The Rocks of Pinnacle Mountain State Park EarthCache

PINNACLE MOUNTAIN STATE PARK IS OPEN DAILY FROM 6:30AM UNTIL 1 HOUR AFTER SUNSET.  PLEASE RESPECT THESE HOURS WHILE SEEKING THE EARTHCACHE.

This is an EarthCache which involves an earth science lesson.  There is no physical container to find.

In order to log this EarthCache, please message or email the cache owner answers to the following questions within 1 week of entering your online log.

1.  Name 2 types of rock that are found at Pinnacle Mountain State Park.

2. Explain the differences in texture and color between the 2 types of rock you listed.

3.  There is a large sandstone rock on the ground near the EarthCache coordinates.  Describe in your own words any veins of quartz you observe.  For example, approximately how many veins do you see and how wide are they.

4.  OPTIONAL: Please post with your log a picture of yourself or a personal item near the coordinates.  Please do not post pictures that include answers to the required questions.  

Pinnacle Mountain State Park includes over 2,000 acres of land and is located near the urban area of Little Rock.  It is a day use only facility that emphasizes preservation rather than development.  A new visitor's center opened in November, 2023, and this is where the posted coordinates take you.  It is not necessary to enter the visitor's center to complete this EarthCache.

Pinnacle Mountain is part of the Ouachita Mountains, which is the only mountain range in the United States that runs from east to west.  The Ouachita Mountains were formed by continental plates colliding against each other.  This collision is called an orogeny and resulted in a "crumple zone."  Prior to the collision changing the region, sediment was accumulating on an ocean floor.  The sandstone and shale that are found in and around the mountain today are the remnants of these ocean sediments.

Although some assume that Pinnacle Mountain is a volcano, it is a geologic feature known as a klippe.  Klippe is German for cliff or crag and is defined as an erosional remnant of an overthrust sheet of rock that is isolated from underlying rock by a thrust fault.  Thrust faults occur when lower and older rock is pushed over higher and younger rock.  This process of thrust faulting created Jackfork Sandstone, which is composed of sandstone and shale.  Sandstone is sedimentary rock that is mainly sand-sized silicate grains, held together by another mineral.  Sandstones comprise about 20–25% of all sedimentary rocks.  Sandstone can contain quartz, which is a mineral.  Quartz is formed when fluids from within the earth flow into the cracks of sandstone.  These fluids contain silica and when silica combines with oxygen, quartz crystals are formed.  Shale also is a sedimentary rock that forms when silt and mineral particles are compacted.  Shale is both fissile and laminated.  Laminated means the shale is composed of many thin layers while fissile means the shale easily splits into thin pieces along the laminations.  Over time, both natural erosional processes and human quarrying have shaped how Pinnacle Mountain looks today.