One of the difficulties of creating landscapes in a realistic way is that real landscapes aren't created all at once. Instead, they evolve from earlier landscapes, which in turn evolved from even earlier landscapes, and so on back for billions of years. There's no good way to simulate this process in a reasonable amount of time, so we need to cheat slightly.
Rather than an infinite regress of older landscapes, I start with a simple 'proto-landscape', built with geometric primitives. This lets me control the broad outlines of the terrain, while leaving the details for the more physical processes to fill in later.
Some useful primitives which we can add together:
- Constant slope - if you want to pretend this is physically motivated, think of it as tectonic uplift on one side of the map
- Cone shapes - these can be islands or mountains, or if inverted, lakes or seas
- Rounded blobs - these make better hills, and can be scattered all around to make a noisy surface