Introduction
With Just a short drive from Roundstone village lies Dogs Bay, one of the most beautiful beaches in Ireland.
Dogs Bay is a horseshoe shaped bay with more than a mile long stretch of white sandy beach. It backs on to Gurteen Bay, and together they form a tombolo which juts out into the Atlantic Ocean. Similar to Gurteen Bay, the sand is not comprised of traditional limestone but rather made entirely of fragments of seashells like microscopic skeletons of one-celled creatures called Foraminifera, which give it a pure white colour.
The Dogs Bay tombolo
Dog’s Bay Beach is a famously beautiful sandy beach in Connemara on the west coast of Ireland and is home to rare ecological features.
The beach is actually part of a geomorphological feature known as the Dogs Bay/Gorteen Bay tombolo. A tombolo is a spit or bar of sand or gravel connecting an island to the mainland or another island. In general terms, persistent winds from the southwest have meant that waves meeting the island and wrapping around it, slow down as they converge on the northeast and landward side of the island, where force and speed of the waves decreases and they deposit their load of sediment. Over time the sediments gradually accumulate to such an extent that they rise as a bar above the water. The sediment bank eventually stretches all the way from the island to the mainland, and the connecting bar is termed a tombolo.
Dogs Bay faces due west. It is a stunning beach with its bright white sands and crystal blue water. Since 1991 the dunes separating Gurteen Beach from Dogs Bay have been the focus of intensive efforts to plant Marram Grass to stem coastal erosion. Visitors are requested to respect the signs restricting access to the dunes. The wild headland which stretches out into the Atlantic between the two beaches is an area of great beauty and is a fantastic walk where you will be surrounded by water on three sides. The diversity of the dune portion of the site is added to by the presence of wetland areas, including brackish pools and a freshwater marsh.
The tombolo, spit and beach features have been formed in the Holocene Period, since the last glaciation, and the bays themselves host soft sand sediment washed into them during that time.
It is likely that the macro-structure of the ‘island’ at the end of the bays, and the mainland bedrock ridges, date back through the Quaternary (Ice Age) to the Tertiary Period.
The bedrock here is Errisbeg Townland Granite (ETG), a pink to pale grey coloured, coarse porphyritic granite, with large pink K-feldspar (potassium-feldspar) crystals. The Errisbeg Townland Granite is one of the granite varieties of the late-Caledonian Galway Batholith. The granite has been dated to 400 Ma.
The sand
Pilat dune, France
The geologist, at the beach, spreads his towel on a loose detrital sediment whose grains are mostly between 1/16 mm and 2 mm, that is to say on sand.
The nomenclature of sands takes into account both the grain and the nature of the dominant elements (quartz, limestone ...) or particular (micaceous, gold ...). The study of the morphology of the grains allows to specify the modes of sedimentation and transport (fluviatile, marine, dune...).
Depending on its composition, the sand can have very different aspects.
Sand samples: Each photo corresponds to one square centimeter of sand. From left to right: 1. Kauai glass, Hawaii. 2. Quartz and spinel, Sri Lanka. 3. Green glauconite, Estonia. 4. Reddish volcanic basalt, Hawaii. 5. Coral, Hawaii. 6. Quartz colored with hematite, Australia. 7. Hematite, California. 8. Emerald, Idaho. 9. Olivine, Hawaii.
The Dogs Bay sand
The sand has become stabilised by the growth of vegetation; and at the present time is a very special and rare type of habitat known as machair. Machair only forms on calcareous soils. At Dogs Bay the sand is composed mainly of minute fragments of the carbonate skeletons of marine animals such as sea urchins and their spines, sponge spicules, bryozoa, seashells, snails, and most remarkable of all, the intricate microscopic skeletons of one-celled creatures called Foraminifera.
Dogs Bay is one of only a few beaches in the world with predominantly Foraminiferan sand. and, on top of that, this is one of the only places in the world where this kind of sand can be found onshore, the site is already an SAC/pNHA (Dog’s Bay, sitecode 001297).
These Foraminifera have been swept up from deeper water into the channel between the small island and the mainland, where they have accumulated to form the tombolo, which, as a result, is comprised of 90% calcium carbonate.
One can find several types of Foraminifera shells in Dogs Bay sand: Acervulina inheres, Fissurina orbignyana, Globulina gibber, Pseudopolymorphina sp.
Types of Foraminifera shells found in Dogs bay sand
Bibliographic sources
- https://www.theirishroadtrip.com/dogs-bay-beach-galway/
- https://foraminifera.eu/remaud.html
- https://natureinfocus.blog/2014/11/01/erosion-of-the-tombolo-at-dogs-bay/
- https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/GY054_Dogs_Bay.pdf
- https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/GY055_Dogs_Bay_and_Gorteen_Bay.pdf
- https://books.google.fr/books?id=Z4d8EAAAQBAJ&pg=PA328&lpg=PA328&dq=geology+dog%27s+bay+irlande&source=bl&ots=kT_2_hWGt7&sig=ACfU3U1jvz7OhfezARw8VjpLSG11zK7Ekw&hl=fr&sa=X&ved=2ahUKEwjiuue64ar5AhVRnRoKHdbgApwQ6AF6BAg3EAM#v=onepage&q=geology%20dog's%20bay%20irlande&f=false
- https://foraminifera.eu/loc.php?locality=Dogs%20Bay
To validate this Earthcache
ATTENTION, you are now entering a special area of conservation, please respect the place and leave nothing but footprints.
Tools needed for field questions: a magnifying glass, one or two sample boxes to collect less than 1 cm3 of sand, some observation and patience.
In order to validate this EarthCache, you will need to answer the following questions correctly and add a photo of yourself, your GPS or your nickname to your log at the cache waypoint. This photo will validate the "found" in case of wrong answers to the questions.
Please send your answers via my profile or via the geocaching.com messaging system, do not give them in your log. The logs recorded without answers will be deleted.
You can log the "found" cache without waiting for my confirmation, I will contact you in case of problem.
Reminder about Earthcaches: There is no container to look for or logbook to fill out. Just go to the location, answer the questions and send the answers back to me.
Question 1 : Can you explain with your proper words how a tombolo is created ?
Question 2 : Collect some sand closed to the see side. Let it dry if necessary. By using your magnifying glass, can you describe the sand you collected? Color? Estimated grain size?
Question 3 : In this sample, do you found other grains but Foraminifera shells ? If the answer is yes, could you tell me what and roughly estimate the percentage of it ?
Question 4 (OPTION) : When you get to the little cliff that marks the edge of the beach, collect another small sample of sand. Can you compare it to the last sample you collected? Grain size ? Percentage of Foraminifera shells ? What can you deduce from it?