Boulder Walls are a form of the oldest and longest lasting type of retaining walls. They are actually natural stone walls. Rather than being made of smaller stones, they are typically built of fieldstone boulders.
Boulder Walls are also the least expensive type of retaining walls. One reason is that fieldstone boulders are a material that is readily available. The cost per square foot for the materials is much lower than the more expensive quarried stone or even manufactured modular blocks.