Ocala National Forest Earthcache EarthCache

This Earthcache involves geology, soil forming processes, and soil mineralogy.  The Ocala National Forest is a special place. Please respect the unique environments, animals, and ecosystems.  The ONF is home to the most extensive area of sand pine scrub in the world. 

Congrats to Fegan/V&T on co-FTF!!!

This excavation in Ocala National Forest reveals many different deposital episodes spanning the Miocene, Teritary, and Quaternary periods.  Age of these sediments ranges from about 5-7 million years ago to, as recent as, 13,000 years ago.  The depositional processes in Ocala National Forest range from recent alluvium along the Oklawaha and St. Johns Rivers to older sediments (at higher elevations), such as beach sediments, aeolian dunes, delatic sediments, longshore bars.  

From observation of the exposure looking northward (at the posted coordinates), there is evidence in the soil material of a high content of iron.  Iron is a very important soil mineral and is responsible for the color in many soils. Typically, soils with red and/or yellow colors are usually well to excessively drained and the iron is in an oxided state.  Soils that are wetter (somewhat poorly to very poorly drained) usually have iron in a “reduced” state and the soil color is gray or yellowish gray.

Several different Fe oxides and hydroxides can be distinguished, which differ in their crystal structure and various other properties (e.g. color, solubility, thermal behavior). 

Following the 6 most widespread Fe oxides and hydroxides are described briefly:

Goethite (alpha-FeOOH): It is the most frequently occuring Fe-oxide in soil and has a characteristic yellowish brown color. In large concentrations it may also appear as dark brown or black. Goethite is found under a broad range of climatic and hydrological conditions and is the thermodynamically most stable of all Fe-oxides. A variety of conditions appear to favor its formation in preference to hematite, which include cool temperatures, moist soil conditions and presence of comparatively large amounts of organic matter. The absence of hematite and abundance of goethite in many soils of cool or temperate regions supports these general observations. Under suitable conditions, it has been suggested that goethite can form from any Fe source.

Hematite (alpha-Fe2O3): It has a characteristic bright red color and primarily occurs in the better drained soils of warm temperate to tropical areas. It is also found in restricted areas of cool temperate areas either as a primary mineral or in more highly weathered soils, where it probably formed under climatic conditions warmer and drier than those operating at present. Its formation appears to be favored by warm dry conditions and small amounts of organic matter (OM). 

Lepidocrocite (gamma-FeOOH): It has a characteristic bright orange color that is evident in soils where it occurs in large concentrations and is not masked by other pigments. It commonly occurs in association with goethite and usually in soils that have restricted drainage. It forms from the oxidation of Fe2+ compounds, which are common in wet soils.  Lepidocrocite formation also appears to be favored by slow oxidation rates and small concentrations of aluminium in the soil solution. Lepidocrocite is rarely found in very acid soils, which typically contain aluminium in the soil solution.

Maghemite (gamma-Fe2O3): It is common in many soils, notably in the tropics and subtropics and varies in color from red to brown. This oxide occurs in soils, especially those derived from basic igneous rocks. Maghemites are commonly concentrated towards the top of the soil profile. Several pathways have suggested to account for their formation in soils, which include:

Magnetite: It is listed in most textbooks as a primary soil mineral that does not form through pedogenic processes. It occurs in soils as singular irregular black grains and is present in most soils. It is fairly resistant to weathering.

  Ferrihydrite (5Fe2O3*9H2O): It is a common Fe-oxide found in soils and appears reddish brown in color if not masked by other pigments. It has a poorly ordered structure resembling that of hematite and was previously referred to as amorphous ferric hydroxide. Its formation is favored by rapid oxidation of Fe in the presence of large concentrations of OM and/or silicate. 

Some other important iron compounds found in soils are:

Siderite (FeCO3): It is found in waterlogged soils, i.e. under reducing environmental conditions. The color is greenish/blue.

Vivianite (Fe3[PO4]2*H2O): It is found in waterlogged soils, i.e. under reducing environmental conditions. The color is greenish/blue.

Pyrit (FeS): It is found in waterlogged soils, i.e. under reducing environmental conditions. The color of FeS is black. 

Source: Florida Department of Environmental Protection and University of Wisconsin, Department of Soil Science.

To log credit for this Earthcache, please answer the following questions:

1. From looking at the north-facing exposure, under which of the named environments (listed earlier) were MOST of the sediments deposited?  Why do you think that?
2. Looking from the top to bottom of the exposure, how many different soil materials do you believe the soil formed in?  
3. In your opinion, from10 to 30 feet below the surface which iron mineral is responsible for the color of the soil?
4. From close observation of the soil materials at this site, there are gray colors present. Do you believe that these colors are due to the soil being saturated? If not, why?
5. Why do you think that soil material was excavated from this site?
6. Take a photo of yourself (and GPS) in front of the exposure and post with your log (optional).