Glaciotectonism

Aquinnah Cliff, Martha's Vineyard, Massachusetts. The 40-m-high cliff displays multi-colored upper Cretaceous and Tertiary strata that were upthrust along the edge of the Atlantic Coastal Plain during late Wisconsin glacier advance. Aquinnah Cliff is among the most famous glaciotectonic sites in the world; many thousands of tourists visit the cliff each year. Photo date 7/05; © by J.S. Aber.

	Photograph of fractured cobbles in gravel core of drumlin at Dollard, Saskatchewan. Brittle fracturing of these cobbles presumably took place at contact points in response to ice loading. Photo date 5/93; © by J.S. Aber.
	Chalk, till, and sand deformed into isoclinal folds, island of Møn, Denmark. Ductile (plastic) folding of materials took place by simple shear beneath the glacier. These material may have been deformed in either a frozen or thawed condition. Ice movement from right to left; shovel for scale (right). Photo date 5/79; © by J.S. Aber.
	Small diapirs of clayey sediment (gray) intruded upward into overlying sand (brown), island of Herdla, western Norway. Clay was mobilized as a result of ice loading on the unconsolidated and water-saturated sediment. This site is in the Herdla moraine of Younger Dryas age. Photo date 5/87; © by J.S. Aber.

Rapid glacier movement may take place over a deforming bed. Such a bed is characterized by high strain rates within sediment that is under high fluid pressure. Sediments deformed in this manner are penetratively sheared. The final product is mylonite with a strongly oriented fabric that resembles lodgement till. Deforming bed conditions may be recognized by stone pavements, which are widespread in the Great Lakes, Great Plains, and Hudson Lowlands regions.

In regions of hard bedrock, small faults and seismic zones are manifestations of glaciotectonism (Aber et al. 1995). These features are thought to be direct results of stress changes associated with glacial loading and unloading. Small faults are commonly developed in slates and other well-consolidated rocks of the Canadian Shield and northern Appalachian Mountains--see Fig. 5-3. Individual faults are usually small (cm-dm displacement), but the cumulative effects of many small faults can be considerable. Larger faults represent glacial reactivation of pre-existing structures.

Seismic zones indicate continued crustal adjustments to glacial unloading, particularly in zones of former glaciation such as northern New York, southeastern Canada, and northern Denmark. In addition, regions beyond the scope of glaciation may be affected by flexing of the lithosphere. For example, the New Madrid seismic zone in southeastern Missouri and adjacent states has a substantially increased rate of earthquakes during the Holocene as a consequence of removal of ice loading to the north (Grollimund and Zoback 2001).

Glaciotectonic landforms may be divided in two general categories on the basis of their morphostructural attributes.

• Ice-shoved hills: constructional hills, ridges, or moraines that rise above the general landscape as a result of structural uplift of deformed rock and/or sediment material. Includes hill-hole pairs, composite ridges, push moraines, and cupola hills.

  View over crest of Prophets Mts., North Dakota. Prophets Mts. is a large ice-shoved hill that rises nearly 100 m above the surroundings. The interior consists of deformed upper Cretaceous and lower Tertiary bedrock along with glacial sediments. Photo date 6/86; © by J.S. Aber.

• Glaciotectonic plains: subdued, streamlined, or smoothed plains. Underlying structures include flat rafts or megablocks, intrusions and diapirs, as well as buried ice-shoved hills and structural basins.

  	Glacial megablocks of deformed Cretaceous sandstone exposed along bluff of the Old Man River, near Tabor, southern Alberta. The source and distance of transportation for these megablocks are unknown. Note flat glacial plain on upland surface. Photo date 8/84; © by J.S. Aber.
  	Jumbled mass of crushed bedrock material in the core of a recumbent fold, near top of section along bluff of the Old Man River, near Tabor, southern Alberta. Photo date 8/84; © by J.S. Aber.

The hill-hole pair is the simplest and most instructive type of ice-shoved landform. It consists of an ice-scooped basin and related hill--see Fig. 5-4. Other kinds of ice-shoved hills are variations of this fundamental form. These hills were often misidentified as kames or bedrock outliers, depending on their internal composition. Bluemle and Clayton (1984, p. 284) described the hill-hole pair as, "a discrete hill of ice-thrust material, often slightly crumpled, situated a short distance downglacier from a depression of similar size and shape."

The depression is the source of material now in the hill, and ideally the volume of the depression should nearly equal the hill's volume. Depressions are now often the sites of bogs, lakes, or estuaries, and so their apparent sizes are often reduced by later sedimentation. The ice-shoved hills may also have been altered by later erosion or deposition, so an exact volume correspondence does not always exist between the hill and related hole. Hill-hole pairs are now widely recognized in many different settings--see Fig. 5-5.

Small hill-hole pair at Anamoose, North Dakota. Lake basin in foreground is the source depression for material shoved into the hill in the background. View in the downglacier direction. Anamoose is a classic locality for this type of landform in North America (Bluemle and Clayton 1984). Photo date 6/86; © by J.S. Aber.

The most typical and distinctive glaciotectonic landforms are ice-shoved ridges found in many glaciated plains. Prest (1983, p. 45) aptly described such ridges as, "a composite of great slices of up-thrust and commonly contorted sedimentary bedrock that is generally interlayered with and overlain by much glacial drift." Composite ridges typically display multiple, narrow ridges separated by steep-sided valleys--see Fig. 5-6. The ridges are the upturned ends of thrust blocks or the crests of folds. Maximum uplift of thrust blocks is may be >200 m, although in many cases no more than 100 m of uplift is present.

Large composite ridges (>100 m high) usually include a substantial volume of pre-Quaternary bedrock. Large composite ridges are topographically and structurally similar to such thrust and folded mountain belts as the Canadian Rockies or Swiss Alps that were formed by thin-skinned tectonics. The only real difference is size, ice-shoved ridges being one or two orders of magnitude smaller than true mountains (Aber et al. 1989).

Strongly folded sedimentary strata of Mt. Kidd, Kananaskis Mountains, Alberta. The style of deformation exhibited in the Canadian Rocky front range is identical to that of ice-shoved ridges; the only real differences are size of structures and duration of deformation. Photo date 8/85; © by J.S. Aber.

Small composite ridges may or may not include deformed bedrock; many, in fact, are composed largely of unconsolidated Quaternary strata. The term push moraine is commonly and loosely used to refer to ice-shoved ridges. Push moraines are a restricted subset of composite ridges that consist largely or wholly of glaciogenic strata. Composite ridges that contain appreciable nonglacial material should not be called push or end moraines.

The typical morphology and structures of composite ridges are displayed quite well in the Limfjord district of northwestern Denmark (Aber et al. 1989; Klint and Pedersen 1995; Pedersen 1996). Paleogene bedrock and drift were folded and thrust into composite ridges during late Weichselian glaciation. The bedrock, consisting of clayey diatomite interbedded with volcanic ash layers, was especially susceptible to ice-push deformation. Large, overturned, rootless folds of bedrock and drift were thrust up forming ridges. Ice-shoved hills display valley-and-ridge topography, in which maximum elevations are up to 100 m above the nearby floor of the Limfjord estuary (source basin).

	View across Limfjord estuary toward Hanklit, a cliff exposure on the island of Mors, northern Denmark. The cliff is about 60 m high and cuts across the end of a large ice-shoved hill, visible behind the cliff. Photo date 6/79; © by J.S. Aber.
	Hanklit reveals an overturned fold of Fur Fm. (Paleocene) bedrock, surrounded by glacial gravel, and resting on till. Note faulting of fold core (left) and extreme stretching of fold nose (right). Ice movement was from left to right. Photo date 6/79; © by J.S. Aber.
	Ice-shoved hill near Salgerhøj, inland from Hanklit, northern Denmark. Narrow valleys alternate with ridges. Maximum elevations are 100 m above the floor of the Limfjord estuary, visible in background (right). The Limfjord basin was presumably the source region for material now thrust into the ice-pushed ridges. Photo date 7/87; © by J.S. Aber.