What does an esker primarily consist of?

An esker is primarily a sinuous ridge of stratified sand and gravel that is formed by the deposition of sediment from meltwater streams running beneath a glacier. As glaciers advance and retreat, they create many different geologic features, and eskers are specifically formed when the sediment-filled meltwater channels become isolated and the material is deposited as the glacier retreats.

The composition of eskers typically includes loose sediments, predominantly sand and gravel, which are transported and sorted by the glacial meltwater flowing underneath the glacier. This movement of water creates well-sorted layers of sediment, often leading to the characteristic wavy or sinuous shape of an esker.

Other materials such as clay and silt may be found in glacial deposits, but they are not the main components of eskers. Granite and quartz are types of rock that may be found in other geological contexts, while volcanic rock is formed from volcanic activity and does not relate to the sedimentary processes associated with glacial environments. Therefore, the emphasis on sand and gravel in the context of an esker accurately reflects its formation process and typical composition.