If a comparison is made between the structure of the mountain and that of an asymmetrical arch, to carry the simile one step further, it may be imagined that the axis of the arch may be either horizontal or inclined. The chances that it is inclined are much greater than the chance that it is exactly horizontal. Thus, most anticlines or anticlinoria are inclined along their axes and the amount of the dip of the line connecting the points along the crest of the fold is called the plunge.
Most of the folds in the Mount Mansfield area plunge about ten degrees to the south. This plunge is expressed in the dip of the crests of the minor folds, particularly in the crenulation of the mica layers. Viewed at a distance the trace of the fold-crests form a series of parallel lines on the smooth mica-rich surfaces. This type of structure is called the lineation and is expressed on the geologic map by small arrows. The dominant lineation is north-south. Although [Figure 5] is a sketch of a small fold showing the different structural elements, it might be taken as a diagrammatic sketch of the regional structure.
Evidence that the structure of the rocks is even more complex is shown locally by the presence of east-west lineations. The intersection of this secondary lineation with the dominant south lineation produces a checker board appearance on some rock surfaces. A system of east-west trending folds is traced by some of the quartz lenses. The significance of the east-west structures is hypothetical, but they are believed to have been mostly obscured by the younger structural features.
Figure 4. Folding and crenulations in the mica-albite-quartz schist near the Chin on Mount Mansfield. As the photograph is looking to the north, it may be noted that the folds are asymmetrical with axial plane of the folding dipping east.
With the description of the rocks completed, the question which arises next is how to represent these three-dimensional contortions on the map. [Figure 6] illustrates how the attitude of a particular layer may be expressed in terms of dip and strike. It is apparent that the dip of the rock layer may vary from 0° to 90° and is measured as the angle between its plane and a horizontal plane. Also, it is apparent that the trend of the bed, or the strike, may correspond to any direction of the compass and can be measured as the intersection of that plane and a horizontal plane. The maximum dip is always at right angles to the direction of the strike.
Figure 5. Diagrammatic three dimensional sketch illustrating the relations between outcrop patterns of folds on vertical planes perpendicular and parallel to the trend of the folding (front and sides of the block) and on a horizontal surface (top of block). Cut-away section of the block shows the folds and lineation lines on a given foliation surface. These folds can be more clearly visualized if the upper portion of the diagram is covered.
The dip and strike are used to measure the position and attitude of the layers of the rocks. In the case of the mica-albite-quartz schists these planes are called foliation planes. If the structure of the rock is an anticline, most of the strikes of the foliation are parallel, but the dips are in different directions on either side of the crest. At the crest of the fold the foliation is horizontal if the fold is not plunging. On Mount Mansfield where the plunge is about ten degrees to the south, foliation along the crest strikes about east-west and dips about 10° south. Away from the crest, the dip of the sides of the anticline begin to be expressed in the readings so that the strike directions “swing back” toward the north-south direction. The majority of the layers on the east side of the mountain strike northeast and dip to the east with the angle of dip increasing away from the crest of the anticlinorium. On the west side of the mountain they trend to the northwest and dip to the west with the dips becoming steeper away from the crest. In addition to these variations in the dip and strike over the anticlinal crest, the smaller folds give local abnormal readings. For these reasons many of the dips and strikes shown on the geologic map represent the averages of a number of readings, and those of the minor folds and crinkles have been omitted in order to simplify the picture.
