ACID SULFATE SOILS - Sawgal.umd.edu - University of Maryland

STOP 4 CLIFF ON THE POTOMAC RIVER AT LOYOLA
RETREAT HOUSE AND RELATED STUFF
By Del Fanning, Dan Wagner, Derrin Lowery, with much help from others
ABOVE: Photo by Darrin Lowery looking to the north, of cliff face which the trail from the
Retreat House leads to, which is off the right side of the photo, where the thickest part of
the kitchen midden, shown in other photos for this stop, also lies. The slope of the hill of the
land, on top of the cliff, leads down to close to the water of the river off the right side of the
photo. A main reason for visiting this site, from an acid sulfate soils perspective, is to see
the boundary between the brown soil materials of the oxidized zone at the top of the cliff
and the dark gray color of the unoxidized zone at the base of the cliff. Lateral oxidation
into the unoxidized zone on the face tends to obliterate the gray colors of the unoxidized
zone on the face, but the boundary is visible to trained eyes, even without scraping away the
thin surficial zone, which in many places contains bitter-tasting salts, produced by the acid
sulfate weathering of pyrite of the unoxidized zone. The salts undoubtedly are iron and
aluminum sulfate minerals such as rozenite, copiapite, and halotrychite. In the soil
materials along the boundary at the top of the unoxidized zone, pale yellow jarosite is
visible in many places, and a thin sulfuric horizon as defined by Soil Taxonomy may be
present, overlain by post-active acid sulfate soil materials. Most of the iron of the brown
iron oxides of the oxidized zone was very likely released from pyrite of those soil materials
prior to oxidation -- as documented for other sites, such as the Burrough’s site data of
Wagner (1982) shown on subsequent pages. The Burrough’s site where the deep
weathering column was studied by Wagner is about a mile north up the river.
27