A tombolo, from the Italian tombolo, derived from the Latin tumulus, meaning 'mound', and sometimes translated as ayre (Old Norse eyrr, meaning gravel beach), is a deposition landform in which an island is attached to the mainland by a narrow piece of land such as a spit or bar. Once attached, the island is then known as a tied island. Several islands tied together by bars which rise above the water level are called a tombolo cluster. Two or more tombolos may form an enclosure (called a lagoon) that can eventually fill with sediment. A tombolo may be considered to be a type of isthmus.
Wave refraction:
True tombolos are formed by wave refraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then refract or bend around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drifting on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there, conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up, the beach shoreline, known as a spit, will connect with an island and form a tombolo.
Lateral longshore drift:
In the case of Chesil Beach or Spurn Head, the flow of material is along the coast in a movement which is not determined by the now tied island, such as Portland, which it has reached. In this and similar cases, whilst the strip of beach material connected to the island may be technically called a tombolo because it links the island to the land, it is better thought of in terms of its formation- as a spit or otherwise.
Morphology and sediment distribution:
Tombolos are more prone to natural fluctuations of profile and area as a result of tidal and weather events than a normal beach is. Because of this susceptibility to weathering, tombolos are sometimes made more sturdy through the construction of roads or parking lots. The sediments that make up a tombolo are coarser towards the bottom and finer towards the surface. It is easy to see this pattern when the waves are destructive and wash away finer grained material at the top, revealing coarser sands and cobbles as the base. Sea level rise may also contribute to accretion, as material is pushed up with rising sea levels. This is the case with Chesil Beach (which connects the Isle of Portland to Dorset in England), notable because the shingle ridge is parallel rather than perpendicular to the coast.
Tombolos demonstrate the sensitivity of shorelines. A small piece of land, such as an island, can change the way that waves move, leading to different deposition of sediments.
"Wellfleet Harbor occupies a depression that was formed when ice, possibly a sublobe of the South Channel lobe, prevented outwash deposition. The islands (A) are composed of Wellfleet outwash plain deposits that filled holes or depressions in the ice. The islands are tied together and to the mainland by spits called tombolos (B) composed of sand eroded from the cliffed Cape Cod Bay shore to the north of the harbor. Once the tombolos were in place, they provided protection from waves and marsh deposits (C) formed in the sheltered water."
Logging Requirements:
Note: Please do not post answers in logs. Please Email answers and then log the EarthCache.
Overlook near Tavern Site:
1.Take an altitude reading.
2.Describe the tide as you look out across the Tombolo. In which direction is the current pulling the waves?
3.Describe the shoreline at this point of the Tombolo. What does it consist of?
Middle Meadow Marsh:
1.Lookout southeast and describe what you see.
2.This area was not originally here. Postulate how it was formed.
Jeremy Point High Tide Marker:
Depending on the tide, walk out as far as you choose. (See link to Tide Table. Be cautious if tide is coming in!)
1.Look at both sides of the split and determine which way the current is flowing?
2.Postulate how the split was formed.
Safety Precautions:
Tide Table: USHARBOR Wellfleet Harbor Tides
Please remain on the designated path, and use the established crossover trail to the beach. Foot traffic erodes sensitive dunes. Remain on designated trails to reduce exposure to disease carrying insects, poison ivy, and other risks. Observe trail conditions and be aware of tree roots, stumps, and other naturally occurring hazards.
Ticks feed on blood and will bite humans. The bites can be irritating and sometimes transmit diseases including Lyme disease, Rocky Mountain Spotted fever and Tularemia. TO AVOID TICK BITES: Stay on designated pathways, avoid grassy or brushy areas. Wear light colored clothing and tuck trouser cuffs in socks. Check clothing and body frequently for ticks. Apply insect repellent to clothing and exposed parts of the body. Keep pets on a leash. Treat with insecticide powders or sprays labeled for tick control. Carefully remove attached ticks immediately. IF YOU SHOULD BECOME ILL OR EXPERIENCE A RASH WITHIN TWO-THREE WEEKS OF YOUR VISIT TO A TICK INFESTED AREA, SEEK MEDICAL ATTENTION. TELL YOUR PHYSICAN THAT YOU RECENTLY HAVE BEEN IN A TICK INFESTED AREA.
Drink plenty of water before, during, and after you are active. This is very important when it's hot out and when you do intense exercise. Fluids such as re-hydration drinks, juices, or water help replace lost fluids, especially if you sweat a lot. Remember it is possible to dehydrate even in the winter.
References:
http://en.wikipedia.org/wiki/Tombolo
http://pubs.usgs.gov/gip/capecod/fig22.html
Thanks to Angela Harris, of the National Park Service, for all her help in getting the EartheCache approved.
Thanks to Foragess for her guidance in helping me to discover Tombolo's
*Congratulations to CaseyToo for FTF!*
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