"Missing Time (Actually, 500 Million Years' Worth)"This locality on the southeast margin of the Adirondacks shows the 500 million year-long break in the rock record of the Adirondack Mountain region of northern New York.
The picture shows a low road cut on the east side of NY Route 4, about 1.5 miles north of Fort Ann village and at the northern intersection of Flat Rock Road and Rte. 4.
The lower half of the road cut is composed of the Hague Gneiss--a metamorphic rock unit formed at high temperature and pressure at a depth of about 25 kilometers (15 miles). This intense metamorphism took place almost a billion years ago during what is generally called the Grenville orogeny (or mountain-building episode).
Gneiss and related rocks are now being formed at great depths under the Himalayan Mountains with the modern collision of India and the southern margin of Asia. The ancient Grenville orogeny is now understood to have resulted from the collision of ancestral North America with Amazonia--the central "core" of South America. The rocks of the Grenville orogeny form the basement of eastern North America from northern Mexico, through much of the eastern US and southern Canada, and into southern Greenland. With the opening of the Atlantic, Grenville rocks have moved east and form the basement of northernmost Scotland and northern Norway.
Erosion accompanied a 25 kilometer uplift of the Grenville mountain belt, and the Hague Gneiss was exposed at the Earth's surface about 500 million years ago. At that time, global sea-level rise brought shallow seas across the southern Adirondack Mountains area. Sands and eroded pebbles and cobbles of the Potsdam Formation were deposited on the much older Hague Gneiss. This contact between highly metamorphosed rock and an overlying sedimentary rock is a type of unconformity (a surface where time is missing) called a "nonconformity." The nonconformity at the Rte 4 road cut is a horizon (marked by yellow arrows) between gneiss that has an east-dipping foliation and almost horizontally bedded, pebbly sandstone of the Potsdam Formation.
Trilobite fossils from slightly higher rocks of the Potsdam Formation near the Rte. 4r road cut are latest Middle Cambrian in age--or about 500 million years old. Thus, the road cut shows a hiatus (an absence of recorded time) of about 500 million years.
*** Important: When visiting this road cut do not damage it in any way by hammering and breaking rock from it. ***
Directions to the Altona Formation:
From downtown Albany: Take I-87 N to Exit 20: NY-149 toward Fort Ann/Whitehall. Turn L onto NY-149E/US 9N. Turn R onto NY-149E for 11 miles. L onto US-4N/George Street. In 1.5 miles, arrive at intersection of US-4N and Flat Rock Road. The roadcut is on the east side of NY Route 4.
More information on the road cut and precise directions to find it can be found in the following publication available through the New York State Museum's web site at: http://www.nysm.nysed.gov/staffpubs/docs/16505.pdf