Montana Earth Science Picture of the Week

Chalk Buttes near Ekalaka

Photo by Gwenith Schultz

A “misnomer” . . .
This photo shows one of the Chalk Buttes located in the remote southeastern corner of the state, about 10 miles southwest of Ekalaka. Although they are called Chalk Buttes, they are not made of chalk, but rather an especially white sandstone. “Chalk” is actually a soft white to gray limestone composed of the hard coverings of microorganisms and some bottom dwelling animals such as ammonoids and pelecypods in a matrix of finely crystalline calcite. As the organisms die their hard parts (shells, etc.) accumulate on the sea floor where they are eventually crushed beyond recognition by layers deposited above them. If the material becomes so compacted that it is hard, geologists would simply call it limestone. Less compaction will result in the softer (less durable) chalk. England’s White Cliffs of Dover are a great example of chalk.

When dinosaurs ruled Montana . . .
The Chalk Buttes of southeastern Montana are made of sandstone deposited in the later part of the Cretaceous Period, which ended about 65 million years ago. The Chalk Butte sandstones are part of the Colgate Formation, which is found throughout much of eastern Montana just below the Hell Creek Formation; the last layers of sediment deposited before the extinction of the dinosaurs at the end of the Cretaceous Period. Evidently the sand that makes up the buttes was deposited in some sort of valley (basin), west of the shrinking interior seaway that split prehistoric North America in two.

Chalk it up to the rock cycle . . .
In order for sand to become rock (lithification) many processes happen over a long period of time. First the sand (usually pieces of quartz) originates from pre-existing rock such as granite. As the granite breaks down (weathering) the sand is transported, often by running water (erosion), to a basin where it is deposited (deposition). As millions of years go by the layers of sand may experience many more “diagenic” processes, including compaction, dewatering, cementation, dissolving of certain mineral components, replacement of minerals by other mineral phases, heat and pressure of deep burial, etc. Not only did these processes change the sand into sandstone, but they also bleached the rock giving it its chalky appearance.

Photo by Gwenith Schultz

Term: matrix


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