trap energy reradiated from the Earth's surface. The Earth's atmosphere warms more than would be expected under normal conditions. These greenhouse gases occur naturally, but human activities, such as the burning of oil and gas for heat, light, and transportation, also contribute to the amount of greenhouse gases in the atmosphere. Sediments, rocks, and glaciers record past changes in the Earth's environment and atmosphere. They often contain annual layers, and in special situations even contain seasonal layers. Corals produce annual growth bands similar to tree rings. These coral bands record temperature changes and changes in water runoff from land. Ocean, lake, or bog sediments can be banded (varved). These bands record the seasonal changes in sediment accumulation and thus reflect temperature and amounts of rainfall and runoff. Sediments often contain skeletal remains of organisms and pollen from plants, which both provide valuable information about past climates and environments. Changes in composition of the Earth's atmosphere are preserved in the ice of glaciers and ice sheets. Atmospheric gases, wind blown dust, and volcanic ash are contained within the annual accumulation of snow, which eventually becomes layered glacier ice.

Changes in storm frequency and tracks, as well as the resulting variations in the amounts of rainfall and evaporation, can change lake levels. (Gary North)

Anticipation and prediction of the changes in climate are necessary for evaluation and design of mitigation strategies. For example, if the precipitation pattern for the United States changes, the Nebraska Sand Hills could again begin to migrate, destroying farm land and covering the wetlands between the hills. The wetlands are an essential part of the central-flyway ecosystem for migratory birds.

Changes in sea level are closely linked to changes in the temperature of the Earth's atmosphere. During cold periods, such as that of 18,000 years ago, water was stored in ice sheets and sea level was 410 feet lower. As the great ice sheets in North America and Eurasia melted, sea level rose as rapidly as 8 feet per century. During the last 16,000 years, sea level has been rising. It is still rising (about 8 inches in the past century), possibly in response to global climate warming. The continuing rise intensifies the concern for coastal erosion nationwide.

Patterns of vegetation, glaciers, and sea-surface temperatures were very different 18,000 years ago. During cold periods, a considerable volume of water was stored in ice sheets. As a result, sea level was 410 feet lower than present-day sea level. Note the size of Florida as compared to today.

Even in the Great Lakes, water levels change. In 1986, water levels in the Great Lakes reached their highest levels in this century. Within Indiana Dunes, four bands of dunes record geologic evidence demonstrating that lake levels in the recent geologic past, several hundred to several thousand years ago, exceeded present-day and known historic levels.

Left: Atmospheric gases, windblown dust, and volcanic ash are incorporated into glaciers with the annual snowfall, such as near Anchorage, Alaska. An annual record of the Earth's atmosphere is present within glaciers. The greenhouse gas carbon dioxide is increasing in the atmosphere and is believed to be contributing to global warming. The present concentration of carbon dioxide in the atmosphere is 25 percent higher than it was at the beginning of the industrial era, in the 1700's. (Gary North) Changes in sea level are closely linked to the melting of glaciers as the temperature of the atmosphere rises, such as the Engabreen Glacier in Norway (top right)(Nils Haakensen) and glaciers in the Chugach Mountains in Alaska (bottom right)(Gary North).

Indiana Dunes contains the geologic history of the past 14,000 years. This history documents that global change occurred as the massive North American ice sheets melted and retreated northward when the climate warmed. Each successive band of dunes is younger and records a progressively lower lake level, starting with the southernmost Glenwood Beach Ridge (approximately 14,000 years old) and continuing to Mount Baldy on the present lakeshore (approximately 1,000 years old).

Indiana Dunes is ecologically unique. Here the Canadian conifers meet the temperate hardwood forests of the Northern and Eastern United States, as well as the tallgrass prairies of the Midwest. If our present climate changes, Indiana Dunes may well be one of the first places to reflect these changes. Some changes have already occurred. In the 1800's, old growth white pine was logged in the Indiana Dunes area. The regrowth of white pine has not done well, possibly as a result of the burning of fossil fuels (coal) in the 1880's and the advent of industrialization. Their tree rings record a reduced growth, and the few white pine seedlings that are produced today are quickly eaten by deer. Not only is the land surface ever changing, but associated plant communities often can reflect even the most subtle changes in climate and environmental conditions.

Location of dunes and ridges showing historic lake levels at Indiana Dunes National Lakeshore. (click on image for a PDF version)