LIBERTY GROVE

At the Liberty Grove Drumlin Field there are spectacular examples of drumlins, which are landforms shaped by the movement of glacial ice.  As the Green Bay Lobe of the Laurentide Ice Sheet retreated from the Door Peninsula region, melt waters from this glacier deposited the sediments that compose the drumlins.  The drumlins were formed into their characteristic teardrop shapes as the glacier retreated back into Canada.  Here we will closely examine these unique glacial landforms.  You will be able to see the characteristic teardrop shape and be able to observe the unconsolidated sediments that compose these drumlins.  Explanations will be given about their origin and formation.                                  

The Liberty Grove Drumlin field is a small field, containing small but distinct drumlins. Two of the prominent drumlins are located at SE 1/4 NW 1/4 sec. 35, T. 32 N., R. 28 E., & SE 1/4 SW 1/4 sec. 35, T. 32 N., R. 28 E, see topographic map. Many of the drumlins here are only 3-6 m (10-20 ft) high, although a few larger ones soar 9-12 m (30-40 ft)   The drumlins are oriented in a slightly southeasterly direction, having steeper north-facing (up-ice) slopes.  The orientation of the long axes of these drumlins averages about  S. 15 deg. E.  The Liberty Grove drumlins were first mapped and described by Kowalke and are included in the glacial map of the Door Peninsula by Thwaites and Bertrand (1957). 

The till in the area is Liberty Grove Till.  Liberty Grove till is a strongly calcareous brown (7.5YR 5/4-10YR 5/3) stony loam till in which the matrix (less than 2 mm) contains 47% sand, 39% silt, and 14% clay.  The clay mineral assemblage is 16% expandables, 67% illite, and 17% kaolinite plus chlorite. (Schneider, 1993)

As described in the soil survey of Door County (Link, et al., 1978), the drumlins are made up of Omena sandy loam.  The Omena series consists of well drained, gently sloping to moderately steep soils on glacial till plains and morainal ridges.  The native vegetation is sugar maple, red oak, and American beech.  Available water capacity and permeability are moderate.  These soils are seasonally saturated at a depth of more than 6 feet.  The depth of the root zone is more than 6 feet.  Natural fertility and the organic-matter content of the surface layer are low.  Runoff is medium.  The hazard of erosion is severe, and it is the main limitation of this soil.

Drumlin Landscapes and Origins:

Drumlins have been described as resembling “inverted spoons” (Martini, 2001).  They have a streamlined shape, with the more blunt end toward the direction of ice flow.  They can be simple or composite, and their sizes usually vary from 1-2 km in length, 0.4-0.6 km in width, and 15-30 m in height.  Drumlins may contain a variety of materials, though many are composed of till.  Drumlins can contain rock cores, stratified drift, or faulted and folded layers.  Rock-cored and sediment-cored drumlins have the same shape and occur in the same drumlin fields. 

Drumlins are best described as oval, streamlined, teardrop-shaped hills with blunt, rounded heads (up-ice) and long, pointed tails (down-ice), usually with a long, straight axis.  Slim, narrow, more elongate drumlins may be the result of higher glacial speeds.  The angle of the drumlin may, however, be influenced by the resistance of the sediment to deformation.  Higher concentrations of cohesive clay may lead to higher, more prominent outlines, but higher concentrations of sand, which is less cohesive, creates more unpretentious profiles.  Drumlins form in groups called swarms, near the terminus of retreating continental glaciers.  They rarely occur in isolation.  They often occur behind moraines and are aligned parallel to the ice-flow direction.  Many glaciated areas do not contain drumlins.

The exact mechanism for the formation of drumlins is subject to debate.  No single hypothesis explains every occurrence, or every feature.  It is likely that drumlins do not form in a single way.  Drumlin formation hypotheses are divided into three groups; erosional, depositional, and melt water  (Martini, 2001). Erosional hypotheses state that drumlins form by glacial erosion either of bedrock (rock- cored drumlins) or of pre-existing stratified drift.  Depositional hypotheses state that drumlins form by sediment deposition either directly by glaciers, or by melt water under the glaciers (Martini, 2001).  The melt water hypothesis states that drumlins form when catastrophic meltwater floods flow underneath temperate glaciers.

Click here for a topographic map of this drumlin field and additional photos.