The boundary between Sedgwicks's Cambrian System and Murchison's Silurian System was never really resolved during Murchison's life time. Sedgwick had attempted to draw his boundary based only on lithological criteria, an unworkable approach. Murchison, using macrofossils, extended his Silurian well into Sedgwick's Cambrian. Charles Lapworth clarified everything in 1879 by recognizing three different graptolite faunas in a complicated Scottish section. Lapworth inserted the Ordovician System between the Cambrian and Silurian because these rocks had a distinctive graptolite assemblage not found in the rocks stratigraphically above or below.
Calcium carbonate is more rigid than calcium phosphate.
Rugose corals and stromatoporoids played ancillary roles. During the Ordovician the equator extended across North America through what is now Baja California to Greenland and across the British Isles and Central Europe. Sediment from the mountains began to fill the basin. The foreland basin sequence of sediments in eastern New York is listed below from oldest to youngest. The gradual deepening of the foreland basin in front of the advancing island arc is documented by the changes in lithology along an east-west transect through the Mohawk Valley.