There is an almost limitless fascination for both children and adults alike with the many endless cycles of change that take place at the edge of the sea, cusps are one of those unique cobble beach features.
Geologists generally think of beaches as moving deposits at the interface between the land and water. The beach that you are visiting is known as a cobble beach. Cobble beaches occur where materials of a course size are being worked by the action of the wave. While the cobble beaches have the same beach forming process as sand beaches there are a number of unique differences. They are composed of materials in a wide range of larger sizes. Generally, the larger cobble is rolled and shaped by the waves, this rolling in the waves makes a magical sound that is unique to these types of beach. The profile of the beachface is generally much steeper without the dunes associated with sand beaches. They have instead a seawall created by storm waves. A natural cobble seawall is established at the landward limit of the worst winter wave influence. The final difference is the formation of cusps, which are evidence of storm surf that can be seen in the form of scooped out depressions forming a scalloped beachface.
Cusps are shoreline formations made up of various grades of sediment in an arc pattern where the wave hit the beach at an angle. The horns (side high points)are made up of coarser materials and the embayment (center section) contains all the finer grain sediment. They generally occur in a regular pattern with cusps of equal size and spacing forming. There are a number of theories as to why beach cusps are formed but current research gives real credibility to only two of these explanations the Self-organization Theory and Standing Edge Wave Theory.
There are two main components to the Self-Organization Theory, which in itself is very complicated theory. There is a positive feedback between the shape of the beach and the flow of the water creating a relief patterns. In the cusp center it is flat with lower relief that not only attracts the wave but accelerates the water allowing for more erosion. This erosion creates the embayment. The horns are areas of higher relief where water slows and sediments deposit on the top thus increasing the impact. The second main point about the regular spacing of cusps is that negative feedback will decrease the amount of net erosion and deposition within a well-formed cusp. As the wave strikes the beach, it will first come into contact with the cusp horns, which will slow the water down. This causes it to lose energy and some of the heavier sediment that it is carrying will be deposited. The loss of this sediment, however, gives the water extra energy and it uses this to remove sediment from the embayment on the backwash.
The standing edge wave theory is based on an interaction between the waves that are approaching and waves that have been set up perpendicular to the shoreline called “edge waves” which are caused by the angle of the approaching waves. These edge waves become trapped near the shoreline and when two of them come together from opposite directions, a standing edge wave is formed. The movement patterns of these waves are fixed and so can be defined as two regions of interest, the nodal (where no movement takes places) and antinodal points (where movement takes place as the water rises and falls.) So the incoming wave approaching at an angle has an almost uniform height but when it collides with a standing edge wave, this is changed. If it collides with a peak, then the wave height is increased and if it collides with a trough, then its height is decreased. The standing edge waves are subharmonic and can have a wave period twice that of the incoming wave, which produces a far more complex system of waves as by the time the incoming wave has completed one cycle from peak to trough, the standing edge waves have done two. Essentially what this means is that there are a regularly-spaced series of peaks and troughs along the length on the incoming wave that are caused by its interaction with the standing edge waves and it is these that caused the development of beach cusps. In areas where the wave height has been increased, the wave now has more power and so can erode more and in areas where the wave height has been decreased, the wave now has less power and so will not erode as much. This is what forms the cusps as the areas with high erosion become the embayments of the cusp and the areas with low erosion become the horns.
Both theories have issues, the standing edge wave theory is that it would only account for the initial formation of the cusp and not their continued growth afterwards because as the cusp increases in size the amplitude of the edge wave decreases to the point where it is no longer a factor. While the problem with the Self-Organization Theory is that this method of cusp formation would take time and if you were observing their formation, then you would see a number of random cusps form along the beach, which then slowly spread along the shore as they even out in size, with small cusps joining together and larger cusps being separated in two. But on the beach, cusps form a regular pattern almost instantly and they all appear at the same time.
This earthcache is best done at low tide. At the posted coordinates you will find yourself on the reflective cobble beach in an area where cusps have formed. As you along the beach in either direction you will find a number of cusps as you travel. These cusps are found in various sizes and shapes. Remember this is an earthcache so there is no container just an earth science lesson at an amazing natural feature. To log this cache, you must email me through my profile the answers to the following questions:
1. Which theory do you feel best describes the formation of cusps?
2. Describe the size of the cusps.
Please make sure to include the name of the earthcache in your email. In your log please take the time to describe what you find that is special about this wilderness beach, also if you include a photo it will give others something to look forward to when they visit.
If you look closely you will find cusps of various ages at different levels on the beach. Also if the waves are coming in watch the wave pattern and listen for the sound of the rolling rocks.