Glaciers are of four chief types. Valley, or mountain, glaciers are tongues of moving ice sent out by mountain snowfields following valleys originally formed by streams. In the Alps there are more than 1,200 valley glaciers. Piedmont glaciers, which occur only in high latitudes, are formed by the spreading of valley glaciers where they emerge from their valleys or by the confluence of several valley glaciers. Small ice sheets known as ice caps are flattened, somewhat dome-shaped glaciers spreading out horizontally in all directions and cover mountains and valleys. Continental glaciers are huge ice sheets whose margins may break off to form icebergs (see iceberg). The only existing continental glaciers are the ice sheets of Greenland and Antarctica, but during glacial periods they were far more widespread. Glaciers may be classified as warm or cold depending on whether their temperatures are above or below - 10°C (14°F).