Geological Trail Intro Part 2

Green Creek Complex 

The Green Creek Complex is an assemblage of metamorphic rocks consisting of granite, granitic gneiss (metamorphosed granite), schist and a few other rock types. Radiometric analysis indicates that the Green Creek granite may be as old as 2.5 billion years (Archean) and is considered to be some of the oldest rock exposed on the continent west of the Mississippi River. This granite has a porphyritic texture, meaning it is composed of crystals of two distinct sizes.The larger crystals are potassium feldspar. They are embedded in a matrix of smaller crystals consisting of quartz, biotite, and plagioclase feldspar. 

Feldspars are the most common family of silicate minerals. They are broadly divided into two groups: sodium and calcium bearing feldspars, called plagioclase, and potassium bearing feldspars, called orthoclase, microcline or sanidine, depending on their crystallographic structure.