A butte is a conspicuous, isolated hill characterized by steep, often vertical sides and a small, relatively flat top. This distinctive tower-like landform is not a result of volcanic activity or tectonic uplift, but rather the outcome of a prolonged geological process. Buttes are formed entirely through the gradual wearing away of rock layers, a slow-motion sculpting of the landscape by the relentless forces of erosion. This article explains the specific mechanisms and sequential stages that transform a vast, flat landscape into these solitary rock features.
Understanding the Landforms
The butte represents the final stage in a progression of flat-topped landforms, each defined by size. The process begins with a plateau, which is an extensive, elevated region with a flat top that can span vast distances. As weathering and river systems cut deeply into the plateau, large, isolated blocks are separated from the main body.
These large remnants are called mesas, a Spanish word for “table,” reflecting their broad, flat summit area. A mesa is a flat-topped hill that is wider than it is tall. As erosion shrinks the surface area of the mesa, it eventually transforms into a butte, the smallest of the three features. A butte is distinguished by the rule that its flat top is narrower than its overall height, creating a pillar-like structure.
The Mechanism of Differential Erosion
The unique, steep-sided shape of a butte is caused by a process called differential erosion, where different types of rock layers wear away at varying rates. This mechanism relies on the presence of a protective layer known as caprock. The caprock is composed of durable, resistant material, such as hard sandstone, basalt lava flows, or limestone.
This hard layer sits atop softer, more easily eroded sedimentary layers, such as shale or siltstone. The caprock acts like a natural shield, protecting the underlying layers from rain, wind abrasion, and water runoff. This protection maintains the butte’s flat summit for millions of years.
The softer rock layers surrounding the feature, exposed on the sides, are quickly attacked and removed by weathering. Water runoff from the summit and wind continually undercut the base of the feature, weakening the structure. This erosion of the softer material creates the nearly vertical cliff faces of the butte. The continuous removal of the soft rock leads to basal sapping, where the unsupported caprock above eventually breaks off in large, sheer blocks, maintaining the steep profile as the butte shrinks.
The Sequential Stages of Formation
Butte formation begins with a wide, elevated plateau composed of horizontal rock strata. Over geological time, the region is uplifted, increasing the energy of river systems that begin to carve deep valleys and canyons into the flat surface. These deep cuts isolate sections of the plateau, forming large, flat-topped mesas.
As the erosion continues, the valleys widen and the rivers cut further into the surrounding landscape, leaving the mesas as isolated remnants. The caprock protects the summit, but the exposed sides are subject to constant weathering, causing the overall horizontal area of the mesa to decrease steadily. This reduction in the summit area is the transition from a mesa to a butte.
The butte remains standing only as long as its caprock survives. Once the caprock is fractured or eroded away entirely, the soft rock beneath is fully exposed to the elements and erodes very quickly. The steep pillar collapses, leaving behind a conical, rounded hill of debris, known as a talus or scree slope. Ultimately, the entire feature is worn down to the level of the surrounding plain, completing the erosion cycle that began with the vast plateau.