Fields Hill - Sedimentary Cross Beds EarthCache

At the listed co-ordinates you will find an example of some of the oldest cross beds in this area, deposited more than 400 million years ago.

In simple terms, a basin is a depression in the Earth's crust which is usually filled with water for most of it's existence. The depth of these basins vary and may even be hundreds of meters deep. Basins are usually fresh water bodies unless they are linked via tidal areas to the sea. From the surrounding higher areas streams and rivers drain into the basin. These erode the surrounding landscape and deposit the eroded materials into the basin. In time, if no other major geological forces occur these eventually fill up the basin. This area might form a new depression, or faulting and shifting may cause other basins to form which may partly overlap. Most of the sediments preserved in basins are not formed in their permanently submerged parts, but in shallower areas of slow moving water. Some sedimentary basins are not formed by water but by wind. In this case driving winds carry eroded materials over a depression where they are deposited over time to form large expanses of sedimentary deposits.

A similar basin such as described in the above paragraphs existed in this area during the Cambrian to Ordovician period, consisting of a relatively shallow depression fed by rivers from surrounding high areas. Until the development of a fault line in this area resulting in the uplifting of the basin and resulting in erosion that led to the features visible today.

Cross bedding can help us to understand ancient water / air current direction. As the water or air current moves downstream / downwind, it picks up particles of mud or grains of sand. If they are exposed to the full force of the current, sand grains are likely to be moved on. However, if they fall into the "lee" of the ripple, they are no longer under the influence of the main current. The water / air is calmer there, and the particles are deposited. The particulate materials accumulate on the steep slope of the ripple-mark, facing downstream. A succession of these "ripple fronts" is preserved as a series of cross-beds.

The arrow shown skimming across the top of the ripples shows the fastest bottom current that carries the particles. Because ripples advance by erosion off the back and deposition down the front (the steep side), the tops of ripples are commonly lost to erosion and the cross-beds preserve only the lower part of the original ripple. Note that the main horizontal layers are often very gently inclined. The steep front faces tilt down-current and thus indicate current flow direction. Cross-beds are commonly curved at the base. All of this gives a guide to determining the original orientation of the layers in more complex bedding examples.

Although the overall layering is horizontal, the structure can be further modified by periods of stronger water or wind currents which will erode already deposited layers, resulting in even horizontal bands. Horizontal Bands may also be deposited in different directions depending on the current flows at the time of creation. Large gently sloping cross-beds could indicate that these sediments were deposited by slow flowing river currents or deposited on large windswept areas. The grain structure is an indication of the materials that were deposited from the suspended source. Larger conglomerate inclusions often indicate a period of stronger current flow as the largest and heaviest particles in suspension are deposited and conversely a fine structure may indicate a slower current. Very fine grain grain structure may also be an indicator of river-delta conditions, these often producing mudstone rather than sandstone.

Matched sets of cross-beds tilting in opposite directions are called herringbone cross-beds. Such beds are most commonly produced by tidal movements in and out of a shallow sea.