About 19,000 years ago, the Earth was in the midst of a massive Ice Age, during which continental glaciers existed as far south as Wisconsin, most of England and Ireland, southern Germany, and most of present-day Russia. Canada, Norway, Sweden, Finland, and most of Russia were uninhabitable, completely covered in ice. In other parts of the world, such as California, alpine glaciers were much larger and more pervasive, carving numerous features into the mountains which remain today.
A glacier forms when snow falls and fails to melt, piling up over the centuries into slabs of ice between a few dozen feet and two miles thick. These bear tremendous weight, crushing the land beneath them as they slowly flow downhill. They carve out pathways known as glacial valleys. Alpine glaciers coalesce in areas where they can flow downhill easily, creating ribbed patterns between glacial "rivers" (cirques) and "hills" between them (aretes). These glacial features remain thousands or millions of years after their initial formation.
Many of the amazing features in places like Yosemite Valley have been formed by glaciers. These areas have large stretches of exposed granite, formed when ice scraped away overlying rock layers to reveal the underlying bedrock. Many of these granite surfaces lack true soils. Trees have to grow in small cracks between the rocks. These granite surfaces are considered ideal for rock climbing, and are visited by hundreds of thousands of people each year for just that purpose. The natural wonder of surrounding glacial valleys provides an excellent backdrop for a challenging climb.
The coasts of many northern countries, especially those immediately adjacent to the Arctic, display fjord-covered landscapes, which are caused by ancient glaciers. Countries with fjord-covered coasts include Canada, Norway, Iceland, and Greenland. Norway is among the most famous, with thousands of beautiful fjords. The world's longest fjord, Scorseby Sund, stretches for 217 miles (350 km). It is found on the eastern coast of Greenland.