Eskers are long ridges of material running in the direction of the ice advance. They have a sinuous (winding) form and can be between 5-20m high. They also consist of sorted, coarse material, usually coarse sands and gravel, due to their fluvioglacial nature. This means the meltwater carries the sediment and deposits it as it loses energy, instead of dropping it all at once. As a result, they are often stratified (layered).
The stream channel is restricted by the ice walls meaning there is high hydrostatic pressure meaning the meltwater has a higher velocity and can therefore transport more material. It also allows the stream to flow uphill for short distances. The bed of the channel builds up and when the glacier retreats (post-glaciation) a depositional ridge is left behind – an esker!