Mount Monadnock

Trails

Pumpelly Ridge (8.8 miles out and back): Park on the mountain side of Lake Road (on Dublin Lake, 42.9011, -72.0750). Longest trail on the mountain. A flat approach and then pretty steep for about 1.5 miles. The trail then follows along the ridge with views to the North.

Dublin Trail (4.8 miles out and back): Begin and end on Old Troy road (42.8857, -72.12505). Personal favorite, not too steep but won't take all day.

White cross and white dot loop (3.8 miles, two trail loop): Begins and ends at the Monadnock State park headquarters. Shortest and steepest accent.

Monadnock-State-Park_Hiking-Map.pdf

Geological History

The following information was paraphrased from Davis, N. (2019). MonadRocks: Mount Monadnock’s Fascinating Geologic History. New Hampshire State Parks: Mount Monadnock.

415 million years ago, the North American continent -- then known as Laurentia -- extended only as far as the Eastern border of Vermont. Present-day New Hampshire was still underwater. Across Laurentia was another continent named Avalonia, whose land eroded and collected at the bottom of the ocean. These new layers would eventually form Monadnock's base. The following diagram, from the same source, depicts this theory, with the star denoting Monadnock's rocks:

25 million years later, Avalonia began to approach Laurentia. In this particular subduction zone, Avalonia slid under, forming new, Himalayan sized mountains! All this heat and stress compressed the rocks at the bottom of Avalonia's plate -- sediments that now make up Mount Monadnock. After 375+ million years of erosion, the mountain range between Avalonia and Laurentia has disappeared, exposing Monadnock's structure.

In fact, "Monadnock" now refers to any mountain that has formed after being more 'erosion-resistant' than what was there previously!

Evidence of these theories is found in sillimanite, a mineral that is formed under intense heat and pressure. Geologists have examined the sillimanite on Monadnock and determined that it was once 7.5 miles below the surface -- in other words, Monadnock's rocks were deeply beneath the mountain range between Avalonia and Laurentia. This suggests why Monadnock still stands when the mountain range no longer does: 7.5 miles beneath the surface, Monadnock's rocks were subject to much higher pressure than anything in its surroundings and, consequently, are more resistant to erosion.

Much more recently, Monadnock was subject to the latest ice age, the Pleistocene Epoch from 2.6 million to 11,700 years ago (2). Large sheets of ice receded towards the North, dragging debris across the Monadnock's structure. This smoothed out Monadnock's shape, as well as creating 'glacial striations' across its face. Read the next section to learn about our own findings! (1)

Our Findings - May 17, 2021

Ridges in rocks running North-South. In this case, we are looking North. We have two theories to explain these ridges. They could be striations scraped into the rock by glaciers or they could be veins in the rock caused by stress. Veins are formed when minerals precipitate from hydrothermal fluids which flow through cracks.

Evidence of sillimanite (1). This harder mineral sticks out of the rock because the other softer minerals have weathered. Christian's boot provides reference for size.

Note the smooth shape of the summit, most likely due to glaciation. Wilder, in the background, taking in the view.

Geologists in action:

Above: Wilder examining a quartz-rich vein that has intruded the surrounding rock.

Below: Christian and Wilder enjoying the summit on a beautiful Spring day!