Published November 9, 2005 in Trumansburg Free Press (Trumansburg, NY)
|Land & People
Joints and Beds
by Bill ChaissonMany first-time visitors to the Finger Lakes region are puzzled by the geometric appearance of exposed bedrock. Outcrops break into right-angled blocks, making parts of the gorges appear to be constructed rather than eroded. It is as if a relentless team of stonemasons had gone to work on the creek beds. One misleading fact is that Civilian Conservation Corps did do a great deal of work in some of the gorges during the 1930s. The CCC built both dry and mortared walls and stairs that in places merge almost seamlessly with the natural architecture of the landscape.
But the rectilinear appearance of the aboriginal rock is actually caused by two sets of vertical planar breaks that plunge nearly straight down through the bedrock. The horizontal planes in the outcrops are created by erosion along the nearly flat-lying sedimentary layers. The vertical breaks are called ‘joints’ and they are made when the bedrock is broken in place and does not move. If movement takes place along these planes, they would be called ‘faults’.
The Finger Lakes region is part of the Allegheny Plateau, the northernmost of several high flat regions along the northern and western edges of the Appalachian Mountains. A event called the Alleghanian Orogeny (~330-260 million years ago) elevated mountains in the southeastern United States where the North American tectonic plate and the African plate were colliding more directly. In the northeastern United States this collision did not so much elevate the existing bedrock as compress it enormously. Water is an ‘incompressible’ liquid, so when the orogenic force squeezed, the water moved violently as the pore space in the bedrock constricted. The water went upward along lines of force parallel to the direction of compression. Therefore the ‘Alleghanian joints’ are a set of parallel planes running roughly north-south, away from the mountain-building in the Southeast.
The second set of vertical joints runs roughly east-west, perpendicular to the first set. These were formed later (~250-140 million years ago) by a different process. The sedimentary rocks of the Finger Lakes region are composed of materials eroded off of the “Acadian Mountains”, which were formed in what is now New England and Maritime Canada in an earlier orogeny (~410-370 million years ago). The
|weight of this sediment depressed the Finger Lakes region (and beyond),
causing the older sediments to be buried miles deep, where, subject to
heat and compression they became sedimentary rock.
Over the last 250 million years the sediments and sedimentary rocks of the northeastern United States have been steadily eroding away and the Earth has slowly rebounded to expose successively older rocks at the surface. This rebounding released the pressure from the deeply buried rocks and they broke in a set of parallel east-west planes. The combination of these ‘release joints’ and the north-south ‘Alleghanian joints’ create the ‘built’ appearance of outcrops in central New York.
In between the visible joints in the bedrock the compression of fluids induced cleavage planes parallel to the joints. The individual joint planes may be hundreds of feet deep and many miles in length, while cleavage planes are much smaller. However, the existence of the vertical cleavage planes means that when stones break off of a rock outcrop, they tend to be roughly right-angled around the sides with essentially flat tops and bottoms, as controlled by the bedding planes.
The Allegheny Plateau, into which the Finger Lakes are carved, is sitting on a layer of salt that was deposited 420-415 million years ago. When the Alleghanian orogeny began shoving northward 90 million years afterward, the compressive forces that formed the pressure solutions and created the joints in the overlying shales and siltstones caused the salt layers to turn into a viscous slush, one big banana peel, as it were.
At Syracuse (the “Salt City”), the salt layers outcrop at the surface. The layers are inclined slightly to the south, so that they drop roughly 38 feet per mile and are over 2000 feet below the surface at Lansing, where they are mined at the Cargill salt mine at Portland Point Road. Therefore the jointing is present in all the rocks above the salt because they were shortened by ~10% during their northward transport by the Alleghanian Orogeny. Rocks below the salt (geographically north of Syracuse), like the bedrock exposed in the gorges carved by the Genesee River at Rochester and by the Niagara River, were not moved by the orogenic force and remain unjointed.
Next month’s column: working with stone; an interview with stonemason Greg Reynolds.
Hibernian Weather Channel Productions
Last revised: January 2, 2006