Mass Movement, Wind, and Glaciers

In survival games, if you dig out the block under sand, the whole stack comes crashing down. Real Earth isn't that dramatic every day, but gravity is always pulling everything downhill. Sometimes slowly. Sometimes all at once.

Mass movement is when rock and soil slide downhill because of gravity. Here's the lineup, from slowest to scariest:

• Creep: super slow, maybe a few centimeters a year. You can spot it by tilted fence posts, bent tree trunks, and cracked walls on hillsides.
• Slump: a chunk of ground slides down along a curved surface, leaving a crescent-shaped scar. Like a scoop taken out of ice cream.
• Mudflow: soil soaked with water flows like a thick milkshake. Common after heavy rain on hills burned by wildfire, because there are no roots left to hold the soil.
• Landslide: a big mass of rock and soil suddenly breaks loose and races downhill.
• Rockfall: rocks break off a cliff and free-fall. That's why mountain roads have "Watch for Falling Rock" signs.

What triggers them? Water (it adds weight and makes soil slippery), steep slopes, earthquakes, and removing plants or cutting into hillsides for roads and houses.

Wind can't lift big rocks, but it's great at moving sand and dust, especially in dry places with few plants.

• Deflation: wind blows away loose sand and dust, leaving pebbles behind. It can scoop out shallow hollows.
• Abrasion: wind-blown sand blasts and polishes rocks, like nature's sandpaper.
• Sand dunes: piles of wind-blown sand. They slowly "walk" downwind as sand blows up one side and slides down the other.
• Loess: (say "luss") fine, windblown dust that settles into thick, fertile layers.

Idaho connection: Bruneau Dunes State Park in southern Idaho has the tallest single-structured sand dune in North America, about 470 feet high. And the fertile Palouse hills in northern Idaho are made of loess, which is why it's some of the best wheat country on Earth.

A glacier is a huge mass of ice that moves slowly over land. It forms where more snow falls each winter than melts each summer. Year after year, the snow piles up and gets squeezed into solid ice.

Two main types:

• Valley (alpine) glaciers: form in mountains and flow down valleys, like slow-motion rivers.
• Continental glaciers (ice sheets): cover huge areas of land. Today, only Greenland and Antarctica have them. If Antarctica's ice all melted, sea level would rise about 58 meters (190 feet).

Glaciers are the ultimate bulldozers. As they move, they pluck up rocks and grind them along the bottom, scratching and carving the land.

• U-shaped valleys: rivers carve V-shaped valleys. Glaciers widen them into U shapes.
• Cirques: bowl-shaped hollows at the top of a mountain where a glacier started.
• Horns: sharp, pointy peaks carved by glaciers on several sides. The Grand Teton is a great example.
• Moraines: ridges of rock and dirt dumped by a glacier, like a pile of junk shoved by a plow.
• Glacial lakes: many lakes formed when glaciers carved basins or moraines blocked valleys. Redfish Lake in the Sawtooths is one.

About 20,000 years ago, during the last ice age, glaciers covered much of North America. Huge floods from an ice-dammed lake in Montana, called Glacial Lake Missoula, roared across northern Idaho and Washington again and again, carving the land as they went.