The beach is a great place to play on the sand, build fantastic castles (like ours), relax, surf, swim or fish, I couldn’t imagine many of us stop to think about Geology while we are here. On this occasion let’s do so.
So what are we looking at?
Mother Nature’s Impressive sandcastles.
By definition a dune is a hill of sand built by either wind or water flow. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter slip face in the lee of the wind. The valley or trough between dunes is called a slack. A dune field is an area covered by extensive sand dunes. Large dune fields are known as ergs.
Simplified formation diagram
There are many types of Dunes.
Tranverse Also known as Crescentic are Crescent-shaped mounds that are generally wider than they are long. The slipfaces are on the concave sides of the dunes. These dunes form under winds that blow consistently from one direction, and they also are known as barchans, or transverse dunes. Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in the China's Ningxia Province.
Linear Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometres long. Some linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional wind regimes.
Star Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally.
Dome Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas.
Parabolic U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation. The largest arm known on Earth reaches 12 km. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of the sand in the dune migrates forward.
Longitudinal Longitudinal dunes often called Seif dunes, they elongate parallel to the prevailing wind, possibly caused by a larger dune having its smaller sides blown away. They are sharp-crested and are common in the Sahara. They range up to 300 m in height and 300 km in length. These dunes are thought to develop from barchans if a change of the usual wind direction occurs.
Reversing dunes Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions.
Types of Dunes.
All these types of dune shapes may occur in three forms: simple, compound, and complex.
Simple dunes are basic forms with a minimum number of slipfaces that define the geometric type.
Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed.
Complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune.
Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.
What are we looking at here?
Well I’m not going to give away the answer you need to decide from the list above what type of dunes you are examining.
What I can tell you;
Beaches and dunes form along sandy coasts located adjacent to shallow seas and usually a predominant onshore wind. A small bulge on a beach can be enough to start dune formation. It is less windy behind the bulge so the wind-blown sand stays put. As long as it doesn't wash away and the wind continues to supply sand, the dune will continue to grow.
Plants help a dune to form. They hold down wind-blown sand with their stems and leaves. A mini dune can form behind a tuft of grass. When the dune has grown around one meter in height, rainwater will be retained in a freshwater bubble under the surface. Native grass will now start to dominate. It grows quickly, holding down even more sand. When the young dunes grow together into a new beach ridge, new dune slacks will form on the land side. Other plants start to grow. Thick bushes and ground covers start to be dominant. Several native plants in the area are nitrogen fixers, these plants add nitrogen to the soil with the help of bacteria which live in the specialised roots, this improved the soil quality allowing other more sensitive plants to grow. Eventually all of these plants together stop the sand from blowing away.
Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea.
To log this cache we require you to message us with the following answers to the best of your ability;
1. What type of Dunes are these?
2. We have purposely left the native vegetation unnamed in the cache notes please name three native plants at GZ inclusive of at least one Botanical name, common names for the other two are acceptable.
3. A photo of your team, GPS or a sandcastle you have built near GZ with your log and answers. (optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers. Cacher’s who do not fulfill the Earth Cache requirement will have their logs deleted.
We hope you enjoy the area, keep a look out for the native grasses that grow along the edge of the beach and the patterns they form with their leaves blowing in the wind we think it is pretty cool. Kudos to Gonefishen07 who has this beach full of fantastic caches. We hope we taught you something. Happy Caching Cheers TeamThommo
Source – Wikipedia, Bass Coast Shire
