What is the reason for a giant metamorphic boulder to be in the middle of a field filled with sedimentary rock? The large rock was probably dragged there by a glacier many hundreds of years ago, a phenomenon known as a glacial erratic. Glacial till is a kind of rock that is generally sedimentary, but is created in basically the same way as the boulder might appear in that field—that is, deposited by a melting glacier. Because of this, unusual types of rock may appear where they are not expected to be. In the case of the rocks in Rock Canyon, one may find sedimentary rock amongst the quartzite crystals. The sedimentary rock found there includes large clastic boulders as well as layers of fine clay and other types of rock.
A glacier is a large body of ice that moves much like a highly viscous river, flowing through the landscape and carving it out as it goes. Both glaciers and rivers are able to carry rocks large and small down the path they take and deposit them wherever it becomes impossible to carry any longer. In the case of glaciers it is most often when the glacial ice melts to a degree that it becomes impossible to keep large boulders (or smaller rocks) frozen in place, thus dropping the sediment where the glacier was.
Another way glaciers deposit new material is by pushing rocks and soils off to the side as the move downhill into a new area. These glacier-created structures are called moraines. A moraine is like a deposition of glacial till. The material that makes up a moraine is deposited there by glaciers as the exposed sides melted and dropped material that had been both picked up earlier and pushed to the side as the glacier moved along.
Glacial till shows no stratification (1). That means there are no visible layers in the rock. When normal sedimentary rock is formed, there are visible layers. The layers are usually formed by the continuous deposition by waves or landslide over a long period of time, each layer representing a separate wave or landslide, for example. Glacial till is deposited basically all at once, so there aren’t any real layers. The material is very scattered and unorganized.
When glacial till becomes rock, it is known as tillite (2). The term for this process is lithification. This usually occurs over very long periods of time, when all the water is able to evaporate or seep out, and a certain amount of pressure is exerted, making what was once a pile of separate rocks, clays, and grains of sand is able to become one solid mass or rock.
- Encyclopaedia Britannica: Till. http://www.britannica.com/eb/topic-595804/till