Glacial Striations

In glaciated regions the rock surfaces will show signs of ice movement over them providing that the surface has not been eroded in the time since glaciation. Signs of movement could be channels, grooves or striations depending on the size of the sediment contained in the glacier. The glacier picks up sediment on its travels by abrasion or by plucking. The sediment is under great pressure from above by the glaciers weight and actually scratches by grinding against the bedrock underneath during glacial movement.

Striations are usually multiple, straight parallel lines which represent the movement of the sediment loaded base of the glacier over. Striae can range from microscopic size on a seemingly polished rock to obvious markings millimeters deep and nearly a meter in length.

The amount of polish depends on the fineness of the abrading material. Fine- grained uniform rocks make and contain the best surfaces. Limestone is easy to form striations on, but it is also easily weathered. A hard rock like quartzite can preserve striations well, but it is difficult to create the striations on. Excellent places to check for striae preservation are quartz veins or fine-grained aplite dikes.

The forms of striae can vary greatly. Variances can be caused by a change in topography, bedrock type, pressure on the debris making the inscription and the orientation of the debris to the bedrock. Most striae are blunt and deep at one end and then taper to a point. These are known as nail-head striations.

Striae form in groups. A small area of bedrock can contain many striae that intersect or crosscut. Multiple striae can be an indicator of the glacier moving in different directions or different periods of ice advances. Weathering between the different striations can indicate different periods of advancements. Glacial movement prediction is difficult to do based solely upon striations, but it has worked in many cases in the past.

The above image is taken from Chamberlin, T. C., 1888, The Rock-Scoring of the Great Ice Invasions; in Powell, J. W., United States Geological Survey Seventh Annual Report, 155-248.

Kirk Heim 2/10/97