A glacier forms in areas of high precipitation and elevation where the snow is allowed to pile up to great depths, compacting the bottom layers into solid ice. The great weight above the bottom ice (along with the force of gravity) pushes it slowly downward like a giant frozen river, scooping out huge valleys and shearing off entire mountainsides. When the rate of advance is balanced by melt-off, the face of the glacier remains more or less stationary. If the glacier flows more quickly than its face melts, it advances; if it melts faster than it flows, the glacier recedes.
Air bubbles are squeezed out of the glacier by this tremendous pressure, which makes glacial ice extremely dense. It’s so compact that the higher frequencies of light cannot escape or penetrate it, which explains the dark-blue tinge. And because of its density, it also melts at fantastically slow rates; a small chunk or two will keep beer in a cooler chilled for a day or two.
As you travel up the coast or hike in the national parks of the Interior , you’ll soon start to recognize and identify glacial landforms. While rivers typically erode V-shaped valleys, glaciers gouge out distinctly U-shaped glacial troughs. Valleys and ridges branching from the main valley are sliced off to create hanging valleys and truncated spurs.
A side valley that once carried a tributary glacier may be left as a hanging trough; waterfalls often tumble from these hanging valleys and troughs. Alpine glaciers scoop out the headwalls of their accumulation basins to form cirques. Bare jagged ridges between cirques are known as arêtes.
As a glacier moves down a valley it bulldozes a load of rock, sand, and gravel—known as glacial till—ahead of it, or carries it on top. Glacial till that has been dumped is called a moraine. Lateral moraines are pushed to the sides of glaciers, while a terminal moraine is deposited at the farthest point of the face’s advance. A medial moraine is formed when two glaciers unite. These ribbonlike strips of rubble can be followed back to the point where the lateral moraines converge between the glaciers.
When looking at a glaciated landscape, watch for gouges and scrape marks on the bedrock, which indicate the direction of glacial flow. Watch too for erratics, huge boulders carried long distances and deposited by the glacier that often differ from the surrounding rock. Glacial runoff is often suffused with finely powdered till or glacial flour, which gives it a distinctive milky-white color; the abundance of this silt in glacial streams creates a twisting, braided course. With a little practice, you’ll soon learn to recognize glacial features at a glance.
The vast majority of Alaska ’s glaciers, like those in many other parts of the world, are retreating as the global climate warms. In some cases, glaciers have drawn back many miles in the last few decades, exposing newly formed bays and producing massive outflows of icebergs. The 1989 Exxon Valdez  oil spill was caused when the ship diverted to avoid ice from nearby Columbia Glacier ; it didn’t help, of course, that the captain was drunk.
One of the few exceptions to the pattern of retreating glaciers is Hubbard Glacier  near Yakutat , whose advance threatens to dam up a large bay, potentially changing the course of rivers. Learn more about glaciers and ongoing Alaska research at the U.S. Geological Survey’s glacier and snow website (http://ak.water.usgs.gov/glaciology ).
To picture permafrost, imagine a veneer of mud atop a slab of ice. In the colder places of the Lower 48, soil ecologists measure how much surface soil freezes in winter. In Alaska , they measure how much surface soil thaws in summer. True permafrost is ground that has stayed frozen for more than two years. To create and maintain permafrost, the annual average temperature must remain below freezing.
The topsoil above the permafrost that thaws in the summer is known as the active layer. With the proper conditions, permafrost will penetrate downward until it meets heat from the earth’s mantle. In the Arctic, permafrost begins a few feet below the surface and can extend 2,000–5,000 feet deep. This is known as continuous permafrost, which almost completely underlies the ground above the Arctic Circle. Discontinuous permafrost, with permafrost in scattered patches, covers extensive parts of Alaska, particularly boggy areas covered by black spruce forests.
Frozen ground is no problem—until you need to dig in it. Russian engineers were the first to encounter industrial-scale problems with permafrost during the construction of the Trans-Siberian Railroad. In Alaska, gold mining, especially in deep-placer operations, often required up to two years of thawing hundreds of feet of permafrost before dredging could proceed.
Today, houses frequently undermine their own permafrost foundations: Heat from the house thaws the ground, causing it—and the house above it—to sink. Similarly, road-building clears the insulating vegetation layer and focuses heat on the frozen layer, causing severe “frost heaving,” the roller-coaster effect common to roads in Interior Alaska .
In the 1970s, pipeline engineers had to contend with the possibility that the 145°F oil flowing through the pipe would have similarly detrimental effects on the permafrost, with potentially disastrous financial and ecological consequences. That’s why more than half of the Trans-Alaska Pipeline is aboveground, supported by a specially designed and elaborate system of heat-reducing pipes and radiators.
Today, there is increasing concern that warming global temperatures could have a devastating impact in the Arctic, causing permafrost to melt and greatly altering the ecosystem.