Figure 1: Location Map of Lessor’s Quarry, The Beam, and Clay Point. Note the
Champlain Thrust Fault, Lone Rock is located to the south on this fault.

Joe Bartlett
12/10/02
The Geologic Evolution of Western Vermont
Abstract
This report will use the measurements and observations taken at Lone Rock Point to create a model for the formation of geologic structures observed throughout Western
Vermont. Lone Rock consists of two large units, Dunham Dolomite and Iberville Shale separated by the Champlain Thrust Fault. The Iberville Shale shows evidence of extreme deformation indicating the tremendous strain produced by movement along the Thrust
Fault. Comparisons of structures and data from three other major outcrops: Lessor’s
Quarry, The Beam, and Clay Point, allow for the understanding of the geologic processes which shaped the region. Strong stereonet trends prove a constant direction of transport which strongly supports the theory that all deformation and motion was in response to the same stress field. This theory allows for the modeling of the strain gradient which was present during the activity of the Thrust Fault.
Introduction
The purpose of this report is to establish a relationship between the geologic formations we observed at four outcrops across Western Vermont. Using Lone Rock as a catalyst, it is possible to relate all the observed structures to one massive sequence of movement along the Champlain Thrust Fault. The structures observed at Lone Rock
Point are very significant because they allow for connections to be made between the much smaller scale formations observed at Lessor’s Quarry, The Beam, and Clay Point
(See Figure 1 for locations of outcrops and Champlain Thrust Fault). Using comparisons

made at each site it is possible to establish a model of the events that shaped the geology of Western Vermont from these connections.
Geology of Lone Rock Point
The large outcrop known as Lone Rock point consists of a two large units, the lower unit is composed of heavily cleaved, folded, and faulted Iberville Shale, and the upper unit consists of Dunham Dolomite which is a much older formation. This structure of older rocks situated on top of younger rocks, separated by a fault, suggests the presence of the thrust fault. The size of the rock units observed at Lone Rock along with the large age difference of the units suggests a major thrust fault. Figure 1 shows the
Champlain Thrust Fault which Lone Rock Point is situated on.
Figure 2: The contact between highly deformed Iberville Shale and Dunham Dolomite known as the Champlain Thrust Fault, as found at Lone Rock Point.
The Iberville shale unit present at Lone Rock Point was composed of fine-grained crumbly silt sized material and was heavily folded, cleaved, and faulted. The faults present in the shale were folded along with the remnants of distinguishable bedding.

Most of the bedding was very difficult to differentiate and it included some isolated chunks of sandstone. The folding in the shale was fairly continuous throughout the exposure and consisted of tight isoclinal folds as shown in Figure 8. The cleavage present appeared to be parallel to the folds evident in the shale.
A major unfolded fault separated the Iberville Shale and Dunham Dolomite units
(Fig 2).This fault is known as the Champlain Thrust Fault and is characterized by deep mullions which indicate movement in a Northwest South East direction. The dolomite was relatively undeformed by faulting, cleavage, or folding. At the contact between the two units, a dense breccia was observed under the fault and mullions were observed above showing the response of the two rock units to movement along the fault.
Lone Rock Point Data Table
Mullions and slickenlines
Faults near the
Champlain Thrust
141 19
142 22
140 14
139 10
141 19
142 22
139 12
141 12
014 11 E
025 09 E
046 12 E
046 32 E
008 04 E
040 15 E
052 04 E
Fold Axes
148 22
135 28
102 30
111 55
137 22
153 21
170 16
150 17
Cleavage in
Iberville Shales
055 14 S
014 27 E
031 39 E
055 14 S
006 15 E
014 27 E
031 39 E
065 32 E
Cleavage Cont.
012 19 S
028 28 E
029 12 E
040 22 E
010 03 E
050 12 E
042 35 E
035 35 E
140 12 150 29
175 31
184 24
048 24 E
051 11 S
068 19 S
058 34 S
042 18 S
020 34 S
039 34 S
030 15 E
047 04 E
018 20 E
078 21 S
099 31 S
064 42 SE
Figure 3: Data table from strike/dip and trend/plunge measurements taken at Lone Rock Point
Analysis of Lone Rock
The structures present at Lone Rock Point show evidence of extreme deformation due to movement along the Champlain Thrust Fault. The cleavage formation is so dense that bedding is only visible in a few locations. The cleavage is almost horizontal and

appears to be axial planar to the heavy folding present in the shale. The cleavage is consistent in orientation and density throughout most of the outcrop and shows little additional deformation with proximity to the contact between rock units. The high density of the cleavage and the proximity to a fault indicate that the cleavage present in the Iberville shale is fault zone cleavage. The very shallow dip of the axial planar cleavage as shown in Figure 3, indicates a combination of stresses. Rotational strain from motion along the fault as well as vertical compressional strain from the immense mass moving resulted in the very shallow dip of the cleavage. Further north along the outcrop the cleavage and other structures show less deformation suggesting that strain was less in the shale further (40-50m) below the fault zone.
Faulting was very prevalent in the Iberville Shale unit below the Champlain
Thrust Fault. Faulting occurred in more than one stage as indicated by faults which are folded and faults which cut across folded structures. Due to the excessive deformation, slickenlines, and fault plane measurements were not possible. Faulting did not appear to occur in any particular pattern. Motion along the Champlain Thrust Fault is well documented by the deep mullions present at the contact. These mullions show a consistent dip to the south-east as shown in Figure 4. The dip direction of the mullions indicate that the movement was to the north-west, up the ramp of the thrust fault. .
The folded structures present at Lone Rock Point also show evidence of extreme deformation. The folding consisted of very tight isoclinal folds which are visible in
Figure 8. Isoclinal folds are indicative of high stress fields capable of producing high degrees of deformation. The folds were almost completely overturned to produced vertically stacked isoclinal folds. Percent shortening of folded structures was used to

estimate volume loss in the Iberville Shale. This calculation estimated that 85% of
Iberville shale was lost to dissolution.
As a whole the structures present in the outcrop show evidence of extreme deformation caused by large-scale movement along the Champlain Thrust Fault.
Cleavage and folds formed concurrently and were aligned on the same axial plane.
Because these folds incorporated some faulted structures, it is known that the folding and cleavage formation occurred after significant strain was acting upon the outcrop. The folds and cleavage were then rotated by the strain caused by motion along the thrust fault.
This rotation caused additional volume loss and continued faulting as evidenced by the faults cutting across folds. Vertical strain from the immense weight of the rock being pushed up the thrust fault caused further rotation of structures in the shale. Isolated chunks of sandstone bedding provide further evidence to the high volume loss and the extreme deformation needed to remove all but a few pieces of a sandstone unit which was present in the Iberville Shale. The presence of two distinctly different rock units demonstrates the different responses of the two rock types found at the outcrop. The shale contained much more calcite and was therefore more affected by dissolution while the dolomite was relatively strong and was affected much more by transport than by deformation.
Regional Data
Data collected from all four of the outcrops in Western Vermont are presented in Figures
4-7.

Regional Interpretation
The geology of Western Vermont, as observed through four outcrops: Lessor’s
Quarry, The Beam, Clay Point, and Lone Rock Point, was strongly influenced by movement along the Champlain Thrust Fault. Motion along this fault created a strain gradient which deformed structures to varying degrees as a result of their distance from the fault. All of the structures we observed in several different locations and through several different rock types indicate responses to the same stress field proving that they all formed in a short period of time on geologic time scales. This theory of a single event shaping all the structures we observed is strongly supported by the consistent direction of transport indicated in the stereonet plots (Figs 4-7). Assuming this theory enables strong relationships to be established between structures visible at each outcrop with those at other outcrops. Relating the observations and calculations taken at all the outcrops allows for an understanding of the geologic processes that shaped the region.
Direction of Transport and Strain Gradient
A very strong trend exists in the stereonet plots indicating a direction of transport trending SE to NW (Figure 4). This trend is present in cleavage, slickenlines, and fault planes from all four outcrops. This strong trend demonstrates that due to a consistent direction of transport, it can be inferred that all movement was as a response to the same stress field. It is expected that the fold axis would not fall under this pattern as they form perpendicular to the direction of movement. . The slickenline cluster in the south-east shows a consistent direction of transport to the north-west, due to the movement of material up ramp faults (Figure 5). The three slickenline measurements labeled as the

wedge, show variation from the cluster. This is due to the formation of faults in the structure known as the wedge as a response to forces not entirely related to the
Champlain Thrust Fault. The wedge formed in the fold plane of the overturned synclinal fold found at Clay Point. Due to heavy folding, the rock was weakened allowing for the formation of a generation of faults which moved at a high dip due to weaknesses in the folded structure. The fault planes also show a similar trend and form in a very strong line as shown in Figure 5. Variation is expected in the strike and dip of fault planes as they form in response to weaknesses in the rock. Different formations have different fields of weakness such as bedding plane boundaries at Clay Point, or the weaker shales located above and below the Beam.
The cleavage planes measured at the four outcrops show evidence of a strain gradient and also formed in the north-west south-east trend as shown in Figure 7. The stereonet clearly shows higher dip angles with increased distance from the Champlain
Thrust Fault. Because all of the poles of the cleavage planes line up in the same trend, it can be inferred that they all formed due to the same compressional forces, but were rotated to varying degrees. This rotation was caused by movement along the Champlain
Thrust Fault. Cleavage closest to the Thrust Fault is near to horizontal while at locations further away, cleavage shows a high dip angle.
The patterns observed in folded structures found in the four outcrops support the presence of a strain gradient centered at the Champlain Thrust Fault. Figure 8 clearly shows a drastic decrease in the interlimb angle with proximity to the Thrust Fault. This is due to the response of rocks at the outcrops to varying degrees of strain. The higher the strain enacted upon the outcrop, the closer the limbs are pushed together during folding.

The increase in faulting during and after folding is increased in the tight folds due to weakness created by bending the rocks to such a high degree. Percent shortening was calculated by measuring the amount of volume lost due to folding at three outcrops.
Percent shortening can be directly related to the amount of strain that caused the volume loss. These strain measurements were plotted against distance from the Champlain
Thrust Fault to establish a strain gradient diagram (Figure 9). This diagram clearly shows an exponential increase in strain with proximity to the Thrust Fault. This diagram shows that strain from movement along the Thrust Fault influenced structures much further away than Lessor’s Quarry.
Comparing the Four Outcrops
Comparisons of structures observed at the four outcrops can be used to interpret larger scale geologic processes which are not able to be directly observed. The faultbend fold mechanism in its entirety is visible at the Beam. This same mechanism formed the structures found at Lessor’s Quarry and Lone Rock, and therefore influenced all of
Western Vermont. The fault–bend fold at the Beam is also very similar to Lone Rock in the demonstration of differential responses of rocks to the same strain. The dolomite unit at Lone Rock is very similar in appearance and behavior to the micrite layer found at the
Beam.
The Beam demonstrated increased strain towards the East where movement had occurred for the greatest duration, similar to how deformation was greatest with proximity to the Champlain Thrust Fault. Using this assumption, the strain gradient present at the Beam can be extrapolated to model the strain gradient of the Champlain

Thrust Fault. The transition from spaced to rotated to fault zone cleavage is very similar to the increased rotation of cleavage between the four outcrops with proximity to the
Thrust Fault (Figure 7). Cleavage found at Lessor’s Quarry and the Beam can be compared to the spaced cleavage found at the Beam. Cleavage found at Clay Point is similar to the rotated cleavage, and Lone Rock demonstrates similarities to the fault zone cleavage. The rotation of cleavage across Western Vermont due to movement along the
Thrust Fault can be used to estimate the strain difference between the outcrops we observed. The strain calculation of 1.38 is an arbitrary comparison of the additional strain present at Lone Rock as compared to Lessor’s Quarry. The strain gradient at the
Beam was vertical, but it can be extrapolated to represent the horizontal strain gradient created by the Champlain Thrust Fault that shaped the four outcrops.
Conclusions

The structures present at Lone Rock Point show evidence of extreme deformation caused by movement along the Champlain Thrust Fault

The high stress fields caused a very large volume loss which primarily affected the shale unit present under the Thrust Fault
 The geologic structures observed at Lessor’s Quarry, The Beam, Clay Point, and
Lone Rock are all closely related to the movement of the Champlain Thrust Fault

The strain gradient produced by this movement shaped much of Western Vermont

It is possible to model large-scale geologic processes by detailed study of smallscale formations