Glacial deformation

James S. Aber

Table of Contents
Glaciotectonism Glaciotectonic structures
Glaciotectonic landforms Model for glaciotectonism
References Glaciotectonic bibliography


Glaciers erode, transport, and deposit rocks and sediments. This knowledge has been the foundation of glacial geology and geomorphology for more than 150 years. The realization that glaciers could also deform rocks and sediments came more slowly. The glaciotectonic origin of certain now classic sites was recognized by the late 1800s: Martha's Vineyard, Massachusetts; Rügen, Germany; and
Mřns Klint, Denmark. Glaciotectonism has become widely recognized as a fundamental aspect of glaciation in recent years (Aber et al. 1989, Aber and Ber 2007).

Long profile of the Rügen cliff exposure at Jasmund National Park, northeastern Germany. Multiple Cretaceous chalk bodies are thrust up from below sea level and deformed along with Quaternary strata.
Close-up view of folded chalk mass exposed in the Rügen cliff. Note people on beach for scale.
Two large thrust faults to left and right within the Rügen cliff section, as seen from a passing ferry.

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. Photos © J.S. Aber.

Kite aerial photographs from Martha's Vineyard, Massachusetts.

The study of glaciotectonics has emerged as a significant subdiscipline within glacial geology and geomorphology during the last 30 years. It is now apparent that glaciotectonic structures and landforms are common and widely present in regions of former glaciation. A variety of glacial landforms are now attributed either wholly or partly to glaciotectonic genesis. Hence, ice-pushed landforms and structures must now be included with depositional and erosional features as primary field evidences for past glaciation.

Some uncertainty surrounds the meaning of the term glaciotectonic, because various deformed structures are common both in glacier ice and in glacial deposits. Glaciotectonism may be defined as, structural deformation of sediment or bedrock as a direct result of glacier ice movement or loading (Aber et al. 1989).

The definition followed here is fairly simple and not overly restrictive, but it does exclude certain kinds of deformation related to glaciation. Deformations caused by drifting icebergs or freezing/thawing of dead ice (including permafrost) are not considered glaciotectonic. Likewise deformed structures within glacier ice are excluded. Glaciotectonic structures do include all deformations created in rock or sediment of the Earth's crust as a consequence of glacier loading, dragging, or pushing.

Glaciotectonic structures

Glaciotectonic structures range in size from microscopic to continental. The depth of structural disturbance is limited to about 200 m (Kupsch 1962), with the exception of lithospheric depression/rebound, which may penetrate 100s of km deep. Deformed materials range from hard, crystalline rocks, to poorly consolidated sedimentary strata, to loose sediments. Deformation occurs in both frozen and thawed material under either low or high confining pressures. Both brittle and ductile deformations of all kinds are present in glaciotectonic settings: faults, folds, fractures, intrusions, etc. Glaciotectonic structures may be created under thick ice, beneath thin ice, or in front of glaciers during advancing, maximal or recessional phases of glaciation.

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. Photos © 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).
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.

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. 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. Glacial loading and unloading of basement structures may lead to fault movements that affect surface landscapes (Sanderson and Jřrgensen 2015).

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 (Larsen et al. 2008). 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 lies just beyond the maximum limit of Illinoian glaciation. This region 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

Glaciotectonic landforms are the morphologic expressions of subsurface structural deformations brought about by glaciation. Such landforms range from conspicuous ice-shoved hills, to smoothed plains, to anomalous depressions (Aber et al. 1989). These landforms may display their original glaciotectonic morphology, where little modified by later events. More commonly, however, subsequent glacial or nonglacial erosion or deposition has altered the initial landform, in some cases obliterating any morphologic expression of the ice-pushed structures. In all cases, some knowledge of subsurface stratigraphy and structure is invaluable for properly interpreting the landforms.

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

These forms represent ideal types within a continuous spectrum of glaciotectonic phenomena. Intermediate, transitional or mixed forms exist between these ideal types and are in fact rather common. The materials of which these landforms are constructed may be classified in three groups: (1) pre-Quaternary strata, that are usually consolidated to some degree, (2) pre-existing Quaternary strata, both glacial and nonglacial, and (3) penecontemporaneous glacial sediment, that was deposited and deformed during the same glaciation. Most glaciotectonic landforms contain all three types of material in varying proportions.

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. 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.

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).

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. 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.

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. Photos © 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.
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.

Kite aerial photographs from the Limfjord region, Denmark.

Brandon Hills are a push moraine located on the Manitoba Escarpment of southwestern Manitoba. The hills contain a core of deformed stratified drift with a discordant cover of till. The internal structure consists of fault blocks or slabs of stratified drift varying from sand to cobble gravel; no bedrock or preglacial strata are present in Brandon Hills. Brandon Hills occupy a rectangular area roughly 10 km east-west by 4 km north-south and reaching an elevation exceeding 590 m. Composite-ridges covered by a thin veneer of till resemble a giant fishhook in overall plan. A conspicuous esker resting on till loops over the eastern end of the hills, and low kames and kettles are located to the southwest.

View northward toward the distal (downglacier) side of Brandon Hills, southwestern Manitoba. Brandon Hills are a push moraine composed entirely of deformed glacial sediments. Lake in foreground occupies a kettle hole south of Brandon Hills.
Steeply tilted and faulted glacial sand and gravel within Brandon Hills. The whole interior of Brandon Hills consists of such dislocated stratified sediments, which were deformed when an ice lobe advanced into its own outwash gravels. Scale pole marked in feet.
Discordant till covers the ice-shoved ridges of Brandon Hills. The till is sandy and contains rounded cobbles that were reworked from deformed stratified sediment of the underlying ridges. Scale pole marked in feet.

Many glaciated hills have the internal structure of ice-shoved ridges, but lack a hill-hole relationship or the typical morphology of composite ridges. A distinctive hill of this type is the cupola hill. Cupola hills have an internal structure similar to composite-ridges, but their external morphology was substantially modified by overriding ice. Cupola hills have three basic attributes.

  1. Interior structure: deformed glacial and interglacial strata with or without detached floes of older bedrock.

  2. External form: long, even hill slopes with dome or drumlin morphology, varying from nearly circular to elongated ovals in plan, 1-15 km long, 20 m to >100 m high.

  3. Discordant till: overridden by ice which truncated deformed structures and deposited a basal till cover over hill.

Cupola hills represent the combined effects of ice-shoving and subglacial erosion, deposition and molding of the ice-pushed hill. Where subglacial modification is slight, a subdued composite-ridge morphology may be preserved. With more modification, a rounded, smoothed cupola hill results. Still greater glacial erosion may create streamlined, drumlin-shaped hills, and eventually all trace of the ice-shoved hill may be removed by prolonged erosion. The typical characteristics of cupola hills are demonstrated on the island of Møn, southeastern Denmark (Aber et al. 1989).

Hvideklint cliff section on the island of Møn, southeastern Denmark. Large masses of Cretaceous chalk are thrust and deformed along with older glacial sediments. The dislocated materials are capped by discordant till (brown) visible at the cliff top. Cliff is about 20 m high.
Pastoral scene inland from Hvideklint. Gently rolling hills give little morphologic indication of the strong glaciotectonic disturbances beneath this cupola hill. Photos © J.S. Aber.

Model for ice-shoved hills

A model for glaciotectonism consists of two stages: (1) proglacial thrusting of ice-shoved hills followed by (2) subglacial modification of overridden hills. Initial proglacial thrusting takes place along a décollement that may be controlled by any of several features: lower boundary of permafrost, lithologic or stratigraphic boundary, position of confined aquifer, etc. Subglacial melt water may either erode tunnel valleys or deposit eskers, while proglacial melt water may cut spillways across the ice-pushed ridge and deposit outwash sediment on the distal side of the hill.

Debris flows, small fans, ablation till and other surficial sediment may accumulate at the ice margin, while basal till may build up under the glacier. Such sediments often become deformed as ice movement proceeds. With continued glacier advance, the hill may be overridden, further deformed, eroded, covered with till, and/or molded into a cupola-hill or drumlin. All traces of the ice-shoved hill may be eventually removed by prolonged erosion.

Glossary or references.

Return to Glacial geomorphology (2020).
All images and text © J.S. Aber.