Welcome to
Roosevelt Campobello International Park, this 2800 acre park was
established in 1964. The main attraction for the park is
the Roosevelt Cottage, while the less visited Natural Area with
its coastal headland, rocky shore, sphagnum bog, field, forest
and beaches make up most of the area of the park and is a
special attraction to the outdoor explorer as those interested
in the geology of natural areas.
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
containing several berms (platforms) from various storms, these
berms are best viewed at low tide. These
are complex erosional and depositional features formed during
storms when water level is elevated above its average
position. A natural seawall is established at the landward
limit of the worst winter wave influence, and is clear testimony of
a stormy past. 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
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 with medium sized waves. At the posted
coordinates you will find yourself on the reflective cobble beach
in an area where cusps have formed. As you head down the
beach you will find a number of them as you travel. 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 post a photo of yourself or your hand with your GPS
showing the view of the cobble beach cusps in the background and
email me through my profile the answers to the following
questions:
- Which theory do you feel
best describes the formation of cusps?
- Describe the size of the
cusps.
Please make sure to
include the name of the earthcache in your email.
If you look closely you will find cusps of
various ages on the different berms. Also if the waves are
coming in watch the wave pattern and listen for the sound of the
rolling rocks.
For further
information on the Roosevelt Campobello International
Park and its features go to the park website.