New England Geology: The Billings Fold of Mount Monadnock




The Billings Fold, a back-folded or recumbant thrust nappe in the Littleton Formation, is evidence that Monadnock is a remnant of a much higher range of mountains.  

Mount

Monadnock is the most prominent peak in New England south of the White Mountains in New Hampshire. Standing 3,165 feet, there are no mountains of similar size in the vicinity which affords expansive views from the treeless summit into the surrounding states of Vermont and Massachusetts. It is the most climbed mountain in the United States and is purported to be the second most climbed mountain in the world behind Mt. Fuji in Japan.

On the west face of Mount Monadnock

is a large cliff near the summit that has been sliced open during the Pleistocene Ice Age. In fact all of New England bears the scars of the advances and retreats of continental and alpine glaciers. The cliff is composed of schist and quartzite, and proudly displays the Billings Fold, a "back-folded thrust nappe."

The Billings Fold was pictured in the first edition of Marland Billings' "Structural Geology" in 1942. Structurally, the fold lies within the uppermost of three southeast-opening isoclinal synclines. Its fold-axis (N58E at 32°) is almost perpendicular to the cliff face. The axial plane is oriented N16W, 36 NE.

THE LITTLETON FORMATION
Schists are metamorphic rocks, forming from clays and muds that have been subjected to heat and pressure over time, lots of time. Quartzite is formed in a similar fashion from grains of sand that have collected in rivers, lakes and seas. Actually, most of Mount Monadnock, in fact all of the Presidential Peaks in New Hampshire, are composed of this schist and quartzite, which we call the Littleton Formation. Well below the Littleton are other rock-types that form the basement structures of New England, but that’s not germane to this discussion.

AN ANCIENT SHORELINE GETS CAUGHT IN A BIG SQUEEZE
How did the Littleton Formation come to be? All of New England, in fact the entire East Coast of the U.S. including the Maritimes of Canada, formed from both small and large land masses that drifted to their present location like slabs of ice floating on a frozen pond. We call that geological process “plate tectonics.” When tectonics plates drift into one another, they interact where they meet, called plate boundaries. The interaction of the plates at their boundaries built a chain of mountains (amongst numerous other geological events). The Littleton Formation began as a continental slope (let’s call it a shoreline) that became squeezed (that’s the heat and pressure part) when the plate containing the land mass of Avalonia slowly collided (the plate tectonic-stuff) into the then-primitive North American (geologists refer to it as Laurentia) northeastern coast. That happened 400 million years ago, give or take, during the Devonian Period in a mountain-building event called the Acadian Orogeny.




A schematic depiction from the Late Ordovician of the impending Acadian Orogeny. It involved the collision between the Avalonian "Volcanic Arc" on the right and the proto-North American continent (Laurentia) on the left. Note the region of future "entrapment" of marine shelf and slope areas of what might hypothetically represent the Littleton Formation. 

Modified from Janet Zeh (http://www.lisrc.uconn.edu/)

FOLDS IN THE ROCK FROM HEAT AND COMPRESSION
As the Littleton Formation began to form during the “big squeeze,” it folded over on itself, like a rug sliding along a slippery floor. Some of those folds were huge (called nappes, which formed Mount Monadnock

and all the Presidentials) and others were small like the Billings Fold, see in the photo. The forces and heat of compression were so great that the rocks behaved plastically. Note that the fold is recumbent or folded-back on itself. The Billings Fold is a small confirmation of the major tectonic-collision, named by the geologist Marland Billings in 1942.



Compressional forces, such as those encountered in plate collisions, can result in a varied array of landforms. 

THE FRACTAL NATURE OF GEOLOGY

The Fold's design is representative of the tectonic process that formed it. So is the larger nappe, and similarly are the metamorphic rocks that comprise the fold. The fold is an excellent example of a concept in science, actually mathematics, called fractals. A fractal is an object or quantity that displays self-similarity on all scales. The object need not exhibit exactly the same structure at all scales, but the same "type" of structures must appear on all scales. The concept readily applies to geology. When a figure is fractal it means that regardless of the scale of observation, enlargening the features and looking more closely, rich detail will continue to emerge. We can see this stratigraphically and structurally with the Billings Fold. 

By the way, the definition of a monadnock is an Abnacki Indian term meaning “a mountain or rocky mass that has resisted erosion and stands isolated in an essentially level area.” Mount Monadnock is a fairly isolated summit projecting above the region with great views especially of Boston sixty-five miles away. In addition, the mountain must contain the same rock-type as the surrounding region, which happens to be (you guessed it), the Littleton Formation.

This is the view looking south from Monte Rosa, a prominent, bald crag on the southwest ridge of Mount Monadnock. Fine views are afforded of the New England coastal plain, central plateau, the Berkshires of western Massachusetts and the Green Mountains of Vermont. The young hiker is croached on a deformed block comprised of schist from the Littleton Formation.