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//__** Basic Geology of Isle au ** __//**//__Haut__//** Isle au Haut and the surounding islands are the result of volcanoes that were active around 450 million years ago. This resulted in various forms of igneous rocks to occur throughout the island. Three main forms of igneous rocks are present on Isle au Haut. The first kind, tuffs, is located on the northwestern part and southwestern part of the island. It is formed by the ashes spewed out of the now dormant volcanoes. It tends to sheer off into angular blocks. The example below was taken at the light house on the the northwestern section of Isle au Haut. The next kind of rock found commonly on Isle au Haut is granite. The majority of the granite is found in the central part of the island. This granite is fine grained and barely recognizable to the common viewer as granite. Lichens and mosses also commonly cover these rocks making them even more unreconizable. The picture below was taken on the part of Isle au Haut owned by Acadia National Park. The last kinds of rocks are found at the eastern side of the island, which includes Thunder Gulch. These rocks are found in alternating deposites of gabbro and diorite. The pictures below were taken at Thunder Gulch. The first picture below shows a stipe gabbro inbetween two stipes of diorite. The second picture shows a close up of where a diorite deposite (on the left) and a gabbro deposite (on the right) meet. **__//How Thunder Gulch Was Formed and Why it Thunders //__** Thunder Gulch was created over thousands of years. The gabbro deposite erodes more quickly than the diorite, leaving two walls of diorite. Over the years this gulch got deeper and longer. The picture below shows the size of Thunder Gulch looking across from about half way down to its bottom.
 * //Thunder Gulch //**

The thundering noise is caused by water rushing under boulders of gabbro that fell into the gulch. Air is pushed under the boulders with the waves. As the water moves under the rocks the air is forced out along with spray, causing the noise which gave Thunder Gulch its name. The pictures below demonstrate this. Unfortunately no pictures were taken when the sea was high enough to cause an actual 'thunder.'

The picture below shows boulders which have not yet reached the bottom of Thunder Gulch and their size relation to a person.

**Created by: Zoey Blittersdorf and Rachael Munroe**