A great escarpment at a rift margin can correspond directly to a major
water divide but at many margins, including in Madagascar, the
escarpment often appears as a steep knickzone on rivers that have their
main water divides in the interior of the high plateau. We hypothesize
that this variability in morphology is a reflection of the frequency and
size of drainage area captured from the high plateau over the
escarpment. To test this hypothesis, we document morphological features
and weathering conditions from river sediment of Madagascar. We propose
that the existence of a weathered weak surface layer of crystalline
bedrock encourages large river captures from the upper plateau, leading
to a dominance of knickpoint type rivers. We demonstrate that this is
feasible, using 2-D landscape evolution models and show that an
easily-eroded surface layer is prone to fast divide migration through
frequent river capture and reversal. A positive scaling relationship
between captured area and escarpment retreat rate is found from models.
We demonstrate that this scaling is also observed in the great
escarpments of Madagascar and India.