About Sioux Quartzite

Quartzite is a metamorphic rock formed from layers of small to medium rounded grains of quartz that have been cemented together by silica, it is so strong that it breaks across the grain instead of around it. Quartzite is between 1.6 and 1.7 billion years old, making it some of the oldest and hardest rock in the world. Though commonly pink or red, the color varies over a wide range due to the presence of a thin film of iron oxide coating the grains of quartz. Quartzite that has been exposed is an outcropping that has been caused by some element of erosion, primarily water, taking away the “blanket” of glacial drift.

Sioux quartzite outcroppings are the area’s most unique geological feature. The outcrops of Sioux quartzite start at Redstone in East New Ulm, MN and extend 180 miles west to Rockport Colony along the James River, south of Mitchell, SD. The north-south band of outcrops is about 40 miles wide. The unexposed quartzite, a sub crop, covers a much broader area extending west of the Missouri River. The whole area is referred to as The Sioux Ridge.

Most of the Sioux formation was deposited in a distal braided river-alluvial plain environment. The  upper third may have formed in a shallow marine, tidally influenced environment. The huge volumes of sand came from the north. Minor quantities of conglomerate quartzite occur in the lower two-thirds and thin beds of silty clay and/or pipestone were “sandwiched” in the upper third.

Quartzite was quarried in the late 1800’s and early 1900’s into building stone and cobblestone.  With the ability to transport stone out by train, quarrying became one of region’s largest industries. Almost every local community had structures made of quartzite.  Because of the stones hardness and durability , these building are still standing today as historic schools, courthouses, churches, fire stations and hotels – many have been converted to museums. Those growing up in the area take these buildings and outcrops “for granted”, but they are not made of granite, an igneous rock.