Layers of Cretaceous and Tertiary deposits exposed in the bank of the Brazos River, Texas. The Tertiary layers overhang the Cretaceous layers. By permission of Norman MacLeod, Department of Palaeontology, Natural History Museum, London.
The geological era that began about 250 million years ago is called the Mesozoic--"Middle life."
FOSSILS
In Mesozoic rocks reptiles of all kinds, including flying reptiles, are abundant. Some--if the dinosaurs were reptiles--are huge. There are diverse free-swimming marine animals. The first fossils of birds and of mammals are found in Jurassic rocks in the middle Mesozoic.
Fossils of many kinds of cone-bearing plants are numerous throughout the Mesozoic. In the late Mesozoic (Cretaceous period), flowering plants and hardwood trees become diverse.
From the Cretaceous (chalky) period, there are numerous layers of limestone formed from the shells of small marine animals that live in shallow, warm seas.
The latest appearance of dinosaurs, ammonites and giant clams is in rocks of the late Cretaceous period. No large land animals survived from the Mesozoic into the Tertiary period.
Layers of Cretaceous and Tertiary deposits exposed in the bank of the Brazos River, Texas. The Tertiary layers overhang the Cretaceous layers. By permission of Norman MacLeod, Department of Palaeontology, Natural History Museum, London.
GEODYNAMICS
At the beginning of the Mesozoic, there is extensive volcanic activity. In the early Mesozoic, the Palisades of the Hudson River valley near New York are formed as a volcanic lava flow. Much of western America is undersea in the mid-Mesozoic. Late in the Mesozoic, mountain building takes place in the western U.S. with uplifting and volcanic activity. There are numerous advances and retreats of the sea.
CLIMATE
Coal swamps form in the late Mesozoic. South America in the Permian is in a glacial age.