Dobs Linn, near Moffat, in southern Scotland is the type locality for that boundary. There, black graptolite-bearing shales are exposed. This chart is mapped, to allow you to travel back to the Cambrian or forward to the Silurian. The Ordovician Period is part of the Paleozoic Era . Ordovician rocks over much of these areas are typified by considerable thickness of lime and other carbonate rocks that accumulated in shallow subtidal and intertidal environments.
Quartzites are also present.
Rocks formed from sediments deposited on the margins of Ordovician shelves are commonly dark, organic-rich mudstones which bear the remains of graptolites and may have thin seams of iron sulfide.