Eventually the water freezes again and becomes attached to the bottom of the glacier, along with whatever dirt and silt it can pick up. The ice and the dirt later separate, forming alternating layers of clean and dirty ice. That process continues all summer long, Evenson says.
So in the winter a glacier picks up new layers of ice on its surface as snow falls in the higher elevations. And in the summer, as it moves down the valley toward the sea, melting somewhat along the way, it picks up new layers of ice and dirt as it grows from the bottom up.
Ultimately, some of those dirty layers will be exposed as the ice melts, much to the consternation of tourists who expect glaciers to be perfectly blue, not black. Incidentally, glaciers are blue because the ice is compacted so tightly that little air is left between the ice crystals, and the glacier thus reflects more blue light than any other color.
These giant pieces of ice are truly among the planet's most spectacular and dynamic structures, but their days may be numbered. Nearly all glaciers in Alaska are retreating, as they have been for thousands of years since the end of the last Ice Age, but these days they are galloping at full throttle.
Los Angeles Ice
Just a couple of decades ago a new visitor's center was built overlooking the Portage Glacier near Anchorage, Alaska's largest city. But within a few years the glacier had retreated so far that it could no longer be seen from the center. Now, to see it you have to get in a boat.
That's pretty remarkable when you think that it hasn't been all that long since glaciers extended as far south as Los Angeles. No kidding.
Lewis Owen of the University of California, Riverside, reveals evidence in the current issue of Geology that shows that a glacier clung to the slopes of San Gorgonio Mountain, just 75 miles east of Los Angeles, just 12,000 years ago. Owen thinks it might have even been there as recently 5,000 years ago, but not everybody agrees with that date.
Too bad it's gone. Southern California, now sweltering in the heat and facing chronic water shortages, could use a big chunk of ice these days.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.