Sedimentary rocks offer a window into Earth’s history, preserving evidence of ancient landscapes and environments. These rocks form in layers, or strata, which are generally deposited in flat, horizontal sheets. However, within these main layers, geologists frequently observe smaller, inclined layers that cut across the main bedding plane. This internal stratification is known as cross-bedding. Studying this feature provides direct evidence of the movement of wind or water currents that were active during the rock’s formation.
Defining Cross-Bedding Structures
Cross-bedding is a type of internal layering that forms at an angle to the larger, surrounding horizontal rock layer, or stratum. This structure is a remnant of the migration of ripples, dunes, or sand waves across the depositional surface. The inclined layers themselves are referred to as cross-strata, and a group of these layers bounded by flatter surfaces is called a set.
A single set contains three primary components. The foreset beds are the most prominent feature, representing the inclined layers deposited on the downward-sloping side of the dune or ripple. These foresets approach the lower bounding surface either tangentially or sharply. The topset beds are the horizontal layers that form on the upstream surface of the bedform, though they are often eroded away. The bottomset beds are the layers that extend horizontally away from the base of the foresets, representing fine sediment carried further out.
The Mechanics of Creation
The formation of cross-bedding is directly linked to the movement of sediment under a flowing medium, which can be either wind (aeolian) or water (fluvial or marine). As the fluid flows over a bed of loose sand or silt, it initiates sediment transport through mechanisms like traction, where grains roll or slide along the bottom, and saltation, where grains bounce across the surface. This movement causes the sediment to accumulate into ripples or larger dunes, which are collectively known as bedforms.
These bedforms have a gentle, upstream side called the stoss side and a much steeper, downstream side known as the lee face or slip face. Sediment moves up the gentle stoss side until it reaches the crest of the bedform. Once at the crest, the grains cascade down the steep lee face in a miniature avalanche, forming a thin, inclined layer of sediment.
This cascading process is governed by the angle of repose, the maximum angle at which loose material remains stable. For dry, well-sorted sand, this angle is typically around 30 to 34 degrees from the horizontal. As the flow continues, the entire bedform migrates downstream due to continuous erosion on the stoss side and deposition on the lee face. The preserved layers—the foreset beds—are the inclined deposits left behind by the migrating lee face, each dipping in the direction of the ancient current.
Geometric Variations in Cross-Bedding
The final appearance of cross-bedding in the rock record depends on the three-dimensional shape of the original migrating bedform. Geologists primarily distinguish between two major geometric types: planar and trough cross-bedding. Planar cross-bedding is created by bedforms with straight or only slightly sinuous crests.
Planar cross-beds are characterized by bounding surfaces between sets that are essentially flat and parallel, appearing as stacked, inclined sheets. This geometry suggests a depositional environment where the current flow was relatively uniform and the bedforms were two-dimensional, extending laterally in a straight line. Trough cross-bedding, by contrast, is formed by three-dimensional bedforms with highly curved, crescent-shaped, or lunate crests.
When viewed in cross-section, trough cross-beds display bounding surfaces that are curved or scoop-shaped, truncating the inclined layers below. The foreset laminae within a trough set are also curved, reflecting the arc-like shape of the original dune. This geometry is associated with higher-energy flow conditions that cause local scouring at the base of the slip face, resulting in a series of nested, scoop-like deposits as the curved dunes migrate.
Interpreting Past Environments
Cross-bedding provides information about the conditions and direction of flow in ancient environments. The orientation and dip direction of the foreset beds directly indicate the paleocurrent direction, which is the direction of the ancient wind or water flow that deposited the sediment. By measuring the dip of multiple foreset sets, scientists can reconstruct the prevailing flow pattern of a river, ocean, or desert at the time of rock formation.
Beyond direction, the scale and character of the cross-beds can help determine the depositional setting. Relatively small cross-beds, measured in centimeters, are typically remnants of ripples formed by water in a stream or on a beach. Conversely, massive cross-beds that reach heights of several meters suggest the former presence of large sand dunes, often found in arid desert environments (aeolian deposits) or deep submarine channels.
The distinction between planar and trough geometry also offers clues about flow conditions. Trough cross-bedding is often associated with the migration of three-dimensional dunes in rivers or shallow marine settings. Specific variations, such as herringbone cross-beds—where foresets dip in opposite directions—are a strong indicator of tidal environments where flow reverses direction twice daily.