Gift of the Glaciers

The peaceful lake country we enjoy today had its origin in great natural disturbances.  At least four times in the last million years the Superior National Forest area was overridden by huge sheets of glacial ice from the north.  Each time the ice advanced, it scraped and plowed the land beneath its crushing weight, and each time the ice retreated it left a changed landscape.  Glacial Striae (huge scratches) are easily visible on the exposed rocks at our camp.

A million years ago, before the first invasion of ice, northeastern Minnesota probably resembled the hill county of Kentucky and Tennessee as it is today.  Streams and tributaries were numerous; lakes were rare.

The only herald of the coming of the first great ice sheet was a hint of the change in the air.  The climate was slowly becoming cooler and drier, and forests of spruce and other conifers-much like those in the area today-began replacing the forests that had thrived here in warmer times.

As the climate cooled, the first glacial ice-which would reach the Superior Forest are 2,000 years later-began forming in Canada.

Snows came earlier and stayed later each year, until there was snow on the ground year round in the mountains of eastern Labrador.  The snow piled up, and underlying snowflakes consolidated into granules of ice under the influence of moisture and gentle pressure caused by the weight of new snow.  As more snowfalls added their weight air was squeezed from between the ice granules until toward the bottom of the accumulating mass, the ice became highly compacted.

The accumulating snow and ice became deepest over the Hudson Bay area, and from there the glacial ice was forced outward in all directions.  Dome-like in structure, the ice sheet eventually became nearly tow miles thick over a large area and covered almost five million square miles.  At its maximum the ice covered all of Canada and reached as far south as the Mississippi Valley.

The moving glacier plowed up whole forests, leveled rock outcroppings, stripped the land of soil, and quarried out the massive blocks of bedrock.  The resulting debris became embedded in the undersurface of the glacier, and rock fragments, acting like the teeth of a file, grooved and polished the bare rock below as the ice inched forward.

Thousands of years passed, and again there was a hint of change in the air.  The climate warmed; winters became shorter; snow ceased to pile up.  Pressure on the moving ice was lessened, slowing the glacier's advance.  Finally the forward movement of the glacier was counteracted by melting action, primarily along the ice front.

Not until about 1870 did scientists learn that huge sheets of glacial ice had formed more than once during the last million years and that long inter-glacial ages had intervened.  At least three other times during this period the climate cooled and glacial ice moved down from Canada, and each time the land lay buried under ice for centuries until freed by warming climate.  The most recent glacier disappeared approximately 11,000 years ago.

The retreat of the last glacier left a stark and barren version of the present lake county.  Rough mounds and ridges of glacial debris, strewn with boulders and surrounded by a labyrinth of lakes and streams, characterized the glaciated landscape.  The hills of pre-glacial times had disappeared, and the normal pattern of major streams and their tributaries and been drastically modified.  Drainage was chaotic as melt water cut through debris in torrential fashion, fillings depressed areas and spilling over basin rims as it sought outlets to ever-lower levels.

Lakes formed in 3 ways behind the retreating ice:
1.    Blocks of ice-some as big as houses-broke off from the glacier and were buried in debris left helter-skelter by the melting ice.  These blocks thawed over many centuries, leaving depressions in which lakes formed.  These are kettle lakes, common throughout the lake country.
2.    The melting glacier deposited masses of debris unevenly.  Lakes formed (a) in natural depressions in the debris and (b) behind natural dams created when debris was deposited across stream beds.
3.    Still other lakes formed where the glacier had gouged out basins in the bedrock.

As glacial climate warmed and the frozen ground thawed, lichens and mosses, and then ferns, flowering plants, and forest trees returned to cover the barren landscape.  Gradually they took possession of all types of surfaces and, and as one generation after another died, built up the quality of the soil.  Now the land areas of the Superior Lake county are almost completely forested.

Decaying plant material and other sediments, collecting in the lake over the centuries, have entirely filled in the more shallow basins.  Today's swamps and bogs-originally crystal clear lakes-are still in the process of filling in.

So nature has fashioned and is still at work in the Superior Lake country that we enjoy today.  Thousands of lakes dot the landscape, and a tangle of streams flows in every direction.  Smoothed, polished, and grooved rock surfaces may be seen on numerous islands, and tops of rocky crags the ice failed to remove.  Most of the larger exotic animals of pre-glacial and glacial times have perished but plant types have remained much as they were before the coming of the ice.

Adapted from: Superior National Forest, Eastern Region, Forest Service, U.S.D.A.