The Ripples
The strange looking rock formations at the bottom of this creek bed are called “ripple marks”. They are formed by shallow water flowing over a bed of sand or other sediments, or by wind-driven wave action in shallow water, again over small sediments.
The ripples formed in shallow water by waves, (imagine small waves lapping at the shore of a very shallow area of a lake) are called wave-formed ripple marks. These are distinguishable by the symmetrical shape of the individual ripples. Another type, called current formed ripple marks are asymmetrical in shape with gentle slopes on the up-current sides and somewhat steeper slopes on the down-current sides of the ripples. Other more erratic ripple marks are formed where two separate wave patterns, traveling in slightly differing directions meet. These are classified as interference ripple marks.
These patterns are usually small and close together, but not always. Very large volumes of fast moving water can form very large ripples. The Channeled Scablands in Washington State were formed when an ancient, very large lake burst through its natural dam and flooded many thousands of acres of land in a short amount of time. The ripple marks it left can still be seen today and may be 100 yards in length or longer, with heights resembling small hills.
The formations seen at this site were almost certainly caused by shallow, slowly flowing water moving over sandy or muddy areas. At some point in the history of these formations, the ripples were covered with other layers of sediment, without disturbing the patterns we see today. In time there were many other layers deposited here, compressing the layers by their own weight effectively forming fossil wave marks in the rock.
Millions of years later, as erosion slowly removed the layers of rock the ripple marks have been exposed and are as we see them today. As erosion is an ongoing process, these remnants will eventually disappear, worn away by the same processes that destroy mountains, turning them into sediments. It is ironic that these marks were formed in shallow water millions of years ago, and today the rock that they have been preserved in makes up the bed of a shallow stream. The water and wind created these formations, and will destroy them in time.
The Rocks of the Area
These formations are made of limestone as are the other rocks seen at this location. Limestone is a sedimentary rock that was originally deposited as fine grained particles that settled to the bottom of a large shallow sea which covered this area millions of years ago. At this location the sea was so shallow that the ripples formed by surface waves. In this water would have lived snails, trilobites, brachiopods, and other invertebrates.
The loose rock here has been washed down from locations upstream. As the rock that makes up the ripples break apart they too will be washed farther down the stream bed.
Logging Requirements: To log this earthcache email me the answers to the following: 1) What type of rock are the ripples formed in? 2) About how many ripples can you see exposed? 3) How tall are the ripples from crest to trough?