On the 29th of April, 1903, 40 million cubic yards of rock slid down from the face of Turtle Mountain and buried the town of Frank, Alberta. This catastrophic episode of deposition created a pile of debris that locally raised the lowland floor into a series of irregular hills and valleys.

The diagram on the left represents 5 adjacent localities whose elevations range from 0' to 220'. The top of each brown bar indicates the elevation of the 'initial land surface' on which deposition will take place.

During the first episode of deposition, each locality is buried by a layer of material just under 100' thick. The elevation of the whole area rises, but the relief and the locations of the hills and valleys remain the same.

During the second episode of deposition, each locality is buried by deposits of different thickness. Locality D receives no deposits, and the depth of the valley increases as the hills on each side rise.

After the third episode of deposition, the valley at D is largely filled in. Locality B, which used to be a hill, is converted to a valley as localities A and C rise on each side. Locality E has achieved the greatest elevation of all.

During the fourth episode of deposition, locality C receives no deposits. Locality B is raised level with C to form one broad valley. One side of the valley is bounded by the hill at A, the other by the slope that rises from C to D to E.

Clearly, differential deposition can radically alter the landscape, changing the local and overall relief, as well as the number, location, and size and shapes of hills and valleys.

David J. Leveson