The term cliff-former is a geologic term indicating that a unit of bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 is more resistant to erosion

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 than overlying or underlying units and consequently outcrops have high slope angles. It is more or less equivalent to ridge-former, and may be contrasted with slope-former

Slope-former

The term slope-former is a geologic term indicating that a unit of bedrock is less resistant to erosion than overlying or underlying units and consequently outcrops have low slope angles. It may be contrasted with cliff-former. Typical slope formers include shales, and limestones in humid...

. In humid environments, sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s are typically cliff-formers. In arid environments, limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s are often cliff-formers also. Recent extrusive igneous units may be cliff-formers as well.

In the Ridge-and-Valley Appalachians

Ridge-and-valley Appalachians

The Ridge-and-Valley Appalachians, also called the Ridge and Valley Province or the Valley and Ridge Appalachians, are a physiographic province of the larger Appalachian division and are also a belt within the Appalachian Mountains extending from southeastern New York through northwestern New...

 of the eastern United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 the major ridge-formers are the Tuscarora

Tuscarora Formation

The Silurian Tuscarora Formation — also known as Tuscarora Sandstone or Tuscarora Quartzite — is a mapped bedrock unit in Pennsylvania, Maryland, West Virginia, and Virginia.-Description:...

, Pocono

Pocono Formation

The Mississippian Pocono Formation is a mapped bedrock unit in Pennsylvania, Maryland, and West Virginia, USA. It is also known as the Pocono Group in Maryland and West Virginia,...

, and Pottsville

Pottsville Formation

The Pennsylvanian Pottsville Formation is a mapped bedrock unit in Pennsylvania, western Maryland, West Virginia, and Ohio. The formation is also recognized in Alabama. It is a major ridge-former In the Ridge-and-Valley Appalachians of the eastern United States...

Formations.