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He next investigated the Devonian rocks and fossils of the Bas-Boulonnais; and in 1839 accompanied Sedgwick and Murchison in a study of the older Palaeozoic rocks of the Rhenish provinces and Belgium, the palaeontological results being communicated to the Geological Society of London in conjunction with the Vicomte d'Archiac.
The hill is formed from mudstones of the St Maughans Formation of the Old Red Sandstone laid down in the Devonian period.
Most of Mountjoy’s research has concentrated on Jasper Park's Miette area, and in particular on its exposed coral reefs from the Devonian period (395 million to 345 million years ago).
The hill has not eroded as fast as the surrounding limestone as it is made of Old Red Sandstone deposited in the Devonian period approximately 400 million years ago.
Rhacophytales are an extinct group of plants from the Devonian period.
Givetian, also known as Tioughniogan or Tioughnioga stage, the middle stage of the Middle Devonian period