Structure
Drumlins are a unique and distinct geologic landform that are typically elongated, spoon-shaped hills or low ridges (Aber, 2015). Drumlins in this region typically rise above the surrounding landscape by roughly 60 to 110 feet, but these values change drastically in different parts of the world. Classic drumlins, such as observed in this region, have a blunt nose (Stoss End) that points in the direction of which ice movement likely originated (up-ice), and possess a gentler slope that likely signifies the direction of ice advance (Lee slope/down-ice); in this case, the general trend is that the long axis is typically oriented at roughly S35E (Reese, 2013). Dimensions of these landforms in this region can vary in both length, width, and elevation. Lengths (which includes the front edge of the Stoss End range from 131 m to 434 m (Saha, 2010). As stated above, height ranges from approximately 60 to 110 feet from surrounding landscape. Note that the Saha data were somewhat subjective and were based on surficial features only. In this case, these data should therefore be considered as an average bulk measurement of identified drumlins based on identifiable surficial features and may not encapsulate the entirety of a given drumlin, as certain characteristics may simply not have been visible or measurable.
It should be noted that not all drumlins in the area contain classic structural characteristics such as an abrupt Stoss End followed by a gentle Lee Slope in the down-ice direction. This was observed over a mapped drumlin off of Interstate 89 in the town of Little Hope, Pennsylvania. See annotated landscape image and supported image below. |
Simplistic cross section of typical drumlin structure. Note steep angle and topographic variability are greatest on Stoss End. Cross section and diagrams (below) by C Miller.
Video taken while traveling east on Kimball Road. This video depits a series of realatively small drumlins in close proximity. *Video taken while in the passenger seat*. Video by C Miller.
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Annotated landscape (above) is an east-looking image taken from I-89. This drumlin differs from others in the area (visually), as its short axis is less pronounced, and the Stoss End is clearly visible. This is not seen in many drumlins in the area due to dense vegetation and lack of access to certain sites (private property). The image to the right shows an abrupt change in topography near at the Stoss End of the above annotated landscape image. This is was one of the best locations visited on during this trip to get a clear picture of classic drumlin structure. It is difficult to tell if bedrock is present in the photo, but if so this may support Accretion Theory described below. Photo by C Miller.
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Process and Time
Drumlin genesis is currently believed to coincide with one of three natural geomorphic processes: deposition, deformation, or erosion. All theory below is re-worded from Saha, 2010 version of respective theory. Depositional Sub-categories: Accretion Theory: drumlins developed by repeatative additions (accretions) of till around pre-existing accumulations of stratified drift or around a bedrock knoll. Frost Heave Theory: drumlins developed as a result of frost heave, where saturated subglacial material freezes and expands, thus pushing material upward to form a mound-like landform. This mound-like landform then acts as an obstacle, around which drift material accumulates to form a drumlin. Deformational Sub-categories: Till Squeeze Theory: suggests that drumlins cavities beneath an ice sheet are filled with underlaying till material and are squeezed into hollows and shaped into drumlins by subsequent ice advance. Mechanics for this theory state that a pressure differential developes and forces subglacial material to zones of lower pressure (e.g. cavities) and is subsequently slowly lost in the down-ice direction, forming the Lee Slope. Boulton’s Model of Subglacial Folding: suggests that drumlins form around a layer of higher competence, such as tightly folded drift material, hard rock, or more competent layers of till material. In this theory, the center obstruction (on the Stoss End) is called the core, and the remaining larger area that surrounds the core is known as a carapace. Erosional Sub-category: Meltwater Hypothesis: drumlins form simply as the resultof currents of floodwaters washing away till material from a localized dense source. This would suggest that zones of localized till material was shoved into position by glacial movement, and subesquently eroded by meltwater motion, and may account for the linear, down-ice Lee Slope. Each individual theory described suggests different times for respective genesis. For example, when compring depositional versus erosional genesis, exposure to weathering was likely much more rapid with the Meltwater Hypothesis. Similarly, Boulton's Model of Sublacial Folding would suggest much slower process, where bothe ice-advance and folding took place over a great deal of time prior to the development of drumlin structures today. It is because of this that determining internal drulim structure may be the key to understanding drumlin genesis as a whole. All of these theories have potential and may one day prove to be correct. It is important to note that not all theories for drumlin genesis are accounted for in this section, and many of them are combinations of all three categories listed above. There are over 1,000 documented theories of drumlin genesis (Saha, 2010). |
Diagram of Accretion Theory. Phases show ice-thrusted till material is blocked by bedrock knoll. Subsequent material is carried down-ice to form gentle Lee Slope.
Diagram of Frost Heave Theory. Phases show infilltration of water into the ground. Ground re-freezes and is forced upward by frost heave action. This creates an obstacle for down-ice movement to form Stoss End and Lee Slope.
Diagram of Till Squeeze Theory. Phase 1 shows underside of ice block containing air-filled cavities. Weight of overlaying ice creates a pressure differential and forces underlaying material up into cavities. As ice retreats, classic drumlin structures form.
Diagram of Boulton's Model of Subglacial Folding. Phases show crustal deformation of bedrock and till material. The core appears as tight folds and is more resistant than overriding ice. Ice sheet advance continues up and over core and forms Stoss End and Lee Slope.
Diagram of Meltwater Hypothesis. Phases show ice-pushed till material and accumulations to form Stoss End. Metlwater moves loose material in down-ice direction to form gentle Lee Slope.
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Related Glacial Landforms
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East-looking image of delicate sand ridges along northern tip of Presque Isle State Park (foreground). Northwestern Glaciated Allegheny Plateau in background. Photo by C Miller.
North-looking image of low sand ridge. Nearby groyne (not see in image) protect against erosion and aid in beach stabilization. Natural geomorphic processes are readily seen at this location, as wave and wind action are consistant in direction (westerlies are commonplace and variable in magnitude). Additionally, sand ridges in this vicinity are the first to see land fall of storm fronts off the lake, further adding to erosion problems in the area. Photo by C Miller.
Schematic of typical groyne structure and function. These structures were observed in several places along the shorline of Presque Isle.
East-looking image of erosion on northern beach of Presque Isle. Note seawalls in right-center of image. Photo by C Miller.
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Webpage presentation for ES 546 Field Geomorphology, Emporia State University 2016. By Corey Miller, Graduate Student.