A Geological Tour of Tumbledown Mountain
Introduction
On any given fine-weather day in any of the four seasons, Tumbledown
Mountain in Township 6 North of Weld (Figure 1), is visited by scores of
people; most in pursuit of the excellent views afforded by the bald peaks,
and perhaps a few mistakenly seeking the solitude of a wilderness experience.
While a few individuals may come specifically to look at the beautifully
exposed bedrock here, over the course of an outing almost every visitor
will develop some interest in the rocks if for no other reason than its
near constant exposure along some of the popular summit trails. It is easy
to guess the origin of the name "Tumbledown" simply by viewing the south
side of the mountain from any of the trails that lead to the summit. Cliffs
of several hundred feet bear the scars, old and new, of rock slides that
have left piles of rubble (talus) at their bases (Figure 2).
Bedrock Geology
Western Maine is underlain by a regular sequence of layered metamorphic
rocks which began their existence as layers of sand and mud on the edge
of an ocean basin about 430 million years ago. As these sediments accumulated
over millions of years, through a number of processes they gradually hardened
into sandstone and shale. During the last great mountain building event
that helped form the northern Appalachians about 415 million years ago,
these layers were folded, thrust upward, and heated. Great masses of molten
rock formed at depth and moved upward through the contorted layers of rock,
cooling into solid rock some distance below the surface. Hundreds of millions
of years of erosion have now exposed the resulting granites and granodiorites
at the surface of the earth.
Figure 3 |
Let's look at some examples of these rocks. Figure 3 is a photograph
showing some of the thinner layers of schist and granofels that can be
found near Tumbledown Pond. The lighter layers are quartz-feldspar granofels,
and the darker layers are andalusite schist. Note that while each light
layer begins with a very sharp boundary on the right, there is only a gradual
change to the corresponding dark layer on the left. This is a characteristic
carried over from the sedimentary origins of the rock and is referred to
as graded bedding. When the sediments that formed this rock were deposited,
the actions of water flow were such that the coarser sand grains (quartz
and feldspar) fell to the bottom first, followed by progressively smaller
grains. Deposited last during each sedimentary event were very small grains
of clay, before the next slug of sediment came down and the process began
anew. Geologists use graded beds to determine which direction was originally
up in the layers. This is important in determining which are the younger
rocks in a sequence. Through the process of metamorphism, the quartz and
feldspar grains changed little, but the clay grains changed into a variety
of minerals including micas, staurolite, and andalusite, the coarsest grains
in the rocks today. So, while the grain size of the sediment originally
changed from coarse to fine as one moved from right to left in each bed,
the grain size change of the metamorphic minerals is just the opposite. |
There is a good deal of variability in the thickness of layers at Tumbledown.
Figure 4 shows some of the most striking layering exposed near the summit.
In many places the layers are several feet thick and continuous for hundreds
of feet. In several places on the mountain, large folds are exposed. One
of the best examples is between Tumbledown Pond and the eastern summit
of Tumbledown Mountain (Figure 5). These folds developed at the time the
northern Appalachians were forming, when the rocks today at the surface
were at considerable depth and subject to higher temperatures and pressures
which were not uniform in all directions. Both sides (limbs) of the fold
are inclined in the same direction in what geologists term an overturned
fold.
Topography
Some who have ventured to Tumbledown have interpreted the shape of the
mountain and the pond nestled among the peaks to be volcanic in origin.
While this certainly is an exciting idea, it simply isn't true. As we've
already explained, the rocks were formed in a sedimentary basin and are
not at all volcanic in nature. The landscape or topography is also not
of volcanic origin but formed through the interaction of several processes.
Figure 6 |
The first important factor in defining the topography is the nature
of the bedrock, here composed of metamorphic rocks and granite intrusions.
From the top of Tumbledown, a most striking feature of the landscape is
the ring of mountains the completely encircle Webb Lake to the south (Figure
6). Webb Lake is underlain by granite while the ring of mountains is underlain
by metamorphic rocks. In particular, these metamorphic rocks were subjected
to even greater heat as the originally molten granite was intruded into
them. One consequence of this process is that the metamorphic rocks immediately
surrounding the granite are slightly more resistant to erosion than any
of the other rocks. Granite, with its large crystals and abundant feldspar,
is actually fairly susceptible to erosion. The result of all this is the
ring of mountains underlain by metamorphic rocks, and the basin of Webb
Lake, underlain by easily eroded granite. At the Mount
Blue State Park visitor's center there is a plaster model of the landscape
which illustrates this ring of mountains well. |
Figure 7 |
The greatest agent of erosion is the glaciers which swept over the landscape
just a few tens of thousands of years ago and represent the second most
important factor in defining the topography. As the glaciers progressed
across the landscape in a southeasterly direction, they scraped soils and
weathered rock off the less erodable rock units and deeply gouged the earth
in the more erodable rock units. Due to the complex processes of glacial
flow, in many places the glaciers polished the northern slopes of mountains
into rounded shapes, while they plucked broken rock without any polishing
on the southern slopes of mountains. This is readily apparent in the landscape
at Tumbledown. On the summit the rocks are well polished, and there are
numerous knobs with smoothly rounded surfaces facing north and steep, unrounded
surfaces facing south (see Figure 4). Tumbledown Pond itself (Figure 7)
was produced by this gouging and plucking action. In many places on the
summit, scratches formed by rocks imbedded in the base of the glacier as
it advanced across the landscape, point to the northwest from whence the
glaciers came. |
Figure 8 |
Many trails (Figure 8) provide free public access to the summit and
Tumbledown Pond through the auspices and stewardship of the Hancock Timber
Resources Group, Inc. |
Originally published on the web as the April 1998 Site of the Month.
Last updated on October 6, 2005
