The Walloomsac Formation - The Bearer of Iron

The Walloomsac Formation was formed from volcanic ash falling on top of a coral ref in the back arc basin of the volcanic arc that formed off the south coast of the North American craton in the mid-Ordovician. Since that time the North American Craton has rotated approximately ninty degrees counterclockwise making the ancient south coast today's east coast. The situation at the time was similar to today's Japan and the Sea of Japan between Mainland Asia and Japan.

The ash fall was intermittent depending on volcanic activity in the adjacent island arc. The prevailing wind would have blown the volcanic ash into the back arc basin. We must assume that the volcanics were mafic to andesitic in nature having ejecta that was enriched in iron and magnesium.

This was originally laid down as interbedded layers of ejecta and coral reefs that grew on top of each layer of ejecta after it was erupted. At the beginning of the eruptive cycle in the island arc the ejecta was more mafic then it became later as it approached andesite in composition. It was these early eruptives that supplied the iron and magnesium that later were converted into dolostone and iron ore.