Besides their peculiar form, the _roches moutonnees_ present all the characteristic features of glacier-action in their polished surfaces accompanied with the straight lines, grooves, and furrows above described. But there are two circumstances connected with these knolls deserving special notice. They frequently present the glacial marks only on one side, while the opposite side has all the irregularities and roughness of a hill-slope not acted upon by ice. It is evident that the polished side was the one turned towards the advancing glacier, the side against which the ice pressed in its onward movement,--while it passed over the other side, the lee side as we may call it, without coming in immediate contact with it, bridging the depression, and touching bottom again a little farther on. As an additional evidence of this fact, we frequently find on the lee side of such knolls accumulations of the loose materials which the glacier carries with it. It is only, however, when the knolls are quite high, and abrupt enough to allow any rigid substance to bridge over the space in its descent from the summit to the surface below, that we find these conditions; when the knolls are low and slope gently downward in every direction, they present the characteristic glacier-surfaces equally on all sides. This circumstance should be borne in mind by all who investigate the traces of glacier-action; for this inequality in the surfaces presented by the opposite sides of any obstacle in the path of the ice is often an important means of determining the direction of its motion.