Large boulders on top of the glaciers were deposited when ice melted.
Boulders can be defined as rocks that are too big for a person to pick up and move. They are often found in "boulder fields" or quite isolated, where they are defined as "erratics." The very large boulders are clearly "out of place" with the surrounding geology of the location and seem to have been dropped there randomly, which is in fact what occurred in the various processes involved in the growth, development and final melting of glaciers.
Boulder Formation
As glaciers grow and begin to move down valleys from the higher, colder regions they carry a load of soil, gravel and rock at the sides, underneath and at the front of the glacier. These act essentially like rough grained sandpaper, scouring the surrounding rocks and breaking off larger pieces which, though large, generally get ground up over time. As a glacier erodes the side walls of a mountain and continually undercuts the walls the rock formations up above are left hanging in the air. These formations eventually break off from the face of the mountain and land on the top of the glacier and begin what can be a journey of many hundreds of miles.
Melting
The boulders are carried along with the glacier until it starts to melt and since they are on the top, when the ice is finally gone they are left in an otherwise relatively smooth area. Since they have dropped from a mountain side and have not been affected much by glacial abrasion they can have straight sides and a fractured outline, like the Airdrie erratic near Calgary, Alberta. Huge boulder fields are not uncommon, since they basically stay together after falling off the mountain side.
Ice Rafting
A secondary transportation effect can take place if the boulders become part of a large iceberg floating in an ancient glacial lake. These rocks will float with the iceberg until it melts and then drop to the bottom of the lake. Depending on the timing they can be completely covered with the finer clays and gravels that continue to be deposited. If the icebergs are still intact when the ice dam that is holding back the glacial lake breaks they will head out on the flood crest, finally coming to rest when the water recedes and the block of ice melts.
Trapped Boulders
Some large boulders become trapped within the ice itself, especially if deposited in mountainous areas where more snow is falling and more ice building. These boulders can gravitate to the sides and the bottom and in the process become much more rounded and worn. They are also the cause of the large grooves in rock that can mark the path of glacial flow.
Scientific Uses of Boulders
Once scientists analyze an erratic boulder they can accurately determine the rock formation from which it came. Therefore the location of boulder fields and erratics can provide data on the extent of glacial ice coverage over an area. Similarly, rocks transported by ice rafting can indicate the extent of the major flood area when the ancient glacial lakes broke their dams.
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