eginning 1.5 million years ago, numerous glacial advances and retreats left stones of many types, shapes and sizes in central South Dakota. Many of these rock fragments originated in Canada and other points north and east of where we find them today. The glacial process ended 10,000 years ago.
These large and small glacial granite boulders have inconspicuously coexisted with the rolling prairie hills in the northwest since then. Semmingly rugged and dirty, and of little importance, we now know of their real beauty- a look inside- each fantastic in color with lovely xinoliths.
Granite is the signature rock of the planet Earth itself. The other rocky planets-Mercury, Venus,and Mars-are covered with basalt, as is the ocean floor on Earth. But only Earth has this beautiful and interesting rock type in abundance.
Three things distinguish granite.
First, granite is made of large mineral grains (which is where its name came from) that fit tightly together.
Second, granite always consists of the minerals quartz and feldspar, with or without a wide variety of other minerals (accessory minerals). The quartz and feldspar generally give granite a light color, ranging from pinkish to white. But that light background color is punctuated by the darker accessory minerals. Thus classic granite has a "salt-and-pepper" look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende.
Third, almost all granite is igneous (it solidified from a fluid state) and plutonic (it did so in a large, deeply buried body or pluton). The random arrangement of grains in granite-is evidence of its plutonic origin. Rock with the same composition as granite can form through long and intense metamorphism of sedimentary rocks. This kind of rock is called granite gneiss.