by John Russell
Travel up any of the riverbeds onto the Millstone grit and ripple marks features are common in the sandstones. They provide geologists with useful information about palate-environments.
Ripple marks are found in many environments e.g. tidal flats, beaches, lakes, seas and rivers. The water depth can vary from very shallow down to a depth of 200m. There are many factors that control the size of the ripples. The three most common are the coarseness of the grain, the water depth and the intensity of the waves. The ripple marks can provide information about the water that created them. There are two main groups of water ripple marks shown in the diagram below.
The photograph shows a symmetrical wave pattern (a) which indicates a marine environment. It is like the first diagram with the crests removed. This reflects a higher energy deposition. The second diagram shows an asymmetrical wave pattern formed in a river. The steeper slope of the wave points downstream. Ripple marks can also be useful in working out the way up of a rock sequence.
Also in the photo are several water-worn riverbed boulders. Their rounded appearance is due to three elements of erosion, attrition, corrosion and solution. These boulders are part of the bedload of the river that is being carried to the sea and deposited. The angular boulder (c1) is a very hard limestone which shows smooth, water worn surfaces. The boulder marked (c2) is of softer sandstone. Its almost-spherical form is a result of corrosion and attrition as it collides with other boulders and the riverbed.
See also:
Overview: The Geology of Swaledale and Arkengarthdale
Stockdale Fault: Where Askrigg Block meets Stainmore Trough
