Horst and Graben (valley and range) refers to a type of topography created when the earth's crust is pulled apart. This process, called extension, can stretch the crust up to 100% of its original size. As the crust is strained in this way, normal faults develop and blocks of the crust drop down to form grabens, or valleys. The end result of this is a vast landscape of alternating valleys and ridges. The western United States is an example of this, in the physiographic province known as the Basin and Range.