What are cross beds, and how do they form? How can you read the current direction from cross beds? (D)

Cross beds are sedimentary structures found in layered deposits, characterized by inclined layers (called laminae or strata) that are inclined at an angle to the horizontal plane of the deposit. These inclined layers indicate the direction of sediment transport and the direction of the depositional current that formed them (such as wind or water).

Cross beds form through the process of sediment deposition in a moving current. When sediment particles are transported and deposited by flowing water or wind, they form small-scale sedimentary structures called ripples or dunes. Over time, these ripples or dunes migrate downstream or downwind, and new sediments are deposited on the downstream faces, creating the inclined bedding planes within the larger sedimentary unit. As layers of sediment are deposited on top of each other, these inclined layers become preserved as cross beds.

To determine the current direction from cross beds, you need to first identify the paleocurrent indicators. In cross beds, these indicators are the inclined layers, or the laminae, that make up the cross-bedded unit. The direction of the inclination of these layers can tell us the direction of the depositional current. To determine the current or paleocurrent direction, follow these steps: 1. Identify the cross-bedding structure: Locate the section of the rock where the cross bedding is visible, which is typically composed of sets of alternating inclined and horizontal layers. 2. Analyze the inclined layers: Observe the inclination of the layers or laminae, and determine the steeper side (also called the lee side) and the gentler side (also called the stoss side) of the layers. 3. Determine current direction: The current direction can be read from the cross beds by observing the orientation of the steeper (lee) side of the layers. The depositional current flowed from the gentler (stoss) side toward the steeper (lee) side. In other words, the paleocurrent direction is parallel to the orientation of the cross beds and points in the direction from the stoss side to the lee side.