In polar regions and in the valleys of high mountains, great masses of ice slowly flow downhill, like cold molasses. These masses of moving ice are called 'glaciers'.
Where, each year, more snow falls in winter than can melt in summer, thick masses accumulate over time.
As the snow thickens, the deeper parts compact and turn into solid ice.
Eventually, as the weight increases on the deeper ice, it begins to deform, slowly flowing like a very 'thick' liquid.
As the ice flows, it presses down on and detaches chunks from the underlying bedrock. These rock fragments are then carried along by the moving ice.
During summer, the glacier thins somewhat and melts back at its lower end, from (4) to (5). The deep ice keeps flowing, eroding bedrock and transporting rock fragment. At the front of the glacier, streams carry away the melt water and fine sediment (mud, sand and gravel) released from the glacier as the ice melts.
In winter, the glacier expands again, perhaps to its former position (4). The ice continues to erode the bedrock, tearing out chunks of rock of all sizes, from huge boulders down to fine sand and mud. This material is continually carried and shoved to the front of the glacier where it forms a great rubble heap.
Summer again and the ice melts back to (5). Bedrock erodes, rock is transported, and the pile of rubble at the front of the glacier grows. The rubble is a mix of all size particles. It is unsorted. However, the rubble loses some of its finer material as melt water washes mud and sand away and spreads it out as layers in front of the glacier.
As the years go by, the glacial front fluctuates between (4) and (5). The rubble heap grows; the layers of sediment multiply, burying the landscape. The shape of the bedrock surface under the glacier becomes greatly altered as it is worn down. Rock fragments dragged across the bedrock scratch and dig grooves into it.
More time passes. The glacier expands and contracts. The ice itself, at all times, continues to flow towards the glacial front. Erosion, transportation and deposition of rock debris never stop. The material deposited from melting ice is an unlayered, unsorted mix. The melt water streams deposit layers of fairly uniform size (sorted) particles.
Eventually, the balance is upset. Snowfall decreases and/or the summers warm up. The glacier shrinks back to (6). The pile of rubble is freed from the ice.
The ice thins to the point where it stops flowing. The glacier is "dead". The remaining ice continues to melt, lowering the rock fragments to the ground. The great pile of rubble is partly buried by sediment deposited by the melt water streams.
The ice is gone. The glaciated landscape is exposed. Boulders are scattered over the ground that as covered by ice. Scratched and grooved bedrock is at or near the surface/ Where the glacial front fluctuated back and forth for a long time. Rubble forms a belt of hills. The sorted, layered melt water deposits form a gently sloping plain.
David J. Leveson