The Llanos sit in a geological cradle formed by some of the oldest and youngest rock on the continent. To the south, the Guiana Shield dates to the Precambrian, its crystalline basement among the most ancient exposed surfaces on Earth. To the north, the Venezuelan Coastal Range and to the west, the Cordillera de Merida represent much younger mountain-building episodes, their genesis spanning from the Cretaceous through the Pliocene. Between these walls, the plain receives the runoff of both, accumulating sediment in a process that continues today. The Llanos connect to the sea through two natural gaps: the Unare Depression, which opens to the Caribbean in the central-east, and the Orinoco Delta, which fans into the Atlantic on the eastern margin. These exits give the basin its drainage logic, tilting the entire plain gently southward and eastward toward the river and the ocean.

Long before the Llanos were plains, they were ocean floor. A marine transgression in the Tertiary period turned this region into an extension of the Atlantic, a wide channel penetrating deep into Venezuelan territory. At the bottom of this interior sea, enormous quantities of organic matter accumulated: the compressed remains of marine plants and animals deposited over millions of years. When the sea retreated during the late Tertiary and Quaternary, it left behind more than dry land. The decomposition of that organic matter, under heat and pressure over geological time, produced the petroleum deposits that now define Venezuela's economic identity. The Barinas-Apure petroleum basin and the Oriental basin, which includes the Orinoco Belt, sit beneath the Llanos, as do significant coal deposits in Guarico and Anzoategui. The flattest landscape in Venezuela floats on some of the most valuable geology on the planet.

Despite the illusion of uniformity, the Llanos divide into three distinct subregions, each shaped by its proximity to the mountains and the sea. The Western Llanos, pressed against the Andean foothills, receive the heaviest rainfall and the richest sediment, making them the most productive agricultural land. The Central Llanos occupy the broad middle of the basin, transitioning from cultivated high plains near the northern mountains to seasonally flooded low plains closer to the Orinoco. The Eastern Llanos stretch toward the Atlantic, where the Orinoco Delta begins to fracture the land into a labyrinth of channels. Each subregion has its own morphology, its own water regime, and its own possibilities for human use, though from cruising altitude, the differences dissolve into a single, overwhelming flatness.

The most arresting fact about the Llanos is their geometry. The slope of the terrain averages about 70 centimeters per kilometer, a gradient so slight that water has no urgency to move in any direction. During the wet season, this means flooding on a continental scale: rivers overflow their banks without finding any topographic reason to recede, and the plains become a vast shallow wetland that can persist for months. During the dry season, the same flatness means that water has nowhere to hide, and the landscape cracks and yellows under relentless sun. This pendulum swing between aquatic and arid defines the ecology, the agriculture, and the culture of the region. The llanero tradition of cattle herding evolved specifically to cope with a landscape that is alternately submerged and scorched, demanding a kind of adaptability that more temperate ranching cultures never needed.