A tombolo, latin for 'mound', is a sandbar that connects an island to the mainland. 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 sand or other sediments transported by the waves are deposited between the island and the mainland as they are swept together by waves from each end of the island. Over time, these sediments form a ridge that eventually becomes higher in elevation than sea level and connects the island to the mainland at all tides. Once attached, the island becomes known as a tied island.
Tombolos can also be formed by lateral longshore drift they are more accurately referred to as a spit in that case.
Morphology and sediment distribution
Tombolos are more prone to natural changes as a result of tidal and weather events than a normal beach is. 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.
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.
To log this Earthcache:
Send an email with answers to the following questions to me via my profile page. Please do not reveal any of the answers in your log. Any logs not accompanied by an email have to be deleted.
Please label your email as “Frank's Tombolo” so I will know which earthcache you are referring to.
1) While standing at the coordinates and looking across to Frank's Island, describe why you think you are looking at a tombolo and not a spit.
2) Using the international scale, describe the major sediment size you see at the tombolo. Would you describe this sediment as a fine, medium, or coarse, grained sand (small: 0.063 - 0.2mm, medium: 0.2 - 0.63mm, coarse: 0.63 - 2.0mm)?
3) Describe the appearance of the sediments that form the tombolo, such as colour and texture. Does there appear to be any quartz or shell fragements? If so, why do you think they are there? Hint, how would they end up as sand on a beach?
4) Did you notice any areas of the tombolo that seemed to have different sized sediment? If so, why do you think so?
5) (optional) Post a photo at the location when you log the earthcache.
Sources:
http://en.wikipedia.org/wiki/Tombolo
http://en.wikipedia.org/wiki/Grain_size