Naturalist Notes - A Pilot Perspective
Matt Maloney, Naturalist/Teacher Tin Mountain Conservation Center
The lay of the landscape is a product of the underlying bedrock and sediments beneath our
feet. The undulations, swamp basins, and pointed peaks of our region all tell of ancient
processes of movement, volcanism, and erosion.
Much of the White Mountain region is composed of schists that derived from seafloor
sediments that were later heated to extreme temperatures as they were shoved under an
ancestral North America. These shiny Littleton schists uphold the heights of the Presidential
Range. Superimposed within all of this schist are large bodies of much younger granites that
cooled deep below the surface over 170 million years ago in Jurassic times. It’s a particular
region of these granites that I’ll focus on for this article.
Many of the large bodies of granite that are exposed at the surface in the White Mountain
region are exposed as ring dikes. Many of us are familiar with the famous ring dike of the
Ossipee Mountains just to our south. Many less visually perfect ring dikes can be seen
through perusing geologic bedrock maps of the region. In the northernmost environs of the
White Mountain National Forest lie the mountains of the Pliny and Pilot Ranges. Together
with the Crescent Range in Randolph and the arched ridge of Cape Horn in Groveton, they
form a closely situated group of ring dikes in the North Country.
On a raised relief map a ring dike looks like a crescent shaped mountain range, a graceful arc
of mountains. The origins of a ring dike is theorized to begin when a large plume of magma
rises into the earth’s crust and starts to melt the crust from below, causing a complete
collapse of the overlying crust into the magma chamber. Because different minerals within
the crust melt at different temperatures, the ones that melt first will rise and fill in the
outlying edges and cracks of the “ring dike”. Magma mixtures that take higher temperatures
to melt will eventually coalesce into later stage magmas that fill more cracks in the ring dike.
Eventually everything cools at depth until millions of years of erosion expose the large body
of magma with rock types (cooled magmas) that follow ring-like patterns.
Rings of rock types that are particularly resistant to erosion will sometimes form mountain
ridgelines as is the case in the Pilot and Pliny Ranges. Like other White Mountain ring dikes,
much of the ridgelines of these two mountain ridges is syenite, a white colored, feldspar
dominated rock that derived from the original magma chamber. In the eastern arc of the Pilot
Range, Conway granite is the exposed rock type.
After a recent November snowfall up high, I took advantage of a day off to climb Mount
Hutchins, a 3,700 foot peak in the Pilot Range. Several years ago I climbed this trailless peak
from the south. On this most recent trek I ascended Hutchinson from the north, seeking the
novelty of a new route. Once into the elevation zone of balsam and spruce an inch or two of
snow cloaked the ground. After a couple hours of ascent I startled a bull moose near the
summit. Just beyond my encounter with the bull moose I found its lay, where it beds down
for the night. At this cold and snowy elevation, what a place to spend a night on the bare
ground!
On this day the dwarfed balsams of the summit ledge were covered in rime ice that welded
together into an impenetrable thicket, so I settled for a standing view in lieu of descending to
an open talus slope below. Nonetheless, there it was as I remembered, the gentle southwest
to northeast arc of the Pilot Range, from which I stood on its southwest end, looking out
toward Mount Cabot at the hinge from whence the range arched to the northeast
culminating in the steep profile of Rogers Ledge. Between Mount Cabot and Rogers Ledge
there is a well-trodden trail, the Kilkenny Ridge Trail.
Looking to the southeast I could see another ring dike arc away to the southeast from Mount
Cabot and then curve back towards the southwest, culminating in Mount Starr King, this is
the Pliny Range. For the purposes of this article I took a photo of the Pilot Range ring dike
curving from Mount Hutchison’s summit out to Rogers Ledge. Like two bodies sitting back to
back, the curvatures of each ridge touch at a sort of apex (Mount Cabot) and then curve in
opposite directions… two ring dikes almost superimposed. Check it out on a topo map if you
can.
The Pilot and Pliny Ranges are just two of many features of our landscape that can tell us so
much about the earth’s past if we observe and discern. There is so much more to learn out
there about the shapes and undulations on our beautiful horizons.
