Pipestone National Monument - Natural Features & Ecosystems (U.S. National Park Service)

NPS Photo

Sioux Quartzite bedrock layer cleared away, revealing an underlying catlinite layer

Geology

The solid bedrock of the Pipestone National Monument is the Sioux Quartzite, a thick stack of ancient layered rocks exposed today in parts of Southwest Minnesota, southeastern South Dakota, and northwest Iowa.

The Sioux Quartzite consists of three rock types that were originally layers of mud (pipestone), sand (quartzite), and gravel (conglomerate) which, according to scientists, were deposited at some time between 1,750 and 1,630 million years ago.

The unique variety of pipestone at Pipestone National Monument is called catlinite. It consists largely of microscopic crystals of pyrophyllite (pie-raw-fill-ite), diaspore (die-ah-spore), muscovite (musk-oh-vite), and kaolinite (kay-oh-lihn-ite). Traces of the iron bearing mineral hematite (heem-ah-tite) give the catlinite its red color. Most other red pipestones found in the world contain the mineral quartz; catlinite has little or none.

Catlinite formed when mud layers were buried within the earth, under temperature and pressure conditions very different from those at the surface. The original minerals were unstable in this new environment, and their chemical components recombined to form new minerals.

The catlininte beds are sandwiched between thicker beds of quartzite. Most of the sand grains of which the quartzite is made are rounded crystals of quartz "glued" together by other quartz crystals that grew between the sand grains after the layers were buried. After millions of years of heat and pressure pressing the grains together, the rock is presently a mineral harder than ordinary steel.

Although Sioux Quartzite is extremely hard, the underlying layers of catlinite are very soft. Since the catlinite contains no quartz, subjection to the same natural heat and pressure metamorphosed it into a very dense material which is roughly the same hardess as a human fingernail. It can therefore be easily carved using only the simplest of tools.

At least five different catlinite layers are now exposed in the quarries at the Monument. These extend in a roughly north-south line which is two-thirds of a mile long, following a zone with the Sioux Quartzite containing many pipestone beds.