Cross-bedding, or cross-stratification, is one of the most informative features preserved within sedimentary rocks, providing geologists with a direct window into Earth’s ancient surface conditions. These structures capture the dynamics of wind or water currents that transported and deposited sediment. By studying the geometry and orientation of these preserved layers, scientists can reconstruct the flow direction, energy, and overall environment of a past landscape. This sedimentary feature is indispensable for understanding the geographical and climatic history of our planet.
Defining the Structure
Cross-bedding is a layering pattern characterized by internal layers that are inclined at an angle to the main, generally horizontal, bedding surface. The entire structure is a preserved remnant of a migrating ripple or dune, which are collectively known as bedforms. The individual layers within the cross-bed are called laminae, which are typically less than a centimeter thick.
A group of these inclined laminae forms a single unit known as a set. Multiple sets stacked vertically create the larger cross-bedded deposit. The inclined layers within the set are specifically termed foresets, and they represent the preserved slip-face of the original bedform. The angle of these foresets often approaches the angle of repose for sand, which is approximately 34 degrees relative to the horizontal.
The boundary between one cross-bed set and the next is usually a flatter, erosional surface, indicating a period of scouring before the next bedform began to migrate and deposit sediment. The physical appearance of these stacked, tilted sheets is distinct because the original depositional layers were tilted, not deformed after they were flat. This morphology, with inclined internal layers truncated by a flatter upper surface, differentiates cross-bedding from layers that have been simply tilted by tectonic forces.
The Mechanism of Formation
The creation of cross-beds is a dynamic process driven by the unidirectional flow of a fluid, such as water in a river or wind in a desert, moving loose granular material. This flow causes sediment to accumulate into ripples or dunes on the riverbed or land surface. The formation involves a continuous cycle of erosion and deposition as the bedform migrates downstream or downwind.
Sediment grains are transported up the gentle, upstream-facing slope of the bedform, which is called the stoss side. Once the grains reach the crest, they avalanche down the much steeper, downstream-facing slope, known as the lee side. This process of repeated avalanching on the lee side creates the inclined layers, or foresets, that are preserved in the rock record.
As the bedform continues to migrate, the crest moves forward, and the newly deposited foresets partially cover and preserve the previously deposited layers. The top of the older set is often eroded away by the passing bedform, leaving a truncated surface that is later capped by the next set of cross-beds. This continuous cycle generates the characteristic internal geometry of the cross-bedding structure.
Interpreting Paleocurrents and Environment
The most significant information preserved within cross-beds is the record of ancient flow direction, known as the paleocurrent. Because the inclined foresets are deposited on the downstream side of the bedform, their direction of dip points directly in the direction of the ancient current or wind. Measuring the orientation of these foresets allows geologists to reconstruct the flow pathways of ancient rivers or the prevailing wind patterns of past deserts.
The geometry and scale of the cross-beds also provide specific details about the depositional environment. Large-scale cross-beds, which can be several meters thick, often indicate the migration of massive dunes, typically formed by wind in an arid, desert environment. Conversely, smaller cross-beds, often only a few centimeters thick, are commonly found in water-lain sediments like those in river channels or shallow marine settings.
Cross-beds are further classified by their shape, which provides insight into the bedform that created them. Planar cross-bedding is characterized by straight, tabular sets that form from bedforms with relatively straight crests, common in some tidal or shallow water environments. Trough cross-bedding features curved or “scooped” erosional surfaces and foresets, which result from the migration of sinuous or crescent-shaped dunes, often found in high-energy river systems.
The analysis of the consistency of the paleocurrent direction is also revealing. A unimodal pattern suggests a river or wind system with a consistent, single direction of flow. Bimodal or bipolar patterns, where flow alternates by 180 degrees, are typical of tidal environments where water moves in opposite directions during flood and ebb tides. By analyzing these geometric and directional attributes, cross-beds become invaluable tools for reconstructing the climate, geography, and sedimentary history of a region.