Louisiana has few naturally occurring surface rocks. The places where they are typically found lying about in the open are upland terraces with sandy, loose soil. If you've visited Hodges Gardens in Sabine Parish, or the hilltops of Harrisonburg in Catahoula Parish, or Kisatchie Bayou riverbed in Natchitoches Parish, you've encountered sandstone rocks there, part of the Catahoula Formation (see image below) built up over the last 23 million years or so, by the deposits of sands, compressed into stone by sedimentary processes, then revealed by alluvial (river) action due to erosion in the time since.
Where you are presently parked or standing as you approach this earthcache is nowhere near those areas, relatively speaking. The present geology of the city of Alexandria is made up of Pleistocene and Holocene (the last several thousands of years) river deposits, as the nearby Red River has shifted and flooded time and time again.
The Community Garden organizers and the nearby Gemiluth Chassodim Synagogue had these stones brought in for landscaping, and that's how they've come to be here, giving us an opportunity to bring an earth science lesson to our community.
Sandstone is primarily made of silicon dioxide, commonly known as sand or quartz sand, the most commonly occurring mineral on the planet. Once sedimented and condensed into stone, it has an average dry density of 146.7 pounds per cubic foot (2.35 grams per cubic centimeter).
Typically sandstone is light brown/tan due to being composed mainly of light-colored quartz crystals. However, it can have altogether different colors based on the presence of other minerals within. Other colors can be Reddish-Orange, due to iron oxides (rust); or Purples due to Manganese; or perhaps even grey or grey-blue due to a significant amount of clay or silt.
You will see here also an organism growing on the stone called a lichen. It is a grey-green hardy organism which feeds on mineral deposits, but it does not enter into our discussion other than for me to instruct you that it is not part of the stone.
LOGGING INSTRUCTIONS:
Email me with the answers to the following:
1. How many individual stones are here? (You may count the small pile of rocks as 'one'.)
2. Do you see any color variations in the stone? If so what causes it?
3. Locate the largest of the stones: measure or estimate its volume.
4. ...and based on the characteristics you've learned above, indicate its approximate weight.
5. Leave No Trace here. Gather only information, and leave only footprints.
6. Photograph of yourself at location is NOT required, though recommended.
Sources: https://www.mineralseducationcoalition.org/minerals/sandstone | http://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=LAOGc%3B0 | Image source Louisiana Geological Survey, Louisiana State University, Chacko J. John, Director and State Geologist.