Heart of Avalonia |
Bedrock frost heave with rotation: Part 1
Level and low-sloping terrain
Level and low-sloping terrain
Systematically rotated frost-heaved joint blocks:
How did these originate?
How did these originate?
The above photo shows a group of closely spaced bedrock frost heave monoliths that have been rotated about a horizontal axis, top toward the left. The frost heave has occurred in what now appears to be an approximately symmetric environment. It must be assumed, however, that the blocks were affected in the past by asymmetric conditions that created a preferred direction for rotation. Identifying the asymmetry guiding the rotation of the blocks can help in understanding the environment prevailing when the frost heave process took place.
Frost heave with rotation in a symmetric environment
When bedrock frost heave affects a single joint block on level bedrock with vertically-dipping foliation, symmetry considerations imply that rotation would not be promoted. If rotation is observed in just one monolith, the cause can be attributed to chance (unrecognized asymmetry), but when several monoliths in a particular area show similar rotations, an explanation of the break in symmetry is warranted. Significantly sloped terrain and non-vertical foliation introduce clearly preferred directions for the rotation of frost-heaved joint blocks. These factors can dominate the analysis of bedrock frost heave with rotation, making it more difficult to isolate other factors.
In the following discussion, asymmetrical frost heave observed in roughly symmetrical bedrock environments will be emphasized. In these instances, occurrences of bedrock frost heave with rotation can be interpreted primarily in terms of asymmetry in either the underlying frost heave process or in the flow or pressure of ambient glacial ice.
In the following discussion, asymmetrical frost heave observed in roughly symmetrical bedrock environments will be emphasized. In these instances, occurrences of bedrock frost heave with rotation can be interpreted primarily in terms of asymmetry in either the underlying frost heave process or in the flow or pressure of ambient glacial ice.
Rotation of a frost-heaved joint block can be expected when ice accumulation rates beneath the block vary from one side of the block to the other. This effect could result when pore water is preferentially fed to one side of the block. Similarly, if the sliding friction on one side of the block exceeds the corresponding sliding friction on the other side, then rotation of the block will be promoted. Both ice-accumulation considerations and side-friction considerations are more likely to be significant when the block is wide relative to its height (width as viewed along the axis of rotation). Greater width increases the moment arm, hence torque, driving the rotation. Furthermore, the likelihood that ice accumulation rates will vary significantly along the base of a frost-heaved block rises with increasing width of the block.
The above photo shows two frost-heaved blocks, each more than 1 meter long. The rotation of the left-hand block could be reasonably attributed to differences in sliding friction at the two ends of the block.
In cases where a frost-heaved joint block is thin relative to its height as viewed along the axis of rotation, ice-accumulation rate differences and sliding-friction differences become unlikely explanations for rotation.
In cases where a frost-heaved joint block is thin relative to its height as viewed along the axis of rotation, ice-accumulation rate differences and sliding-friction differences become unlikely explanations for rotation.
The above photo shows several relatively thin rotated frost-heaved monoliths. It is not probable that these monoliths were systematically rotated by differences in side friction or ice accumulation rates. The preferred explanation for the rotation is asymmetry in the ambient ice environment. Specifically, glacial ice moving tangential to the ground with velocity directed toward the left (as seen in the photo) appears to have guided the frost heave process. Alternatively, the blocks might have been lifted vertically by frost heave and then deflected by subsequent leftward-directed tangential ice motion. The bedrock foliation in the area hosting the blocks dips at an approximate vertical angle. Independent assessment of glacial ice flow at the location illustrated above conforms with the ice flow direction suggested by the deflection of the blocks. The blocks illustrated above provide a probable example of subglacial bedrock frost heave with rotation.
Rigid rotation vs. shear
The above diagram shows the rotation of a frost-heaved monolith where the block rotates as a rigid body. All rotations of blocks that are not fully displaced from substrate require deformation of rock. This deformation might comprise crumbling or fracturing of the edges of the frost-heaved block, or breakage of the edges of the bedrock adjacent to the rotated monolith. In the above diagram, breakage of rock is not shown. Rather, the bedrock is shown as having undergone compressional deformation adjacent to the rotated block on the right-hand side. This mode of accommodating rock conflict is not intended to reflect common occurrence and is illustrated mainly for simplicity. The stress causing rotation of the monolith must be sufficient to overcome the yield strength of the rock comprising the monolith and/or the yield strength of the adjacent bedrock.
Gaps in the ice underlying the rotated monolith and adjacent to the monolith are opened as rotation of the monolith progresses (lighter colored areas in above diagram). If there is no ambient hydrostatic pressure (subaerial environment) then the stress needed to create these gaps (tensile strength of ice) is small compared to the strength of rock. However, in a subglacial environment with tens or hundreds of meters of overlying ice, the stress required to open the gaps becomes significant (subglacial hydrostatic stress, adjusted by a geometric factor). Thus, in a subglacial environment, there is a potential preference for deforming a monolith in shear, rather than rotating the block as a rigid body.
Gaps in the ice underlying the rotated monolith and adjacent to the monolith are opened as rotation of the monolith progresses (lighter colored areas in above diagram). If there is no ambient hydrostatic pressure (subaerial environment) then the stress needed to create these gaps (tensile strength of ice) is small compared to the strength of rock. However, in a subglacial environment with tens or hundreds of meters of overlying ice, the stress required to open the gaps becomes significant (subglacial hydrostatic stress, adjusted by a geometric factor). Thus, in a subglacial environment, there is a potential preference for deforming a monolith in shear, rather than rotating the block as a rigid body.
Rotation with pure shear, as illustrated above, does not generate a gap in the ice beneath the rotated monolith nor does it require significant creep deformation of the underlying ice. Two circumstances favor rotation with shear deformation. These are: 1) The monolith has a low yield stress (for shear) and hence is vulnerable to shear deformation. This circumstance would commonly result from intense foliation induced by prior tectonic stress acting on phyllosilicate-rich bedrock. 2) The rotation of the block is caused by lateral tangential loading by ambient glacial ice, rather than by differences in side-to-side ice accumulation rates or differences is side-to-side sliding friction (see discussion above).
Continuous shear of rock is a less likely occurrence than discrete step-wise shear. Step-wise shear achieves similar deformation geometry while breaking a much smaller number of molecular bonds in the deforming rock. The occurrence of step-wise shear in rotated frost-heaved monoliths might be recognizable by observing the subsequent weathering of the monoliths. Rock failure in step-wise shear generates repeating faults which then become vulnerable to penetration by meteoric water and subsequent freeze-thaw weathering.
The excessive joint widening seen in the above-pictured rotated frost-heaved monolith might be the result of step-wise shear deformation of a foliated block, followed by Holocene freeze-thaw weathering and other weathering. This block was observed at a location where several other nearby frost-heaved monoliths were similarly rotated, foliation dipped vertically, and glacial flow direction (based on independent observations) was found to conform with the direction of block rotation.
Step-wise shear, combined with the displacement of one or more sheared blocks from bedrock substrate, provides a favorable (low tangential stress needed) arrangement for the occurrence of bedrock frost heave with rotation.
The ejection from bedrock of one or more blocks from a sheared monolith facilitates the rotation of the remaining blocks with reduced rock deformation. The large (~1 m high) frost-heaved joint blocks seen in the foreground of the above photo are the apparent remnants of a single block that was subjected to side loading from the left (direction as seen in the above photo). Two sections of the original block were displaced from bedrock or broken off at the base, allowing room for rotation of the entire row of blocks. The precursor block accommodated the rotation by failing repeatedly in step-wise shear.
The arrangement shown above reflects tangential glacial ice loading of a frost-heaved monolith in strongly foliated bedrock. A single block was divided into a row of thin fragments separated by multiple faults. The faults, now widened by erosion, were induced by step-wise shear deformation. The right-most block was ejected from bedrock leaving room for the other blocks to rotate without any further rock deformation. Note also other instances of bedrock frost heave with rotation visible in the background.
Special Case
When foliation in bedrock dips at an angle significantly less than 90 degrees (typically on the limbs of folds), frost-heaved joint blocks can appear to show rotation when, in fact, there is no rotation.
The frost-heaved joint blocks shown above occur in bedrock with foliation dipping at a shallow angle. Superficially, these blocks might appear to demonstrate rotation although their orientation is little changed from their initial orientation in bedrock. This type of occurrence is mentioned here for completeness only, and will not be analyzed further in this section.
Rotation in the plane of foliation
The above diagram illustrates bedrock frost heave with rotation occurring in the plane of foliation. This type of occurrence is infrequent, but is nevertheless readily observed in areas where frost-heaved monoliths are abundant. Often, the most plausible cause for rotation of blocks in the plane of foliation is differing sliding friction at the ends of an elongated block. However, in some circumstances, the rotation can be reasonably attributed to differences in the rate of ice accumulation along the base of the block. The diagram above shows a vertically-dipping cross joint running along the left-hand side of the raised joint block. If this cross joint was a preferential source of groundwater feeding the frost-heave process, then the block might be expected to rotate clockwise (as shown in the diagram) because of a higher ice accumulation rate beneath the left-hand side of the block.
The photo above shows four large frost-heaved joint blocks (marked with magenta arrows) that have been rotated in the plane of foliation. The direction of rotation is indicated by the arrows. The white stripe extending diagonally across the photo marks the apparent path of one or more closely spaced parallel cross joint(s). It appears as though water feeding from the cross joint(s) spread laterally outward along longitudinal joints following bedrock foliation and accelerated a frost heave process occurring beneath each of the rotated blocks. In all four blocks, the side of the block nearest the cross joint(s) experienced greater uplift than the side furthest from the cross joint(s). A closeup of the two rear-most blocks is shown below.
In the above photo a large joint block is shown, divided into two sections by separation along a cross joint. The two sections have been rotated in opposite directions creating the V-shaped feature shown. The hole visible between the sections accesses a deep cavity beneath the rotated blocks. An interpretation of this site based on the Subglacial Ice Plume model appears reasonable. Within this interpretation, pressurized pore water accumulating and migrating within the cross joint augmented a lens of segregated ice beneath the blocks, driving the blocks upward into an overlying glacier while simultaneously rotating them. However, it remains possible that the feature originated in a subaerial environment, with the cross joint serving to concentrate meteoric water. The substantial open hole in the center, between the blocks, tends to favor the subglacial origin model. Observations of erratics near this site also point to the frost heave having occurred in a subglacial environment.
Rotation in the plane of foliation: More examples
In each of the above three examples, a major cross joint (fissure) runs along the side of a rotated frost-heaved joint block in a manner suggesting that preferential groundwater feed from the cross joint aided the rotation.
Note the slot in the ice-disrupted bedrock just to the right of the tallest of the rotated frost-heaved joint blocks seen above. This slot potentially reflects a concentration of groundwater feed (ice accumulation) that preferentially lifted bedrock in the immediate vicinity . A joint block was ejected and displaced from the slot (note corner of block protruding from vegetation to the right of the hammer; estimated length and width approximately match the slot), indicating that subglacial frost heave occurred at this site.
The large rotated blocks seen behind the hammer in the above photo form part of a severely ice-disrupted bedrock feature. A plume of rising subglacial ice, concentrated in an area behind and just left of the hammer, could account for the rotation in this instance.
The block shown above was subjected to vertical frost heave, rotation by about 45 degrees and horizontal shift away from the camera position. A plausible interpretation for this feature incorporates subglacial frost heave driving the block upward into ice while the ice moves via creep in the direction indicated by the horizontal shift of the block.
As with the previous example, the block shown above was subjected to vertical frost heave, rotation, and horizontal shift. The original position of the left-hand edge of the joint block was at the extreme left of the frame shown above. The feature is a probable example of subglacial bedrock frost heave where ice creep directed left-to-right (direction as seen in the photo) shifted the block horizontally. Within a subglacial-origin model, in both of the above two examples the rotation of the blocks is attributable, at least in part, to the increasing creep velocity of cold based glacial ice at distances further from the ground.
The frost-heaved joint blocks seen above were translated to the left and rotated, probably by the horizontal creep of overlying glacial ice. Two factors complicate this interpretation. Firstly, the fissure to the right of the frost-heaved blocks might have been a significant accumulation line for segregated ice, and ice moving via creep from the fissure could potentially account for the horizontal shifts and rotations of the blocks. Secondly, there is a small (~1 m high) cliff to the left of the blocks, making any frost heave process beneath the blocks more likely to shift the blocks leftward. Why, in the row of joint blocks pictured above, did only two blocks experience horizontal shift and rotation? It is possible that only the two shifted blocks underwent vertical frost heave and that vertical frost heave was critical in moving the blocks into a zone of cold glacial ice that was further from the ground where horizontal ice creep velocity was significantly greater than zero.
The cause for the rotation of the two joint blocks shown above is not clear. The disrupted rock seen in the right foreground suggests that migration of ice upward from the ground might have been concentrated on one side of the rotated joint blocks. The local area where these blocks were observed includes a large network of major ice-induced fissures, implying abundant ice under pressure permeating the bedrock. The block between the two frost-heaved blocks was almost certainly also frost-heaved, but the central block settled back into the underlying cavity after ice departed. The same circumstance likely applies to the numerous other small blocks seen in the foreground.
Rotation normal to foliation: Small groupings
Strongly foliated bedrock is weak with respect to shear in cases where the applied shear stress acts to cause slippage along planes of foliation. It is common for large foliated joint blocks to fail repeatedly and separate into layers divided by faults when undergoing frost heave with an aligned component of ice-induced shear stress. When one large foliated joint block divides into layers separated by faults, it becomes ambiguous or arbitrary as to whether to interpret the result as a single step-wise sheared joint block or as a group of separate frost-heaved joint blocks.
In the example shown above, the illustrated group of frost-heaved blocks has undergone systematic rotation perpendicular to the plane of cleavage in the host bedrock. This feature would appropriately be labeled as a group of separate rotated frost-heaved monoliths. The sizes, positions and separations of the individual blocks make this description reasonable. The rotation of the above-pictured group of blocks was likely caused by horizontal loading by glacial ice moving in creep.
The closely-spaced group of frost-heaved rock fragments shown above could appropriately be described as comprising a single block, although the block is distinctly separated into layers by weathering. The pattern of weathering appears to follow faults induced in the rock by subglacial frost heave and rotation with step-wise shear.
Note regarding rock deformation induced by near-ground ice creep:
The time-dependent strain experienced by cold glacial ice moving via creep near the ground is primarily a shear strain. It is thus not surprising that bedrock vulnerable to shear adopts shear strain when protruding into ice at the base of a cold glacier advancing by creep. The pattern of accumulating strain in the glacial ice is, to an extent, preserved in the sheared bedrock fragments after the ice departs. This will be a theme of the Frost-heaved Bedrock Project going forward. In specific instances, a potentially detailed picture of past ice-creep activity at or near the base of a cold glacier can be inferred from remnant patterns in ice-disrupted bedrock.
The time-dependent strain experienced by cold glacial ice moving via creep near the ground is primarily a shear strain. It is thus not surprising that bedrock vulnerable to shear adopts shear strain when protruding into ice at the base of a cold glacier advancing by creep. The pattern of accumulating strain in the glacial ice is, to an extent, preserved in the sheared bedrock fragments after the ice departs. This will be a theme of the Frost-heaved Bedrock Project going forward. In specific instances, a potentially detailed picture of past ice-creep activity at or near the base of a cold glacier can be inferred from remnant patterns in ice-disrupted bedrock.
The blocks shown in the above photo are likely the remnants of a single frost-heaved joint block that underwent step-wise failure in shear when exposed to horizontal loading by cold glacial ice moving via creep.
The above two photos show views of a single group of rotated frost-heaved blocks, taken first looking south, then west. Rotation of the blocks was accompanied by repeated failure in step-wise shear. In the second (right) photo, Placentia Bay appears in the background. This bay was the likely end point for glacial ice moving off the pictured foreground highlands in the late stages of the Younger Dryas cold interval.
A second set of photos taken near the location pictured just previously shows another group of frost-heaved blocks apparently rotated and sheared by ice moving toward the coast. The first (left) picture shows a north-looking view. Many large and fragile frost-heaved bedrock features in the immediate area imply that frost heave was occurring here at a time post-dating any significant local movement of glacial ice via basal sliding. The foliation in the bedrock underlying the illustrated area dips vertically.
The three blocks seen in the right foreground of the above photo appear to be pieces of a single block that sheared into sections following frost heave and horizontally-directed ice loading. The higher a joint block becomes lifted in a subglacial frost heave event, the more shear strain it will experience as overlying glacial ice deforms in creep directed tangential to the ground. Note the inconsistencies in frost-heaved block rotation visible in the photo. Blocks in the background are either not rotated or rotated oppositely to the block shown in the foreground. The area where this photo was taken is characterized by indications of inconsistent ice creep direction. Either creep patterns were time-varying, as were the frost-heave episodes, or else ice emerging from the bedrock was creating a complex ground-level pattern of local creep velocities. An analysis of this site (discussion pending) suggests that the second explanation is more likely.
The photo above shows the rotated remnants of a large frost-heaved block that appears to have failed repeatedly in shear under horizontal ice loading.
Four views of a large frost-heaved bedrock feature are shown above. The central row of blocks is presumably the remnant of a single block, raised by frost heave and then subjected to faulting via shear loading. One or more sections of the frost-heaved central block appear to have been ejected completely and removed from the site by ice flow (note also the missing block in the side wall visible on the third {bottom left} photo), leaving room for the remaining blocks to settle back into the underlying cavity without jamming. Alternatively, the entire layout could have been widened by ice pressure. The greater rotation of the blocks seen at the upper left in the second {top right} photo might have resulted from these blocks reaching further upward into overlying moving ice during a subglacial frost heave event.
The systematic rotation and shearing of the frost-heaved blocks seen in the center foreground of the above photo suggests tangential loading of the blocks by glacial ice advancing right to left (direction as seen in photo). This ice flow direction conforms with independent determinations of local ice flow direction. The general fragility and rough edges seen in the pictured group of blocks and in other nearby frost-heaved bedrock features implies that ice movement by basal sliding did not occur after frost heave affected the area. It appears likely that all of the frost-heave features seen in the entire field of ice-disrupted bedrock pictured above were generated beneath a cold-based glacier.
The block with the hammer resting on it shows evidence of horizontally-directed glacial ice loading and step-wise shear failure.
A question arises as to why it is common to observe frost-heaved blocks that have been rotated, apparently by ice moving horizontally, but not also uprooted and translated horizontally from their original point of origin. An explanation lies in considering the creep velocity profile of cold glacial ice near the ground. The frost-heaved blocks are "frozen in", and the ice at ground level does not move. The ice significantly above ground level moves and applies load to the tops of the frost-heaved blocks. The blocks, if not decapitated by the differential loading (examples of this are seen in the field, see below), respond by shifting down-ice at the top, while remaining rooted at the bottom. Once the blocks have rotated, an equilibrium is reached where rotation stops and any subsequent motion of a block would necessarily require pulling the block out of substrate. This uprooting process undoubtedly occurs to some extent in some instances, but the balance of forces could readily favor leaving the block in place while overhead ice, continuing to move in creep, deviates around it.
A question arises as to why it is common to observe frost-heaved blocks that have been rotated, apparently by ice moving horizontally, but not also uprooted and translated horizontally from their original point of origin. An explanation lies in considering the creep velocity profile of cold glacial ice near the ground. The frost-heaved blocks are "frozen in", and the ice at ground level does not move. The ice significantly above ground level moves and applies load to the tops of the frost-heaved blocks. The blocks, if not decapitated by the differential loading (examples of this are seen in the field, see below), respond by shifting down-ice at the top, while remaining rooted at the bottom. Once the blocks have rotated, an equilibrium is reached where rotation stops and any subsequent motion of a block would necessarily require pulling the block out of substrate. This uprooting process undoubtedly occurs to some extent in some instances, but the balance of forces could readily favor leaving the block in place while overhead ice, continuing to move in creep, deviates around it.
Two oppositely-directed views of a possible instance where differential horizontal ice loading fractured a frost-heaved joint block are shown above. The large block seen at the center of the first (left) frame above appears to have broken off from its base and rotated. An examination of the site indicated that the grouping of rocks shown above is a rotated frost-heaved bedrock occurrence, and that the surface of failure beneath the large block is an irregular surface, not a cross joint. While it is possible that the breakage of the large block was caused by Holocene freeze-thaw weathering, it is also reasonable to attribute the failure of the block to the same horizontal ice loading that caused rotation of nearby blocks. The broken block was the highest-extending block in the group and would have experienced the greatest torque from horizontal loading by glacial ice moving in creep.
The section of rock shown above has undergone joint widening and minor vertical frost heave on the right-hand side, whereas a greater amount of frost heave accompanied by rotation with step-wise shear has occurred on the left-hand side. The difference between the two sides can likely be attributed to the more pronounced vertical frost heave that affected the left-hand side (height difference between the two sides is more apparent in photo below). Two possibilities could account for the rotations of the left-hand blocks. 1) The left-hand blocks were displaced completely from substrate and eventually toppled into their present orientation. 2) The more elevated blocks on the left (direction as in above photo) experienced horizontal loading and were shifted by overlying moving glacial ice. A view of the arrangement from the side opposite to that shown above is provided below.
The second explanation given above (deflection by glacial ice) seems the more likely explanation based on the horizontal distance that the furthest-shifted block (right-most in the above photo) has been displaced.
The trail of displaced blocks seen in the above photo appears to be comprised of rock fragments sourced in a frost heave event that occurred in the disrupted bedrock visible at the center left edge of the field of view. The blocks were seemingly ejected from substrate by vertical frost heave action in a subglacial environment and then carried horizontally by creep motion in the overlying ice. The large amount of shift of the blocks suggests that they were lifted high enough to escape the low creep rates characteristic of ice motion close to the ground. Significant flow of ice from bedrock into an overlying glacier could account for both the ejection of the blocks from bedrock and their transport upward into faster-moving glacial ice. Note that "faster moving" in the present context could imply motion of the order of millimeters or centimeters per year.
A row of frost-heaved rock fragments, apparently rotated and shifted by glacial ice creep, is shown in the above photo. These thin joint blocks probably originated from the gravel-filled depression visible in bedrock just below and to the right of the right-most frost-heaved fragment. As described in the previous instance, these fragments were likely lifted upward into glacial ice moving via creep, transported, and finally deposited during departure of ice from the location.
The row of frost-heaved fragments shown above follows an eroded cross joint that can be seen diagonally crossing the field of view in the above photo. The fragments are not only rotated about a horizontal axis, but they have also been rotated about a vertical axis and are no longer precisely aligned with the foliation in the underlying bedrock. Both the horizontal and vertical rotations were likely forced by creep motion in overlying glacial ice. The rock in the foreground of the above photo is an erratic.
Moving glacial ice (ice moving via creep) appears to have redirected a chain of rotated frost-heaved blocks seen just right of the hammer in the above photo. The blocks to the left of the hammer remained rooted in bedrock and were rotated about a horizontal axis only. The blocks to the right were displaced from substrate and were rotated about both horizontal and vertical axes. Presumably, the right-hand blocks were brought into alignment with the creep velocity vector in the overlying ice.
If the above-pictured grouping of frost-heaved blocks were observed at an unglaciated location experiencing suspected past or present periglacial climate conditions, the feature might be construed as a frost-heaved mound, formed in a subaerial environment. It would be puzzling, though, as to why all of the blocks have been rotated or shifted in the same direction. The oriented collection of frost-heaved blocks is likely a remnant subglacial ice plume feature, where a cold-based glacier moving in creep top-right to bottom-left (as seen in photo) rotated some of the blocks and transported some others that had been ejected from bedrock and lifted into the ice stream.
A few more examples
All of the above instances of bedrock frost heave with rotation represent potential examples of subglacial frost heave where glacial creep has deflected the blocks from a vertical orientation.
Possible instances of glacial erosion by basal sliding?
The above photo shows a group of frost-heaved joint blocks that have been rotated, presumably by the action of glacial ice moving left to right (direction as seen in photo). The edges of the individual fragments appear to be rounded. Furthermore the entire group of blocks is somewhat streamlined. It appears possible that, after frost heave occurred, the blocks were overrun by warm-based glacial ice and eroded by basal sliding.
A closeup view of the frost-heaved rock formation seen at the upper left of the preceding photo is shown above. This fragile layered frost-heave feature may have been planed off by basal slip erosion. Abrasive erosion by moving ice would account for the smooth, relatively uniform profile of the top surface of the feature. The rotation of the blocks could have been caused directly by interaction with warm-based glacial ice, but it is considered more likely that cold ice caused the rotation and warm ice smoothed the result.
All of the above rotated frost-heaved bedrock features show evidence of rounding by erosion that took place after frost heave had occurred. It cannot be determined with certainty that the rounding was caused by basal slip glacial erosion. Runoff from glacial melting or other flooding could also round the features, particularly if the runoff carried rock debris. Nevertheless, the shape of the rounded features (particularly notable in the first {top, left} photo) suggests that abrasive erosion by warm-based glacial ice played a role in streamlining the features. Very few examples of this type of rounding of frost-heave features have been observed on the Avalon Peninsula. If the features were eroded by warm-based glacier flow, then a transition from cold-based glacial conditions (needed for subglacial frost heave) back to warm-based glacial conditions is indicated for some small local areas.
Part 2 of Bedrock frost heave with rotation continues to focus on level and low-sloping terrain, but presents examples of larger-scale features and features explicitly demonstrating ice flow.
Part 2 of Bedrock frost heave with rotation continues to focus on level and low-sloping terrain, but presents examples of larger-scale features and features explicitly demonstrating ice flow.
heartofavalonia.org Exploring Geologic History