Geologists still debate (of course they do, they’re geologists) what triggered the last Ice Age, but they agree how glaciers form.
The lovely carol “In the Bleak Midwinter,” with Christina Rosetti’s lyrics set to music by Gustav Holst, pretty much sums it up: “Snow was falling snow on snow on snow.”
There you have it. The easiest concept in science: More snow fell than melted.
This happened year after year. It didn’t melt in the summers, so "snow was falling snow on snow” for thousands of years.
The mass of each new layer compacted the layers below, air was squeezed out and snow grains recrystallized into ice crystals. More and more layers, more and more compaction, and dense, blue glacial ice was formed.
Under the extreme pressure of its own weight, the glacial ice began to move outward covering the northern half of our continent.
The final verse of “In the Bleak Midwinter” offers touching thoughts about gifts.
Because “snow had fallen, snow on snow on snow, in the bleak midwinter long, long ago,” we received five of the most precious gifts imaginable.
When the mile-thick glaciers of the Wisconsin Advance receded from our region, five freshwater seas: Lake Michigan, Lake Huron, Lake Erie, Lake Ontario, and Lake Superior remained.
They are Great Lakes. And if we were wise, we would do our part to care for these precious gifts.
