The term “cross rocks” is the common name for a geological feature known scientifically as cross-bedding or cross-stratification. This structure is a primary feature found in sedimentary rocks, such as sandstone, indicating that the sediment was deposited by a flowing medium like wind or water. Geologists use the patterns within cross-bedding to reconstruct ancient environments, determining the direction and energy of currents that existed millions of years ago. These preserved layers record migrating ripples, dunes, or sand waves.
What Defines Cross-Bedding?
Cross-bedding is characterized by inclined layers, called cross-strata or foresets, that are contained within larger, generally horizontal rock layers, known as master beds. Unlike the master beds, the cross-strata are visibly tilted at an angle to the main depositional surface. The structure appears when the inclined layers are truncated, or cut off, by the horizontal surface of the next layer above it. This visual pattern distinguishes cross-bedding from rock layers that were simply tilted after deposition.
The two most common forms of this structure are tabular and trough cross-bedding, which are distinguished by the shape of the lower bounding surface. Tabular cross-bedding features planar or flat bounding surfaces, with the internal inclined layers being straight or nearly planar. This form is typically created by the migration of large, straight-crested dunes or ripples. Trough cross-bedding, conversely, is defined by curved, scoop-shaped bounding surfaces, and its internal layers are also curved, reflecting the migration of dunes with sinuous or irregular crests.
The Mechanics of Sediment Migration
The formation of cross-beds begins with the interaction of a fluid, either air or water, and loose sediment like sand. As the fluid flows over a bed of sand, it organizes the sediment into wave-like features called bedforms, such as ripples or dunes. The fluid flow causes grains to move up the gentle, upstream-facing slope of the bedform, which is known as the stoss side.
Sediment travels up this slope through saltation, where grains bounce along the surface, or by traction, where they roll or slide. Once the grains reach the crest of the bedform, they fall out of suspension and cascade down the steep, downstream-facing slope, called the lee side. This lee side is maintained at a specific angle, called the angle of repose, which is the steepest angle at which loose granular material remains stable (typically 30 to 35 degrees for sand).
The gravitational settling of these grains down the lee face creates the inclined layers that are preserved as cross-strata. As the fluid flow continues, the entire bedform slowly migrates forward, with constant erosion occurring on the stoss side and deposition building up on the lee side. Preservation occurs when the migrating bedform is covered by subsequent sediment before it can be completely eroded by later currents.
Environments Where Cross-Beds Develop
Cross-bedding forms in any environment where a fluid moves over mobile, non-cohesive sediment, such as sand. In river systems, or fluvial environments, cross-beds are commonly formed by migrating ripples and sand bars on the riverbed. These fluvial cross-beds are often smaller in scale, reflecting the size of the bedforms in a flowing channel.
Aeolian, or wind-driven, environments like deserts produce some of the largest cross-beds found in the geological record. The massive sand dunes in ancient deserts create cross-bed sets that can be tens or even hundreds of feet thick, distinguished by their large scale and typically high angles of inclination. In marine and coastal settings, cross-beds develop in shallow seas and tidal flats. These environments sometimes feature reversing currents, which can result in complex structures like herringbone cross-bedding, where adjacent sets of layers dip in opposite directions.
The size of the cross-bedding provides clues to the ancient environment, as larger structures indicate larger bedforms and higher energy fluid flow. For example, large-scale trough cross-beds suggest a high-energy environment, such as a deep river channel or a major dune field. By analyzing the orientation of the inclined layers, geologists can accurately reconstruct the direction of the ancient wind or water current, known as the paleocurrent.