Tumultuous Tumulus EarthCache
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Grand Manan is well known for its high cliffs and rocky coves, with only a few sandy beaches. Geologists use rocks in their original locations, where they hope to read the story of how this part of the Earth was formed. This feature will give you a bit of insight into what happened here 200 million years ago during the Early Jurassic period.
Grand Manan has a “split personality” regarding its physical geology. The western 2/3 of the island contains thick lava flows, which are little changed from when they cooled. They are part of the enormous “flood basalt” that underlies most of the Bay of Fundy, and which erupted 200 million years ago. The same lava flows crop out along the western shores of Nova Scotia, where they are known as the North Mountain Basalt. Here it is referred to as Dark Harbour member of the Grand Manan Basalt.
North Mountain Basalt of the Fundy Basin has a present area of about 16,500 square km and thickness of 200 to 1,000 m. An average of 400 m thickness indicates at least 6,600 cubic km in volume. This is the largest known lava eruption. The basalt is thickest in the central, deeper part of the Fundy basin, indicating a giant pond flow or lava lake.
The lava as it flowed did not always flow smoothly. Periodically the flow would form small domed structures or a pressure ridge. As the surfaces of the lava flows on a flat or gentle slopes, domed structures called tumuli are formed. A tumulus is created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward. Since the solid crust is brittle, it usually breaks to accommodate the "inflating" core of the flow. Such fractures generally extend along the length of a tumulus, and are frequently accompanied by smaller irregular cracks down the sides. Lava commonly squeezes out through these fractures, and sometimes drains from the tumulus to leave a hollow shell.
Early settlers in the area discovered these formations to be a good source of copper. If you look at the formation carefully you will see the green color of the copper minerals in the rock. You should be able to find evidence of primitive mining.
At the posted coordinates you will find yourself looking up at an ocean side cliffs that has been exposed to the action of the waves and weather for years. You should be able to easily identify the arched features in the bedrock at low tide. You should be able to identify the layers of lava and see the cracks in the top section.
Remember this is an earthcache so there is no container just an earth science lesson at an amazing natural feature that needs to be protected. To log this cache, email me through my profile:
1) The name of the Earthcache and the number of people in your group.
2) Estimate the length of this pressure ridge?
3) Can you guess the direction of the flowing lava?
In your log please take the time to describe what you find special about this location and its unique features, it is also nice to include photo so other have some idea what to expect.
Do not try to take the shortest path to this location, as you will find while it maybe short it is not quick. Use the “Red Trail” that circles the Island along the shore. It can be accessed from the Southwest Head Lighthouse travelling to the west along the cliffs. You will see a red markings as well as an ATV trail. This earthcache is best visited at low tide and remember the tide moves very quickly.
If you would like more information on the geology of the area, Greg McHone has an outstanding webpage called Grand Manan Geology at (visit link)
Additional Hints
(Decrypt)
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Treasures
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