Computer generated terrain usually begins with a base of random noise, or perhaps a fractal. These look nice in small sections, but are unconvincing for a continent: noise has none of the major features it should, like mountain ranges, and fractals are nothing but feature -- they're too regular. Here's a method for generating continent-level trerrain using distortion fields.
To many, the most perfect shape is the circle -- emblematic of unity -- or the square -- for combining with itself in a tile. But in the city Pherasa, the people have long held the spiral above all other forms. For them it is the basis of all aesthetics, and used in architecture, art, religion and war.
Here I present a surprisingly easy way to fill valleys in a rasterized elevation map, for the purpose of creating realistic rivers quickly.