Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/192

166 to the border of the blocks, extremely large folds (overfolds or overthrusts) are formed, but none in echelon; if it be oblique to the boundary, folds in echelon are formed which will be narrower and of less altitude the more parallel the direction of movement is to the margin of the blocks. With exact parallelism, there will be a gliding surface with lateral displacement. Finally, if the movement possesses a component which is directed away from the margin of the blocks, there will be either oblique or normal rifting, which then manifests itself in a rift valley. We can illustrate in a very satisfactory manner the relation of the

normal folds to the folds in echelon with a table-cloth, if we weight down that portion which will represent the immovable block and displace the other part relatively to it.

These general considerations show that folding and rifting are only different effects of one and the same process, namely, the displacement of the parts of a block relatively to one another, and that they pass continuously into each other through echeloned folds and lateral displacements. It is therefore right that we should now consider also the process of rifting.

The East African rift-valleys form the most beautiful example of such rifts. They belong to a great