Fort Hill Mound lies on what geologists call the "Onondaga Escarpment." This east-west line runs the width of New York State across the head of the Finger Lakes, and is marked by a series of low hills which are the first rise of the Alleghany Plateau. North of the Fort Hill Mound, the rolling sandy hills of the Ontario Lake Plain are underlain by softer rocks, mostly shales, formed in the Silurian Period of the Paleozoic Era over 400 million years ago. South of the Onondaga Escarpment, the land begins to rise upwards over harder layers of limestone bedrock laid down in more recent geological eras. These rocky ridges—deeply scored by glaciers in two recent Ice Ages into long north-south valleys—rise steadily to form the vast Alleghany Highland Plateau of southern New York.
The Finger Lakes themselves are a special and unique geological feature of this region. They are nestled in a series of long, narrow valleys which were deeply carved into the Alleghany Plateau by fingers of ice from the immense continental glacier of the last two Ice Ages. The southern ends of these valleys were blocked by 250 foot thick glacial deposits of sand and gravel, called "terminal moraines" in geologic jargon. These moraines formed natural dams impounding the melting glacial waters in the Finger Lakes. These uniquely beautiful lakes are strung like carefully chosen azure jewels on the emerald cloth of the Appalachian headlands. They sit across the opening of the Hudson-Mohawk passage into the Great Lakes heartland of North America.
The tilted bedrock beneath this northern end of the Appalachian Plateau is young, formed in the Devonian Period 350-400 million years ago. Visible at many places along the head of the Finger Lakes, including the Owasco area, are rocky outcrops of hard Onondaga limestone, which forms the Onondaga Escarpment. This calcerous rock is ten to fourteen feet thick, light gray, tough, small grained, with regular vertical right angle joints, and numerous organic remains (especially encrinal stems). There are some pink hued and white varieties. Although it is generally pure, some localities contain flint nodules, and in others the lower layers contain black, water worn pebbles. Since it resists air, water and frost, it is usually the bottom rock of streams and commonly forms waterfalls.
This bed of limestone lies exposed in a narrow band five to ten miles wide and 250 miles long. It extends from the crest of Helderberg Mountain south of Albany to the Niagara region around Buffalo in the west, its continuity unbroken except by valleys and watercourses. It sits above Oriskany Sandstone, which is of variable thickness, while above it is Seneca Limestone, which is very flinty and of unreliable quality.
Several feet above this is Tully Limestone, which appears in Chenango, Onondaga and Cayuga counties. This hard, insoluble limestone forms the Tully Escarpment, the next sharp rise of the Alleghany Highlands (see map). This is the last complete bed of limestone; all others above this are partial masses which don't span the entire geologic region.
Onondaga Limestone is valuable for a variety of purposes. Its relative purity, regular joints and ability to take a good polish made it a valuable building stone. In the fIrst centuries of European settlement, it was used as "marble" in the construction of many public buildings. It was considered comparable to Potomac Marble for this purrpose. Much of the Erie Canal was built with this hard, durable stone; much of the western Canal was built with stone from Split Rock Quarry west of Syracuse. South of Syracuse, this rock has been extensively quarried for industrial processing into cement and industrial chemicals. This area is also known for its salt deposits. Dark blue stone is preferred for industrial burning, as it is easier to break and requires less heat.
Although limestone is hard and durable, it is slowly soluble to the action of water. Consequently, it is often worn away along its natural joints by water streams to form underground channels and caverns. On the Earth's surface, this wearing action frequently produces "sinkholes." The limestone then often precipitates out of solution as water drips and dribbles through the cavern, forming slowly growing columns of stalactites and stalagmites. In New York, one of the more famous of this underground limestone architecture is Howe Caverns near Herkimer.
Marble is limestone which is liquified and compressed by heat and pressure in massive earth movements. This "metamorphic" rock uniquely valued as a building material. Crushed and burned by human industry, limestone becomes cement, the "plastic rock" that is modem civilization's primary building material.
The Devonian Period was interrupted with what geologists call "orogeny", a time of earth upheavals and mountain building on the planet. The 70 million years of geologic history which followed were Carboniferous Periods, when much of the planet's coal, oil and gas deposits were formed. These appear in bedrock farther south, forming the coal strata in Pennsylvania and West Virginia which are interbedded with shale and sandstone.
The possibility exists that Fort Hill is intentionally sited on the boundary that separates Onondaga limestone from Salina shale bedrock. Although both are forms of sedimentary rock, they have markedly different chemical and biological character. Shale is formed from muddy deposits laid down on an ancient ocean bottom. Although shale often contains an abundance of fossilized animal and plant life, it is composed of inert mineral matter layered on the primeval ocean floor.
On the other hand, limestone is made of compressed organic crystals created by biological life. It is made from the shells of ancient sea creatures that thrived in the primeval ocean during certain brief periods of planetary history. At these times, a hotter climate created warm oceans in which plant and animal life blossomed and flourished. These ancient creatures transmuted and concentrated calcium to form their shells and skeletons. In death, they rained down on the ocean floor to form thick sediments which in later geologic ages became limestone. Thus, this dense rock consists of minute organic crystals formed by ancient ocean life. These tiny biological crystals, largely calcium, are compressed into a hard, alkaline, durable bedrock.
There is one difference between shale and limestone which needs special comment. This is their electro-chemical properties. Shale is always acidic (+ = pH < 7.0) and contains excess positive ions. On the other hand, limestone is composed of calcerous shells of ancient sea animals, and very alkaline (- = pH > 7.0) and imparts an abundance of negative electric ions. In agriculture, overworked farmland which has become acidic is sweetened with crushed limestone.
The bedrock junction between acid Silurian shale (H+) and basic Onondaga limestone (OH-) is an electric junction where ions from two oppositely polarized rock meet. This creates a very low voltage but massive size battery from these stacked layers of geological history. The possibility that Fort Hill is intentionally sited on the electic junction of this giant geological battery should be given serious consideration in understanding its purpose.
Study of a Cayuga County soil map reveals that Fort Hill consists of a soil type which is nearly unique to the northern Owasco Lake plain. Most of the soils in the northern Owasco Lake plain are Cazenovia silt loam. This moderately fine, reddish, glacial till is closely related to the Onondaga Limestone Escarpment, so it occurs mostly on the slopes of Cayuga and Owasco Lakes. The second most common soil around Auburn is Ovid silt loam.
All of the Mound, and much of the land around it, lie on Arkport fine sandy loam soil (AtC), with a high sand, low silt and high calcium composition. There are remarkably few other occurrences of Arkport soil in and around Auburn, although it's more common in the deep sandy soils north to the Seneca River.
In many places around Fort Hill, this soil lies exposed, a unique light brown, very fine grained soil. It is very light in texture, too, and at most places at Fort Hill, it is easily penetrated to a depth of several inches by a metal rod. This light texture contributes to the idea that the entire Fort Hill Mound complex is largely a construction. At present, there is no apparent significance to this unusual variation of soil type in Fort Hill's design.
|The Northern Slopes
Fort Hill Cemetery, Auburn, NY