21. Two land masses collided at the end of the Ordovician Period about 466 million years ago. 22. These are from the Ordovician period and consist of chert, phyllite and quartz rich micaceous sandstone. 23. During the early and middle Ordovician period ( 485 to 460 Mya ), volcanic activity increased. 24. By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats. 25. The valley lends its name to the Borrowdale Volcanic Group, a geological development from the Ordovician period . 26. These deposits occurred during the Ordovician period . 27. The "'Fish Haven Dolomite "'is an Ordovician period geologic formation in southern Idaho, northeastern Nevada, and northwestern Utah. 28. These fast-moving low-level epifaunal deposit feeders lived in the Cambrian and Ordovician periods , from 494.0 to 478.6 Ma. 29. These fast-moving low-level epifaunal deposit feeders lived in the Cambrian and Ordovician periods , from 498.5 to 478.6 Ma. 30. Graptolites with relatively few branches were derived from the dendroid graptolites at the beginning of the Ordovician period .
