“What’s a tombolo”, you ask? Well, that’s a good question, because it is not a commonly used word. In simple geological terms, a tombolo is an island that has become attached to the “mainland” by a narrow spit of land, forming a peninsula or isthmus. Sometimes several islands may become attached in a series, tied together by bars which rise above the water level. This is called a tombolo cluster, and that is what I think has taken place here in Ferryland.
Another possibility is that the linkages between the shore and the islands are man-made - where the road is now leading out to the Lighthouse - but I can find no historical record of this being the case, and very old maps (from the time before front-end loaders!) show the tombolo cluster as it is today.
How exactly are tombolos formed?
Over time (and this can take thousands of years, or it may or take place almost overnight, as in the case of a major storm rearranging sand bars) the shoreline moves toward an island due to sediments accumulating in the lee of the island where wave energy and longshore drift are reduced and therefore sand deposits occur.
Essentially, tombolos are formed by wave refraction and diffraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then bend around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drift 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.
One good example of a tombolo in this part of the world is the (currently) unified island of Miquelon, which actually consists of three geological islands: Miquelon, Langlade (also called Petite Miquelon) and Le Cap, connected with sand dunes. A 13 km long tombolo sandspit called La Dune connects Miquelon and Langlade. In the 18th century it was still possible to sail a boat between Miquelon and Langlade, but by the end of the century La Dune had closed in to form an isthmus between the islands.
Take a look at this satellite photo of part of Ferryland Head, and see how there are two very narrow points. One is at the posted coordinates for this Earthcache, and the other is about 1.25 kilometers directly east at N47° 01.287 W052° 51.932 (second waypoint), where there is a “cove” on either side of the narrow strand that leads out to the larger portion of land (the second island!) where the Lighthouse is.
To me, it really looks like Ferryland Head actually originally consisted of two islands, which eventually became joined together as a tombolo cluster. Either that, or the innermost section of the strand was a tombolo that has gotten thinned in sections with the action of waves and current.
As further evidence of this, check out this antique but very detailed map of Ferryland, dating back to 1769, which clearly shows two almost-separate islands joined together by very narrow bars of land.
Logging requirements
Please remember this is an EarthCache, so there is no physical container to find at the posted coordinates.
Your mission, to qualify for this Earthcache, is to gather some basic data at the two narrow spots along the spit of land that joins Ferryland Head to the shore, and let me know whether or not you agree with my theory that this is a tombolo.
To claim this Earthcache as "Found", please contact me using the “message this owner” link at the top of the page, with your answers to the following items:
- At the posted coordinates, estimate the width and height of the strip of land between the ocean waters on each side.
- Estimate the width and height at the second set of coordinates: N47° 01.287 W052° 51.932 (you can drive to and park close to that spot).
- Which of the two locations is narrower?
- What time of day were you there, and what were the waves and tides like along the adjacent shore?
- Based on your observations, do you think this strip of land may have been formed as a tombolo?
- After sending me the required answers, go ahead and log your find. Please do not post your answers in your online log.
- Optional (but highly appreciated) – in your online log, attach photos of yourself taken along this strip of land – it is a ruggedly beautiful location, isn’t it!
Resources:
Geology.com
Wikipedia.org
Geology.about.com
Endnote - Ferryland is where my father's family has lived for generations, since my ancestor Matthew Morry emigrated here from Dartmouth in England in the late 1700's.
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