A hill is a landform that rises above its surrounding terrain, distinguished generally from a mountain by its lower elevation and gentler slope profile. Hills typically result from a combination of forces that build up the Earth’s surface and forces that wear it down. The formation of these features is a continuous geological process involving both internal, constructive activity and external, destructive, and depositional agents. Understanding these mechanisms allows categorization of how different types of hills are created across diverse landscapes.
Hills Formed by Crustal Movement
Internal forces, driven by plate tectonics, create hills through the large-scale movement and deformation of the lithosphere. These constructive forces compress, stretch, and fracture the Earth’s crust, pushing rock layers upward to form elevated landmasses composed of structurally uplifted bedrock.
Folded hills occur when tectonic plates collide, subjecting rock layers to immense compressional stress. This stress causes the layers to buckle and bend into wave-like structures called anticlines (upward folds) and synclines (downward folds). Over geologic time, the upward-arched anticlines become the hills and ridges that define the landscape.
Fault-block hills arise from tensional forces that pull the crust apart, causing the brittle bedrock to fracture into blocks along fault lines. When sections of the crust are pushed upward relative to adjacent blocks, they form elevated features known as horsts, which appear as abrupt, steep-sided hills.
Volcanic activity also creates unique hilly landforms through the constructive process of magma rising to the surface. Cinder cones, which are among the smallest and simplest types of volcanoes, are classic examples, forming from the explosive ejection of gas-charged lava fragments (pyroclastic material). This material piles up around the vent to create a steep-sided, conical hill.
Hills Formed by Differential Erosion
External forces like wind, water, and ice sculpt the landscape, creating hills through differential erosion. This occurs because rock layers possess varying degrees of hardness and resistance to weathering. Softer, less resistant rock erodes faster, leaving the harder, more durable rock standing as a residual hill.
In arid regions, differential erosion forms flat-topped hills known as mesas and buttes. These features begin as plateaus where a cap of hard, resistant rock protects the softer sedimentary layers beneath. As the surrounding softer material is stripped away, the protected core remains as an isolated, steep-sided hill.
When the top-protecting layer is completely eroded, the feature shrinks in area, eventually forming the smaller, column-like structure of a butte. This continuous process gradually reduces the size and height of the isolated landform.
The same principle applies to monadnocks, which are isolated hills of resistant bedrock, such as granite, standing prominently above an otherwise flat, eroded plain. These residual hills represent the remnants of a former landscape lowered over vast periods of time.
Hills Formed by Deposited Materials
Hills can be constructed entirely by the accumulation of loose sediment transported and deposited by natural agents. These depositional hills are distinct from bedrock hills because they are composed of unconsolidated materials. Glacial activity, wind, and mass movement are the primary forces behind these structures.
Glacial Deposits
Glaciers leave behind a variety of depositional hills as they melt and retreat. Drumlins are elongated, streamlined hills composed of unsorted glacial till, aligned parallel to the direction of the ice flow. Kames are irregular, steep-sided mounds of poorly sorted sand and gravel. They accumulate where meltwater streams deposit sediment in depressions within the ice.
Aeolian Deposits
Wind action creates aeolian hills, such as sand dunes and loess hills. Sand dunes are small hills of sand that form when wind slows down and drops its sediment load. Loess hills are formed from the deposition of fine, wind-blown silt particles. These particles accumulate in thick layers, forming gently rolling hill regions far from the original sediment source.
Mass Wasting
Hills can also be formed by mass wasting events, such as rotational slumps. A coherent block of material moves downslope along a curved failure surface. This movement often results in a distinct, hummocky terrain at the foot of the slope. These gravity-driven deposits create small, localized hills composed of jumbled rock and soil.