21. Approximately 24-36 million years ago these areas were subjected to massive volcanic activity during the Tertiary Period . 22. The geologic formations in this area date to the early Tertiary Period , some 30 million years ago. 23. This is due to the diversity of its geological-tectonic structure, particularly with the new vulcanization during Tertiary Period . 24. Finally in the later part of the Tertiary period the local climate entered a trend of increasing temperatures. 25. The Cenozoic is sometimes divided into the Quaternary and Tertiary periods , although the latter is no longer used officially. 26. These fossils range from the Triassic to the Tertiary periods , but are mostly restricted to the Triassic and Jurassic. 27. After being periodically submerged during the Jurassic and Cretaceous periods, the region finally became land in the Tertiary period . 28. The formation of the Rocky Mountains began in the Late Cretaceous Period and finished in the Early Tertiary Period . 29. This basin was formed by a prehistoric lake ( " Uinta Lake " ) during the late Tertiary period . 30. There are few if any rocks from the ensuing Tertiary period of the Cenozoic era in Pennsylvania's geologic record.
