What Makes This Place Special
What makes this place special is not what is here - but what is not here! Sedimentary rock has been deposited on top of metamorphic rock. Nothing unusual about that. But the metamorphic rock is about 500 million years older than the sedimentary rock that lies directly on top of it. And not only is 500 million years of time missing but there are also 25 km (15 miles) of rock missing!
How Do We Account for Missing Time and Missing Rock?
Earliest Rocks - Ancient Seas and Sediments
North America was formed billions of years ago. This ancestral landmass is referred to as Proto-North America. New York’s oldest rocks were deposited about 1.3 billion years ago. Sediments eroded off the continental margins of Proto-North America into the adjacent shallow seas. The sediments deposited on the eastern margin of the continent formed the Grenville belt.
A BILLION Years Ago: The Grenville Orogeny and the Formation of Hague Gneiss
Beginning about 1.1 billion years ago, a collision occurred between a continent approaching from the east, and Proto-North America. Continental crust is light so when two continents collide neither completely overrides the other. Instead they crumple like a multi-car collision, deforming and piling up on one another. This “collision” continued over the next 100 million years or so, resulting in a wide area of uplift, highly deformed rocks and a thickened crust. A large mountain range was formed with a broad plateau similar to today's Himalayan Mountains and the Tibetan Plateau. This mountain building event is referred to as the Grenville Orogeny.
from Isachsen et.al, 1991
The sedimentary rock of the Grenville belt was buried about 25 km (15 miles) underneath subsequent rock and was subjected to high pressures and temperatures, causing the rock to recrystallize to form the metamorphic Hague Gneiss.
The rocks of the Grenville Province form the basement for all of New York State. This basement is buried by younger rocks over most of the State. It has been re-exposed in the Adirondack Mountains, the Hudson Highlands, and right here in Fort Ann!
A Billion to 500 Million Years Ago - Erosion of 25 km (15 miles) of Sediment!
When the Grenville Orogeny ended, the Grenville Plateau began to collapse and spread sideways. As the erosion continued, the unloading of this mass of rock allowed the land to rebound. This process of erosion and rebound, played over and over again, resulted in the removal of about 25 km of rock. By about 500 -600 million years ago the mountains and plateau had worn away to a low flat plain. The Hague Gneiss eventually came to be exposed at the surface.
500 Million Years Ago - Sedimentation Resumes to Form Potsdam Sandstone
Around 500 million years ago shallow seas covered the southern Adirondacks. Sand and eroded pebbles and cobbles settled to the bottom of these seas. Layers of sedimentary sandstone and conglomerate were deposited on the gneiss on the sea floor. This makes up the Potsdam Formation that lies unconformably on the Hague Gneiss. Unconformably? Let’s look at what that means.
Unconformity, Nonconformity?
When layers of rock are deposited without interruption they tell the story of the geologic record like pages of a book. When there is a break in the geological record it's as though pages of that book are missing.
The Dictionary of Geological Terms defines an unconformity as “a break or gap in the geologic record, such as an interruption in the normal sequence of deposition of sedimentary rocks, or a break between eroded metamorphic rocks and younger sedimentary strata.” The second entry adds that it is “the structural relationship between two groups of rock that are not in normal succession; also, their surface of contact.”
EOSC 326 Active Reading by F.Jones, L.Longridge, 2014-2016. Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Several types of unconformity are recognized (refer to figure above):
- Disconformity: exists where the layers above and below an erosional boundary have the same orientation
- Nonconformity: develops where sediments are deposited on top of an eroded surface of igneous or metamorphic rocks
- Paraconformity: strata on either side of the unconformity are parallel, there is little apparent erosion
- Angular unconformity: strata is deposited on tilted and eroded layers (e.g. GC9P4CJ Taconic Unformity - Am I Missing Something?)
What all these types of unconformity have in common is a long period where depostion ceased, erosion removed previously formed rocks, and then deposition resumed..
The Adironack Missing Time Formation
From the New York State Education Department. Adirondack “Missing Time” Formation. Internet. Available from http://www.nysm.nysed.gov/research-collections/geology/resources/adirondacks-missing-time-formation; accessed 21/03/2023.
At this location we can observe a 500 million year interruption of the geologic record. The lower half of the road cut is composed of the Hauge Gneiss which, as we learned above, was formed about a billion years ago when the Grenville Orogeny subjected the previous rock to tremendous heat and pressure 25 km (15 miles) below the surface.
Erosion and uplift brought the Hague Gneiss to the surface about 500 million years ago. Sedimentation resumed. The Potsdam Sandstone was deposited on the billion year old gneiss. This contact, with 500 millions years of missing time, is an unconformity. And since it is a sedimentary layer resting unconformably on a metamorphic rock exposed through erosion, this unconformity is a nonconformity.
The Potsdam Sandstone visible here was deposited in a shallow marine environment and contains significant amounts of quartz. It represents the upper layer of Potsdam Sandstone, deposited by a second occurence of sea-level rise, and is referred to as the Keeseville Member of the Potsdam Formation.
Logging Requirements:
The questions/observations are meant to be a review. All answers are in the Description or based on direct observation. Don't stress over the answers. Do your best, have fun and enjoy the site!
Q&A (short answers, essays not required)
1. What type of unconformity is recorded in the rock record at this site? What makes it this type of unconformity?
2. How long is the break in the rock record?
Observations
3. Is the Hague Gneiss horizontal, vertical or does it dip? (Not required but for extra credit and bragging rights: If it dips, what is the angle and direction of the dip?) Hint: Pour water on the rock. The direction that the water flows is the dip direction.
4. Is the Potsdam Formation horizontal, vertical or does it dip? (Not required, but for extra credit and bragging rights: If it dips, what is the angle and direction of the dip?) Hint: Pour water on the rock. The direction that the water flows is the dip direction.
5. Look for quartz pebbles in the Potsdam Sandstone. What are the approximate dimensions of the largest one you find?
Smile (or not)
6. Include a photo with yourself (face not required), your GPSr, or a personal item indicating the contact between the billion year old Hague Gneiss and the ~ 500 year old Potsdam Formation. (hint: you can use the background image around the borders on the web page, or the NYS Education Department image above)
Glossary
Basement - The undifferentiated rocks, commonly igneous and metamamorphis, that underlie the rocks of interest, commonly sedimentary, in a given area.
Dip - The angle that a stratum or any other planar feature makes with the horizontal.
Grenville - A provincial series of the Precambrian of Canada and New York
Metamorphic rock - Any rock derived from pre-exisitng rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state, in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in the earth's crust.
Nonconformity - An unconformity between stratified rocks above and unstratified igneous or metamorphic rocks below.
Orogeny - the process of the formation of mountains.
Sedimentary rock - A layered rock resulting from the consolidation of sediment (solid material that has settled down from a state of suspension in a liquid)
Unconformity - A break or gap in the geologic record, such as an interruption in the normal sequence of deposition of sedimentary rocks, or a break between eroded metamorphic rocks and younger sedimentary strata.
References
Ansley, J. 2016. The Teacher-Friendly Guide to the Geology of the Northeastern U.S. www.priwen.org
Bates, R. And Jackson, J. 1984. Dictionary of Geological Terms, The American Geological Institute.
Fisher, Donald. 1984. Bedrock Geology of the Glens Falls - Whitehall Region, NY. NYS Museum, Map and Chart Series Number 35.
Isachsen, Y. et.al. 2000, Geology of New York A Simplified Account. http://www.geo.hunter.cuny.edu/courses/geog383.19/geology_nys.pdf
Lutgens, F.K., Tarbuk, E.J. and Tasa, D. 2012. Essentials of Geology Precambrain Geology of the Whitehall Area, Southern Adirondacks.
Selleck, Bruce. Stratigraphy, Sedimentology and Diagenesis of the Potsdam Formation, Southern Lake Champlain Valley, New York. www.nysga-online.org/wp-content/uploads/2019/06/NYSGA-2008-13.-Stratigraphy-Sedimentology-and-Diagenesis-of-the-Potsdam-Formation-Southern-Lake-Champlain-Valley-New-York.pdf
Whitney, P., Stacher, G. And Grover, P. 2002. Precambrian Geology of the Whitehall Area, Southern Adirondacks. In New England Intercollegiate Geological Conference 94th annual meeting New York State Geological Association 74th annual meeting Guidebook for Fieldtrips in New York and Vermont
