Welcome to Purgatory Chasm
Danni and Irene may or may not have gotten stuck in “Fat Man’s Misery”, which coincidentally, is also a fantastic example of ice wedging!
Sliding down a wind eroded exposed rockto the right of the chasm entrance
Project by Danni Lu and Irene Terpstra
Avalonian terrane
Areas of New England that the Avalonian terrane is responsible for forming.
Laurentide Ice Sheet
Map of North America showing the maximum extent of the Laurentide Ice Sheet outlined in the thick dark blue line.
The first theory was proposed by Dr. Edward Hitchcock in his report, Geology of Massachusetts
(1841) (Purgatory Chasm Geology
). He states that the chasm was formed by prolonged wave erosion which occurred when the sea level was higher (Purgatory Chasm Geology
). Hitchcock believes that the fractures of rock found in the Chasm come from intense wave energy and movements (Purgatory Chasm Geology
). Hitchcock further supports this notion with Purgatory Chasm in Rhode Island and explains that this formation was created similarly by wave motion (Purgatory Chasm Geology
).
Problem: This theory was proposed prior to the theory of ice ages so the possibility of glacial erosion was never considered (Purgatory Chasm Geology
). Additionally, all the evidence found in Purgatory Chasm show that this theory is incorrect. The Chasm reveals no signs of the rounding and smoothing of rocks that waves would have generated by erosion. The example above supports this in showing many jagged rocks that are prevalent throughout the Chasm. Also, if the Chasm had been created by water, sediment would have become sorted in the process and there would be marine deposits throughout the area as well, neither of which are apparent (Purgatory Chasm Geology
)
The second theory was put forth by Professor W.O. Crosby of MIT and his son Irving B. Crosby in Bulletin 36
of the Geological Society of America (1925). (
Purgatory Chasm Geology
). The Crosby’s correctly rule out any reason that water created the Chasm due to its large, jagged blocks of rock (Purgatory Chasm Geology
). Although this is correct, they also rule out glaciation because the Chasm is oriented perpendicular to glacial movement, despite the fact that glaciers pluck rocks out of the ground (which can create chasms either perpendicular or parallel to movement) and do not necessarily gouge or bulldoze rocks out (Purgatory Chasm Geology
).
The Crosby’s then turn to tectonic activity for the creation of Purgatory Chasm. They state that the formation is an example of a Keystone fault (Purgatory Chasm Geology
). A Keystone fault occurs when two bounding Y-shaped wedges of rock undergo tension stress due to earthquake vibrations (Keystone Faults
). When this happens, these two wedges may pull apart, leaving the middle V-shaped wedge of rock (or keystone) unsupported and drops downward as a whole (Keystone Fault
). Compression stress of the next vibration then powerfully squeezes the keystone, effectively crushing much of the rock (Keystone Faults
). This successive pulsation causes the keystone to drop and get crushed over and over again (Keystone Fault
). The diagram below further illustrates the orientation of rocks in a Keystone fault. They also claim that nearby Tophet chasm formed in the same way (The Mystery of Littleton's Tophet Chasm
).
Problem: The evidence we gathered at the chasm indicated that this theory is also incorrect. Although the chasm is covered with boulders, they fell in from the walls after the event that caused its formation (Purgatory Chasm Geology
). Another issue with this theory is that a Keystone fault, in which the bottom of the chasm completely drops (bedrock and all), occurs at much larger scales than the quarter-mile long chasm (Purgatory Chasm Geology
). This type of faulting also does not occur in this type of rock (Purgatory Chasm Geology
).
The third theory proposed by Professor R.J. Lougee in 1951, was widely accepted for many decades, although it is now considered to be outdated and impractical (Purgatory Chasm Geology
). It stated that Purgatory Chasm is part of an ice-marginal channel cut through a ridge of bedrock (Purgatory Chasm Geology
). These channels are cut by flowing water along the ice margin and are often left dry after ice has melted away (Some Definitions for Glacial Geology
). The water that supposedly rushed through to form the Chasm originated from an ice marginal lake in Casey Brook Valley just northeast of the Purgatory Chasm Visitor Center shown below on the map (oriented with north at the top) (Purgatory Chasm Geology
). When the glacier retreated out of the area and an ice dam was melted away, a torrent of water from the ice marginal lake was released (Purgatory Chasm Geology
). This water discharged into Purgatory Brook Valley through the Chasm from a falls, creating the rock formations seen there today (Purgatory Chasm Geology
). This falls descended 75 feet over a horizontal distance of 400 feet near where visitors enter the chasm (Purgatory Chasm Geology
). The pounding action of large boulders that tumbled over this falls broke out rock joints and although all but the largest of these rock fractures have been washed away, post-glacial weathering of the chasm walls account for the large boulders strewn across the floor (Purgatory Chasm Geology
). It is also believed that Tophet Chasm in Newton, MA was formed 25,000 years ago in a similar way when an ice-dammed lake drained suddenly (Tophet Chasm, The Drainage Waterfall of Glacial Lake Nashua
). An ice-dammed lake is a body of water located in, on, under, or at the ice margin and is dependent on damming by glacial ice (Ice-Dammed Lakes
).
Problems: Similar to the first theory, Lougee does not address the fact that there are no signs of water erosion in the chasm as there is no smoothing of rock surfaces that should come with such fast-moving water (Purgatory Chasm Geology
). Also, the water would have had to be transported from Casey Brook Valley to the Purgatory Brook Valley (Purgatory Chasm Geology
). This length was not completely downslope and instead, included multiple low points (valleys) and higher points in between (Purgatory Chasm Geology
). All of these low points could have only been filled with ice to allow the river to pass through with enough force to erode the chasm out (Purgatory Chasm Geology
). Even if this was true, such a large, erosive, and dynamic stream would have certainly left a distinct pathway, yet there is no trace of a raging river in that stretch between Casey Brook Valley and Purgatory Brook Valley (Purgatory Chasm Geology
).
The fourth theory was put forth by Dr. Mauri S. Pelto, a professor of Geology at Nichols College (Purgatory Chasm Geology
). His theory explains how a glacier (part of the Laurentide Ice Sheet) plucked the rock from an area already weakened by fault activity beneath the Chasm (See Faults page) (Purgatory Chasm Geology
). Plucking commonly occurs with glaciers when water is found at its base due to the pressure of the ice moving upon hard rock surfaces, causing pressure melting. It then refreezes while it seeps into joints of the rock and essentially pulls the rock out of the ground and mobilizes it to transport the rock along the bottom as it travels. The diagram below, helps illustrate glacial plucking. This initial plucking carried the fractures of rock to Purgatory Brook Valley where many of them were dropped (Purgatory Chasm Geology
). Rocks found in the valley were large, angular, and widely scattered, which means that a very large amount of force would have been needed to move such rocks and that no massive flood, river, or anything having to do with water could have accounted for this(Purgatory Chasm Geology
). The fact that there is no evidence of water erosion or marine deposits in the chasm also lead to the conclusion that glacial plucking was responsible for the formation of this chasm (Purgatory Chasm Geology
). This plucking created the general shape or shell
of Purgatory Chasm and the large boulders that exist within the Chasm fell in from the walls after the initial formation (Purgatory Chasm Geology
). This theory corresponds with what was seen throughout the Chasm and takes into consideration many of the flaws in the other three theories. For these reasons, this is the most plausible theory as to how Purgatory Chasm was created.
Ice Wedging
Root Wedging
Organic Acids and Oxidation
Spheroidal Weathering and Abrasion
Ice wedging is a very common form of weathering in areas where the temperature fluctuates between below and above freezing. When a rock is found in such an area and has small cracks in it, water can seep through and fill these cracks. Later, the water could freeze with a temperature drop and when it solidifies, the ice forces the sides of the rock and expands the crack. This is due to the fact that water expands its volume by 9% when it freezes into a solid (ice).
In the photo, the rock may have began years and years ago with just a very minute fracture. Runoff and rain filled the crack with water each time it freezes and expands the crack, until the rock has been forced apart to a present width of about three inches.
Root wedging is very common in areas with heavy vegetation (usually larger, well-established trees), such as Sutton, MA, where Purgatory Chasm resides. This type of mechanical weathering occurs when a tree’s roots grow beyond the soil layer, approaches bedrock, and continues to grow into its cracks. These roots push through the rock and forces it apart. Eventually, it could completely separate slabs or slices of the rock.
In the photo above, the roots of this tree seem to have split the rock towards the bottom left and has created a long crack where the roots grew into. Roots also grew into the crack in the bottom right to create a split in the rock there as well.
Organic acids are produced from lichen and moss growing on rocks and eat away at the surface of the rocks. In the photo, above Danni’s head, there is a line of light green splotches and even further to the right of these splotches of darker brown. The lighter splotches are lichen and the darker ones are moss; both are slowly wearing away at the surface of this rock.
Oxidation occurs when oxygen dissolved in water reacts with metals in the rock. This chemical reaction usually results in some type of color change. To the left of Danni’s head, is a large patch of dark brown. This is an example of oxidation because of the obvious color change. There is also a little moisture on the rocks just below this patch, which means that water commonly passes through this small area of rock in the form of runoff or rain. Parts of this patch also seem streaky, which further supports the idea that water was dripping or seeping down the side of the rock to allow for the oxidation to occur.
Spheroidal weathering is a type of chemical weathering where minerals round away the edges of cracks. The image above shows that the sides of each crack are no longer straight edges that fit together. The edges and especially the corner of the rock are rounded off.
Abrasion is a type of mechanical weathering that occurs when wind or water picks up small grains of sand and rubs it against rock. Wind abrasion would create a pitted but polished surface while water abrasion would make it rounded and smooth. The rock in the image above shows sign of wind abrasion with a pitted and slightly polished surface.
Granite Gneiss
Faults
From the bedrock map below, the bedrock of the area in and around Purgatory Chasm (Sutton, Medford, Dedham area) has been determined to be Scituate Granite Gneiss (Massachusetts Geology
). This rock is a gneissic granite containing small clots of biotite and is represented by the letters Zsg
on most bedrock maps and is colored in light pink in the map above (Scituate Granite Gneiss
). This rock type is estimated to have been formed in the Late Proterozoic eon or Neoproterozoic period, which was from 1 billion to 541 million years ago (Proterozoic Eon
). More specifically the rock from that area originated on Avalon in the Proterozoic Z, which was from 800 to 570 million years ago (Proterozoic Geology
, 310).
Granite is an igneous rock that is felsic and phaneritic. The fact that it is felsic means that it is silica-rich (low-temperature material), mostly light in color, and originates from continental crust. Its phaneritic quality means it is coarse-grained and spotted, so it is an intrusive rock that was allowed enough time and heat for large minerals to form. Granite is also a protolith of gneiss, which is convenient considering a combination of the two is found in the Chasm. This means that the granite experienced considerable heat, pressure, and chemical reactions to partly transform into gneiss. Gneiss is a foliated metamorphic rock with its minerals arranged in discrete bands under high heat and pressure with a high metamorphic grade. It is possible that the gneiss formed during the event when the Avalonian terrane was sutured to the coast of North America because the collision caused a large amount of heat to be generated.
The sides of the chasm show northward dipping joint surfaces as is presented in the image above. The fact that the joints in the walls do not line up to each other indicate that the walls were displaced at some point (Purgatory Chasm Geology
). This leads to the conclusion that the chasm exists along a fault plane since faulting activity is responsible for these type of joints(Purgatory Chasm Geology
). The tectonic forces in the area that could generate such faulting have been silent for 200 million years, so this activity could have only created a weakness in the rock and not the entire chasm because the chasm was formed much later (Purgatory Chasm Geology
). This weakness was then opened up by the glacial activity of the Laurentide Ice Sheet during the last ice age to form the chasm (Purgatory Chasm Geology
).