In the first period of the phanerozoic eon, the cambrian period, we find north america lying on the equator, and the midcontinent (including ohio), completely submerged in a shallow sea (epeiric or epicontinental sea). Note: the volcanic island arcs offshore of the east coast of north america, the trench visible along those volcanic island arcs, between the islands and. North america: the passive margin of the west coast, compared to the active margin of the east coast. By late ordovician time (about 450 million years ago) note what has happened to north. America on ron blakey"s (ron blakey, nau geology) plate tectonics reconstructions. This is the first big mountain building event that starts the development of the appalachian. Mountains: the taconic parkway in eastern new york runs through exposures of this. The taconic unconformity is an angular unconformity showing the folding and faulting at this time: look at ohio-still a tropical, shallow sea.