Mr Marsden, without pretending to be a natural historian, gives us a very good picture of the water-worn surface of Sumatra.  History of Sumatra, page 20.

“Along the western coast of the island, the low country, or space of land which extends from the sea shore to the foot of the mountains, is intersected and rendered uneven to a surprising degree, by swamps, whose irregular and winding course may in some places be traced in a continual chain for many miles till they discharge themselves either into the sea, or some neighbouring lake, or the fens that are so commonly found near the banks of the larger rivers, and receive their over flowings in the rainy monsoons.  The spots of land, which these swamps incompass, become so many islands and peninsulas, sometimes flat at the top, and often mere ridges; having in some places, a gentle declivity, and in others descending almost perpendicularly to the depth of an hundred feet.  In few parts of the country of Bencoolen or of the northern districts adjacent to it, could a tolerable level space of four hundred yards be marked out:  about Soogey-lamo in particular, there is not a plain to be met with of the fourth part of that extent.  I have often from an elevated situation, where a wider range was subjected to the eye, surveyed with admiration the uncommon face which nature assumes, and made enquiries and attended to conjectures on the causes of those inequalities.  Some chose to attribute them to the successive concussions of earthquakes, through a course of centuries.  But they do not seem to be the effect of such a cause.  There are no abrupt fissures; the hollows and swellings are for the most part smooth and regularly sloping, so as to exhibit not unfrequently, the appearance of an amphitheatre, and they are clothed with verdure from the summit to the edge of the swamp.  From this latter circumstance, it is also evident that they are not, as others suppose, occasioned by the falls of heavy rains that deluge the country for one half of the year.  The most summary way for accounting for this extraordinary unevenness of the surface were to conclude, that in the original construction of our globe, Sumatra was thus formed by the same hand which spread out the sandy plains of Arabia, and raised up the Alps and Andes beyond the regions of the clouds.”  Our author then, after reprobating this idea, endeavours to explain the appearance here examined by the constant though imperceptible operation of springs.  The present purpose is not so much concerning the explanation of those appearances, as to inquire if these be the general appearances of things over all the surface of the earth.

The general appearance here is that of land washed away upon the surface by water, which has every where left the marks of its operation in the shape of the ground.  As for any particulars in the shape of this water-worn surface, this can only be explained in knowing the nature of the soil and solid parts, and the circumstances of the operation in which they have been wasted.