It was a time of global high sea levels that began at the end of the Jurassic Period about million years ago and the beginning of the Paleogene Period about 66 million years ago. During the Cretaceous, warm waters of epeiric seas, seas that flooded continental crust during sea level highs, existed in many parts of the world. Warm waters of the epeiric seas facilitated chalk deposition because calcium carbonate is more soluble in cold water rather than warm water, and because organisms that produce calcium carbonate skeletal debris will more actively produce in warm water.
More chalk formed during the Cretaceous Period than in any other period in geologic history. The Cretaceous received its name after the Latin word creta , which means "chalk".
Coarse Chalk: A specimen of chalk with a coarse grain size from the Cretaceous-age Kristianstad Basin collected at a gravel pit near the community of Luneburg, northern Germany. This specimen is from the geological collection of the City Museum of Berlin, and the image is used under a Creative Commons license.
Click to enlarge. The keys to identifying chalk are its hardness , its fossil content, and its acid reaction. At a glance, diatomite and gypsum rock have a similar appearance.
An examination with a hand lens will often reveal the fossil content, separating it from gypsum. The acid reaction will surprise you if you are used to testing other types of limestone and have never tested chalk.
When you apply a drop of acid, capillary action pulls it deep into pore spaces of the specimen. There, the enormous surface area of calcium carbonate that contacts the drop of acid usually produces a spectacular effervescence. Instead of holding the specimen in your hand during the test, place it on surface that will not be damaged by the acid, with a couple paper towels beneath it.
Fields are shown in yellow, well locations are shown in green and red. Image by the United States Geological Survey. At a microscopic level, there can be a lot of space between the fossil particles that make up chalk. Chalk is an extremely soft sedimentary rock that forms under the sea due to the gradual accumulation of plates of calcite a mineral form of calcium carbonate and very small amounts of clay and silt. The famous white cliffs of Dover are an extraordinary example of a chalk rock formation, from which these little chunks of rock were gathered.
Although now replaced by another mineral form of calcium called gypsum, chalk is often associated with its use as the material for drawing on black boards. When pressed against rough surfaces, the mineral readily crumbles leaving a clear mark on the surface. Subsequently buried under layers of sediment and compressed into rock, the Niobrara Chalk was eventually exposed at the surface when overlying rock layers eroded.
As the exposed chalk began to erode, the remnants at Monument Rocks and Castle Rock were spared when more resistant, localized beds above them helped shield the softer layers below. The western Kansas chalk beds became famous in the 19th century for largely complete fossils of giant swimming and flying reptiles known as mosasaurs , plesiosaurs , and pterosaurs as well as fossils of aquatic birds with teeth, foot-long fish, and clams up to six feet in diameter. Things like bones and teeth and shells are generally made of this amazing stuff, and when the critter dies, the calcium carbonate is returned to the earth for recycling into new and wondrous forms.
There are many types of creatures on earth, and quite a few of them at least by the sheer number of individuals are composed of a single living cell. There are quite a few different types of these primitive life forms.
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