Lateral moraines are parallel ridges of alluvial debris pushed up by the gradual action of a glacier. As the glacier advances downhill, the immensely heavy mass of ice pushes rocks, silt and other material to the sides of the glacier's course. According to the National Snow and Ice Data Center, lateral moraines are typically sharper in profile than the more rounded terminal moraines that form at the glacier's head.
As a glacier slides downhill, it can't help but collect massive quantities of debris from the surrounding and underlying terrain. Alluvial silt under the glacier, rocks sheared from the walls of the glacial valley and any other material, including trees that happen to be standing in the glacier's way, are likely to be lifted away from their position and carried along with the glacier as it travels.
This loose collection of rocky and organic debris tends to work its way out of the glacier over time, and it is eventually deposited along the sides of the glacier as piles of rubble. These piles, or lateral moraines, mark the maximum lateral extent of the glacier's path and form long parallel ridges on either side, according to the National Snow and Ice Data Center.