Glacial Grooves And Striations
A surface exposed with glacial grooves in Ste-Marguerite, Quebec.
Glacial Grooves are formed through a process known as glacial abrasion where scratches are observed which cut into the bedrock (Goldthwait, 1978). These grooves or striations are further seen as multiple parallel grooves which represent the movement of sediment loaded base of the glacier. All grooves contain striation marks up to 1-2mm deep and they are gouged laterally by sharply curving (Goldthwait, 1978). They further have been said to have formed by the dragging of coarse gravel and boulders underneath the glacier which results in the cutting of the grooves. Finer sediments and particles also move at the base which causes the glacier to scratch the bedrock, thus forming grooves (Goldthwait, 1978). Researchers have also found that these glacial grooves were exposed during the Last Glacial Maximum. As a result, these glacial grooves and striations provide evidence to the flow of glacial ice and extent of weathering which may have occurred (Goldthwait, 1978).
This picture was taken in Churchill, Manitoba and depicts glacial grooves and striations, which are known as scratches from glaciers on the rock. In addition striations also show ice flow direction.
Scientists have found that the glacial grooves located towards the center of the ice stream were longer and maintained parallel conformity. Researcher Stokes (2000) illustrated that this is what would be expected from an ice stream in which the velocity increases in the onset zone and decreases towards the divergent end. Therefore Stokes (2000) maintained his thought that with increasing velocity will result in greater grooves and striations which are clearly seen in satellite images, however only at the initial stages. However, researcher Clark (1993) indicated that striations form when keels at the ice sheet drag through the soft sediment, resulting in grooves. If there is flat till sheet containing soft sediments, it can be said that the strength of the ice is much greater then that of the sediment (Stokes & Clark, 2003). In such an event, the heavy and bumpy ice at the base will groove out sediment as it passes downstream. If the keels, also known as ice bumps last long enough then they will plough through all of the softer sediment resulting in the formation of grooves and ridges (Stokes & Clark, 2003).
Glacial striations, Lac Blanchet, Canada.
Formation of Striations
In regions of glaciation signs of sediments and rocks moving within the ice is evident. The glacier picks up the sediment and as ice moves it either abrades the surface or can further result in plucking of sediment (Chamberlin, 1888). The ice is heavier and therefore puts more pressure on the sediment below which leaves scratches or striations on the bedrock below. This consequently results from the movement of ice. Striations are often seen as numerous straight lines which appear on bedrock as a result of movement of sediment within the glacier (Chamberlin, 1888). Striations can range from being microscopic in size to deep marks which are quite visible. Limestones, have been known to form striations easily, but are also weathered easily. On the other hand a rock like quartzite is difficult to create striations on, however when they do form they are highly preserved (Chamberlin, 1888). The different forms or types of striations can vary i.e. deep or light marks, visible or not so visible. This variation is due to the change in topography, pressure on the debris, and bedrock type to name a few.
When a rock shows scratches or lines on the surface as a result of glacial erosion, it is classified as having striae. Striae are formed when a glacier containing clasts embedded at the bottom of the ice passes directly over a rock, abrading the surface as it moves (Glasser, 2004). How these clasts carve the ice is categorized by three different types:
- Type 1: Through pressure, the embedded clast abrades the surface of the rock becoming wider and deeper as it moves down the rock. The striations cease only when the clast is released due to pressure or force on the clast (Iverson, 1991).
- Type 2: This type is classified by a slowly rotating clast which increasingly causes deep gouges until it reaches the midpoint, in which the clast rotates at a higher page until the clast has the same velocity of the ice, causing the striations to be less deep, and smaller in size. The deep gauges leading to the midpoint are a result of a large ploughing angle, however this angle decreases as the glacier passes over the rock (Iverson, 1991).
- Type 3: Formed when a clast initially comes into contact with the rock surface, and as the clast rotates down ice, it become displaced and the ploughing angle decreases. This causes the striations to decrease in size as the glacier continues to move (Iverson, 1991).
The formation of striae are commonly used to determine previous ice movement direction, since it is believed that the striae are parallel to direction of ice flow. Advance and retreat of a glacier may be shown by a cross cut pattern in the rock, since the pattern would develop from different flow directions (Glasser, 2004). The thickness of the striae may also determine the age of advancement, with thicker lines being older, and finer striations showing a younger age. Finer striae are assumed to be a younger age since it would be destroyed by thicker striae if abrasion occurred earlier (Glasser, 2004).
Morphology of Striations
Striae occur in a variety of sizes, depending on the clast that abraded the surface. These gouges can be from a few millimetres deep to several metres in length (Glasser, 2004). According to Iverson (1991), there are three types of striae that are categorized by their formation and morphology.
- Type 1: This type resembles deep gauges in the rock as the striae become wider and deeper as the glacier continues to advance (Iverson, 1991).
- Type 2: This type has a centre point in which the highest width and depth occurs, with thin, indistinct lines that widen and decrease leading to and away from the median (Iverson, 1991).
- Type 3: This type consists of deep gouges that are initially at its widest, and become narrower and less deep as the glacier moves down the rock (Iverson, 1991).
Despite these three classifications of striae, striations may show breaks or gaps in the lines. These may be due to a temporal loss of contact between the clast and the rock surface, possibly from a formed cavity or debris cushion (Glasser, 2004).
Striations help us to determine that glaciers contained a great amount of debris, including clasts that can abrade rock surfaces, that glaciers experience melting, since striations are formed from the moving ice, and that there is a good amount of pressure exerted onto the rock from the glacier, since the clasts only carve into the rock from pressure and torque keeping the clast in contact with the rock surface (Glasser, 2004).
Detection of Striations and Grooves in Satellite Imagery
Satellite imagery is very significant as it leads to the discovery of previously unseen glacial landforms, and furthermore those landforms which cannot be seen through aerial photography or field investigations. Recently, evidence of glacial grooves were found through ETM + imagery which allowed researchers to look more detail into what could have caused this formation as well as high resolution photographs of these landforms (Stokes & Clark, 2003).
This image illustrates the glacial grooves and striations seen through satellite imagery. In addition the direction of flow of ice is also seen