BRIDGES OF ROSS
Until around 100 years ago there were two natural bridges here.
Hence the name - Bridges of Ross (plural). But one collapsed
leaving the impressive arch and lagoon before you. For interest's
sake, a picture of the now collapsed bridge appears below (courtesy
of The Print Collector / Heritage-Images).
To log this earthcache you have to take photos and measurements
at the location and answer further geological questions. There is
no information board at the site so you will have to do your own
research.
Logging the cache will involve a short cliff-top walk. Please wear
footwear appropriate for the conditions.
GEOLOGICAL CONTEXT
The Bridges of Ross occur at a rocky outcrop which forms part of
the Ross Sandstone Formation. The Ross Formation (for short)
accounts for the majority of the coastline of the Southwestern tip
of County Clare (and parts of the Southern shore of the mouth of
the River Shannon) as indicated in yellow on the map below.
The Ross Formation dates from the Namurian period of the
Upper-Carboniferous era and is world renowned as an example of a
turbidite geological formation. Turbidity current deposits are like
an underwater avalanche which spills down slopes and through
gulleys or canyons forming sedimentary layers where it settles. In
general terms, the Ross Formation comprises a series of stacked,
relatively small, sand-rich “submarine fan” systems
– see the illustration below.
The nature of the rock that is formed is determined by the
material flowing in the turbidity process. In the Ross Formation,
the sand-rich nature of the deposits means that it is largely
sandstone (65% at Loop Head) although there are examples of
interbedded shales and other deposits of mixed origin. The Ross
Formation is at its thickest in Southern Clare where it is
typically between 380-460 metres thick.
THE BRIDGE
There are many processes involved in the formation of a natural
arch or bridge such as the one here. However, the most important
contributors to the creation of this bridge / arch (and the
ultimate demise of the other one) include:
Bedding plane expansion. The boundaries between
the sedimentary layers of the Ross Formation, known as bedding
planes, are similar to joints or cracks. Water seeping between the
bedding planes can cause chemical exfoliation and freeze expansion.
This often leads to the growth of a horizontal air gap between the
layers of rock. In this way, the expansion of a bedding plane in a
rock exposure can contribute to the formation of a natural
arch.
Wave action. The waves that batter the shoreline
of a large body of water, such as an ocean, sea, or great lake, are
a major force of erosion on any coastal rock exposures that are
present there. Waves trigger and accelerate several erosional
processes. Particularly relevant here are chemical exfoliation and
differential erosion. In addition, particles carried in the waves
(e.g. sand) act as an abrasive on the rock. As a result, coastal
rock exposures experience erosion rates ten to a thousand times
higher than those inland. Therefore, coastal natural arches are
formed and destroyed relatively quickly and frequently. They are
short-lived compared to most inland natural arches.
Chemical exfoliation. Water that is in contact
with rock will, over time, dissolve the lattice of fine crystalline
grains that cement the larger grains of the rock together. In
effect, the water dissolves the rock into grains which can then be
removed either by the water itself, gravity, wind, or other
mechanisms – a process known as chemical exfoliation.
Differential erosion. When erosion proceeds at two
different rates at the same location, e.g. on adjacent rock
surfaces, it is called differential erosion. This can happen
wherever the grain and cementing properties of rock vary from place
to place in a rock exposure. Such differences commonly occur when a
rock exposure comprises more than one geological formation or
member. Each member will erode at its own pace. However, many
geological members form as the result of a long period of
sedimentary deposition. Such a member may consist of several layers
laid down at vastly different times. Differences in graining and
cementing can certainly occur between such layers. Therefore,
differential erosion can occur in a rock exposure that consists of
a single member.
Freeze expansion. When seeping water that has
permeated a rock joint freezes, it expands. This puts stress on the
rock and frequently fractures the rock adjacent to the joint. As
the water thaws and is replenished from whatever source is
involved, it gains access to these fractures. In this way, repeated
cycles of freezing and thawing will break up the rock along a joint
into small pieces that can then be removed by gravity or water
pressure.
LOGGING THE CACHE
Before you log this cache you must email me the following
measurements / information:
Q1 How long is the arch (from one side of the lagoon to
the other)?
Q2 How far is it from the centre of the arch to the
furthest end (West) of the lagoon?
Q3 The Ross Formation is overlaid by another geological
formation (shown in grey in the first diagram above). What is its
name?
Q4 How many millions of years old is the Ross
Formation?
Once I have given permission, you may then log your find. Please
feel free to upload photograph(s) evidencing your visit to your
log.
Happy caching!