Welcome to Outer Point, this short yet scenic trail will take you to a beach out on Douglas Island. Parking can be found at the noted coordinates. Please enjoy the views, take some notes and learn a little bit about the geological forces acting on this quasi-island! Note thas as this is an earthcache there is no container to be found at the posted coordinates. In order to log this cache as "found" you must send the cache owner answers to the logging requirements at the end of the description, this is most easily completed with the message center feature. If you have any questions or concerns, please reach out to me!
Tidal Islands and Tombolos:
A tidal island is a piece of land that is connected to the mainland by a natural or man-made causeway that is exposed at low tide and submerged at high tide. A tombolo 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 or tidal island. The tombolo you are observing at the posted coordinates is a tenuous one: it is only there at low tide! At high tides the island is fully cut off from the mainland.
Formation of Tidal Islands:
Tombolos are prone to natural fluctuations of profile and area as a result of tidal and weather events; even moreso than a normal beach! Due to this susceptibility of weathering, tombolos are sometimes made more sturdy through the construction of roads or parking lots. Note that this is not the case here! You are standing at a natural and unaltered tombolo! 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.
Wave Refraction and Diffraction:
True 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 (as demonstrated in the image below). 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.
Tombolos are perfect examples of how sensitive a shoreline can be! A small piece of land, such as an island, can easily change the way that waves move, leading to different deposition of sediments and a fundamentally different shoreline!
LOGGING REQUIREMENTS:
Please answer the following questions, to your best ability, using the information above and your observations at the posted coordinates. Any "Found It" logs that fail to submit the answers to the CO in a timely manner are subject to deletion. I will attempt to reach you to resolve any discrepancies prior to taking this course of action. While all questions can be answered from the posted EC coordinates, I highly encourage you to visit at low tide and walk out to Shaman Island, the natural terrain and views of Chilkat Mountains from the far side is a sight to behold!
Please include the GC code of this cache and the number of people in your party in your message.
1) What time of day did you visit? Is the land between the island and the mainland visible? (In other words did you visit at high or low tide?)
2) Describe the action of the waves against the stretch of land and the island. If you visit the Tombolo at high tide and the land is not visible, you can still answer this requirement by observing the water over the spit of connecting land. Is the water calm or tumultuous? Is the process building or eroding the Tombolo?
3) Optional: Take a photo of yourself on or in front of Shaman Island!
Sources:
- Glossary of Geology and Related Sciences. The American Geological Institute, 1957
- Easterbrook, Don T. Surface Processes and Landforms, Second Edition. 1999 Prentice Hall Inc.
- Neal, William; Orrin H. Pilkey; Joseph T. Kelley (2007). Atlantic Coast Beaches: A Guide to Ripples, Dunes, and Other Natural Features of the Seashore. Missoula, MT: Mountain Press Publishing Company. p. 272.
This cache has been authorized and is covered under Permit #15-KA-1994.