The assertion that geological structure exerts a dominant control on landforms and is reflected in them is a fundamental principle in geomorphology. Geological structure refers to the arrangement, attitude, and deformation of rocks in the Earth's crust, encompassing factors such as rock type, bedding planes, folds, faults, joints, and dips. These structural elements dictate how rocks respond to weathering and erosion, thereby shaping the surface topography.

Dominant Control Mechanisms:

1. Differential Erosion: The most significant way geological structure controls landforms is through differential erosion. Rocks vary widely in their resistance to weathering and erosion. Hard, resistant rocks (e.g., granite, quartzite, well-cemented sandstone) erode much more slowly than softer, less resistant rocks (e.g., shale, clay, unconsolidated sediments). This differential resistance leads to the formation of varied topography, where resistant rocks form uplands and ridges, while softer rocks are excavated into valleys and lowlands.

2. Rock Type:

   • Igneous Rocks: Massive igneous intrusions like batholiths often form rounded, dome-like hills (e.g., Stone Mountain, USA). Extrusive igneous rocks like basalt can form extensive plateaus (e.g., Deccan Traps, India) or volcanic cones.
   • Sedimentary Rocks: The layered nature of sedimentary rocks is crucial. Resistant layers (e.g., sandstone, limestone) often form cliffs or caprocks, while interbedded softer layers (e.g., shale) form gentler slopes. Chemical weathering of limestone creates distinctive karst topography with features like sinkholes, caves, and underground drainage.
   • Metamorphic Rocks: Often highly resistant, metamorphic rocks like gneiss and schist frequently form the cores of mountain ranges.

3. Folds: Where crustal compression has folded rock layers, the resulting landforms often mirror these structures:

   • Anticlines (upfolds): Can form ridges if the crest consists of resistant rock, or valleys if the crest is eroded away, exposing softer underlying layers.
   • Synclines (downfolds): Can form valleys if resistant rocks are at the trough, or inverted relief where resistant rocks in the trough form a ridge due to differential erosion of surrounding softer anticlines.
   • Appalachian Topography: A classic example where parallel ridges and valleys are formed by the differential erosion of folded sedimentary strata.

4. Faults: Fractures in the Earth's crust where there has been significant displacement also profoundly influence landforms:

   • Fault Scarps: Steep slopes or cliffs formed by vertical displacement along a fault line.
   • Rift Valleys (Graben): Down-dropped blocks of crust between parallel faults, creating elongated depressions (e.g., East African Rift Valley).
   • Block Mountains (Horst): Up-thrown blocks between parallel faults, forming elevated mountain ranges.
   • Linear Valleys/Ridges: Erosion often preferentially occurs along fault lines, creating linear valleys or guiding river courses.

5. Joints: Fractures in rocks without significant displacement. Joints provide pathways for water penetration, accelerating weathering and erosion. They can lead to blocky disintegration of rocks, columnar jointing in basalt, or influence the development of rectangular drainage patterns.

6. Dips: The angle at which sedimentary layers are tilted significantly affects slope morphology:

   • Cuestas: Asymmetrical ridges with a steep scarp slope and a gentle dip slope, formed on gently dipping resistant strata.
   • Hogbacks: Symmetrical, steep-sided ridges formed on steeply dipping resistant strata.
   • Mesas and Buttes: Flat-topped landforms with steep sides, formed by horizontal resistant caprock overlying softer, easily eroded layers.

Reflection in Landforms: The internal geological structure is directly expressed in the external morphology of the landscape. The orientation of ridges and valleys, the steepness of slopes, the patterns of drainage networks (e.g., trellis drainage in folded regions, rectangular drainage along joint systems), and the overall 'grain' or fabric of the landscape are often direct reflections of the underlying rock types, their attitudes, and their deformational history. Thus, by observing landforms, geomorphologists can often infer the geological structure beneath, demonstrating the dominant and pervasive control that structure exerts on the Earth's surface.