A hill is a landform that rises above its surroundings, typically defined by its lower elevation and gentler slopes compared to a mountain. While there is no universal height standard, hills generally rise less than 610 meters (2,000 feet) above the adjacent terrain, possessing a smoother, more rounded profile. These topographical features are dynamic products of slow-moving geological forces. Hill formation processes are broadly categorized into those that sculpt the land by removing material, those that construct it by depositing material, and those that uplift it through internal forces.
Hills Formed by Erosion
Many hills are residual structures, meaning they are the remnants of a once-higher plateau or plain that has been extensively worn away. This process is known as differential weathering, where different rock types erode at varying rates. Water, wind, and ice act primarily on the softer, less-resistant rock layers surrounding a harder core.
Over time, the surrounding sedimentary or weak metamorphic rock is gradually stripped away, leaving the more durable material standing isolated above the lowered landscape. These isolated hills are often called monadnocks or inselbergs. A well-known example is Stone Mountain in Georgia, a large dome composed of highly resistant quartz monzonite that remained after the surrounding terrain was eroded.
Hills Formed by Material Accumulation
A major category of hill formation involves the addition of material, where external agents like glaciers or wind deposit large volumes of sediment. These depositional hills are composed of loose, unconsolidated material rather than solid bedrock.
Glacial activity creates several distinct hill types, including moraines and drumlins. Moraines are ridges or mounds of unsorted debris, called glacial till, that are pushed and deposited at the margins of a glacier as it melts. Terminal moraines mark the farthest point of the ice sheet’s advance, often forming hummocky, low-relief hills.
Drumlins are streamlined, elongated hills composed of glacial till that form beneath the moving ice. They are typically shaped like an inverted spoon, with the steep, blunt side facing the direction the glacier advanced. These hills frequently appear in large groups called “swarms,” with their long axes precisely aligned with the ancient ice flow.
Wind can also be a significant building force, creating hills from finer sediments. Loess hills, for instance, are formed from the accumulation of wind-blown silt, often referred to as “glacial flour.” This material was ground up by ice sheets during the Pleistocene epoch, swept from exposed river floodplains, and deposited in thick layers. This process forms the uniquely dissected topography of the Loess Hills in Iowa.
Similarly, sand dunes are hills of wind-driven sand. They form where a consistent wind, a supply of sediment, and an obstacle cause sand grains to accumulate. Large star dunes can reach heights exceeding 230 meters (750 feet) as shifting winds cause the sand to pile vertically.
Hills Formed by Tectonic Activity
Internal forces within the Earth’s crust, collectively known as tectonic activity, can elevate the land to form hills. This occurs primarily through folding, faulting, and localized volcanism.
Hills can form through compression, where horizontal forces squeeze the crust, causing rock strata to bend into wave-like structures called folds. The upward arching sections of these folds, known as anticlines, can create low, rolling hills. This is common in regions where ancient mountain belts have been heavily weathered, such as the Appalachian foothills.
Faulting occurs when sections of the crust fracture, and blocks of land are pushed up or down along the fracture line. Smaller fault-block hills result when a specific block is elevated between two faults, a process known as uplift.
Volcanic activity can produce relatively small, cone-shaped hills that do not reach the stature of stratovolcanoes. The simplest of these are cinder cones, which form when gas-charged lava is violently ejected and solidifies into fragments called cinders or scoria. These fragments fall back around the vent, creating loose, steep-sided cones.
These cones rarely rise more than 300 meters above the surrounding landscape and are often built during a single, short-lived eruptive event. Even smaller are spatter cones, which are low mounds typically only 3 to 5 meters tall. They are built from the accumulation of sticky, welded lava fragments that pile up near a vent.