Geologic
History
of
Lake Superior
Michigan,
Minnesota and Wisconsin are not thought by many Americans to be lands of
waterfalls. Minnesota is know as the land of 10,000 lakes and even Michigan is
quite proud of all its lakes. Northern Minnesota, particularly Minnesota's
'Arrowhead Country' - that area north of Lake Superior - has a string of
significant waterfalls due to the unique geologic history of northern North
America. Many waterfalls are also found along Wisconsin - the South Shore - and
'Upper Michigan' has many more significant waterfalls
Some
1.5 Billion years ago the area was covered by a shallow ocean which laid down a
fine, but heavy, sediment. As the layers increased in number and weight a kind
of shale was formed. Then a great continental drift started pulling the earth
apart in the area now covered by Lake Superior. This allowed molten lava to emerge forming
perpendicular sheets or 'dikes'.
Then
the planet became cooler, and cooler and we had the 'ice age'. A huge ice cap
reached all the way down from the Artic. Then, as the earth warmed again, this
ice cap began to melt forming individual glaciers which, very slowly, slid
downhill crunching everything in their paths. Aggravated by the rushing of
melting ice, torrents of water added to the erosion washing away the softer
rocks and the more easily removed minerals. What remain are the harder layers left
by the hardened lava. Now our rivers and streams, flowing toward Lake Superior,
encounter the drop-offs created by all this erosion and we have waterfalls even
though there are no true mountains.
For
a more complete history, see the excellent web site of
the Superior Hiking Trail Association:
Trail
Geology of the North Shore
