Upper Cretaceous

Eon	Era	Period	Epoch	Start Million years ago
Phanerozoic	Cainozoic	Palaeogene	Palaeocene	66
	Mesozoic	Cretaceous	Upper Cretaceous	100.5
			Lower Cretaceous	145
		Jurassic	Upper Jurassic	163.5
			Middle Jurassic	174.1
			Lower Jurassic	201.3
		Triassic	Upper Triassic	237
			Middle Triassic	247.2
			Lower Triassic	252.17
	Palaeozoic	Permian		298.9

The Late Cretaceous

Chalk cliff of Mers-les-Bains, at high tide. You can just see slanting dark lines on the chalk: these are formed by lines of flint, the only other constituent of chalk.

The Upper Cretaceous was a period in Earth history, from 100.5 to 66 million years ago.

The Cretaceous is traditionally divided into Lower Cretaceous (early), and Upper Cretaceous (late), because of the different rocks. The rocks reflect the conditions in which they were formed as sediment.

The Upper Cretaceous is the chalk. It is composed of countless millions of calcareous (CaCO₃) plates called coccoliths. They are so small they can only just be seen with a light microscope; details require an electron microscope. The plates are formed by single-celled planktonic algae called coccolithophores, and were laid down in the off-shore seas.

The only other rock found in chalk is the flint, which is siliceous (silica, SiO₂). This derives from those algae and animals which have skeletons of silica.

The Cretaceous was the last period when dinosaurs were the dominant land animals. Triceratops, Tyrannosaurus and Velociraptor lived at this time. The huge Mosasaurus was the dominant marine predator. In the Cretaceous period, birds became more diverse. Flowering plants developed more, and became the dominant plants on land. The Upper Cretaceous ended with the K/T extinction event.

References

Geologic History

Source

International Chronostratigraphic Chart 2013. International Commission on Stratigraphy, retrieved 8 April 2013. Divisions of geologic time – major chronostratigraphic and geochronologic units USGS, retrieved 8 April 2013.