An esker is a long, sinuous ridge that is made up of stratified drift. It is one of the more interesting of glacial landforms, and also one of the least intuitive in formation for most students; they form when meltwater at the base of the glacier preferentially erodes to ice instead of the bedrock, forming a tunnel at the glacier bed (known as an R-Channel). When sediment is carried through the tunnel by the meltwater, it is deposited along the way, continually accreting upward. When sufficient ablation of the glacier has taken place, a ridge is exposed.

Eskers can be quite large, achieving heights of tens of meters or more, although commonly they are only a few meters in height. The lengths can range from just a few meters to several hundred, and their paths are interesting. Since they are affected not only by gravity but also the hydrostatic pressure of the overlying ice, they can flow downhill, sideways, or even uphill. There are even instances where an esker flowed up the side of one valley wall and crossed a drainage divide, where it was picked up by another glacier!

Most eskers form on relatively broad, flat surfaces, where the velocity of the stream is slow enough to deposit. Most were formed at the stagnating margin of ice sheets, although they are not uncommon in mountain glacier settings. They are usually a warm-based glacier phenomenon, as an abundance of meltwater is needed to form these interesting glacial landforms. Today, many eskers are mined for their gravel deposits, and in many parts of the upper-Midwest, roads are even constructed on them.