It was a Swiss geologist named Louis Agassiz who first proved that glaciers moved.  Faced with the longstanding hypothesis that they did not move, he devised a simple test as shown in the diagram.  He not only showed that the glacier had moved appreciably within a year, he also showed that it did so by plastically deforming itself or PLASTIC FLOW.  The shape of the ice had changed much as a ball of silly putty flattens itself if left on a table.

The two photographs below show a model that explains the motions of the stakes.  The photo at left shows a mass of a plastic material called "silicon putty" shaped into a roughly rectangular block to model a glacier, and placed in a PVC "valley".  The "valley" slants at about 40 toward the bottom of the pictures.  Red lines on the sides of the "valley" mark the approximate terminus of the "glacier" and the beginning position of the toothpick "stakes".  Note that these are essentially vertical.  The photo at right shows the model some time later.  Plastic flow has moved the terminus of the "glacier" down-valley, the "stakes" are now in an arcuate arrangement (as is the terminus), and they have tilted substantially down-valley.  Clearly the center of the "glacier" has moved faster than the edges.  The reason is that there's less friction at the center (Ff is lower there) so Fd is even greater than Ff here and flow easier.  The stakes tilt because Ff is lower there than at the base and so flow is also easier at the top.  If the "stakes" were flexible and the "glacier" thicker they would bend as seen in the earlier diagrams instead of simply leaning forward.


Alpine glaciers such as the one Agassiz studied can also move by sliding along their bases in a process called BASAL SLIP.  even though we know from later experience with real glaciers that it happens, neither Agassiz's real glacier (to his surprise) nor the silly putty model shows evidence of basal slip.  (What would the evidence be?). 

The diagram below shows how (and why) an alpine glacier moves.  The Fp gravitational component that we studied in mass wasting creates the force that causes the motion.  Ice added at the top increases the strength of Fp and makes the ice move downhill.  The motion is accomplished by a combination of plastic flow and basal slip.