I've worked it out, I was using the wrong node for getting the high points from the fractal, so it was effectively doing nothing. Thanks go to Volker for a massive help there!
Ok, here's what the nodes do:
Canyon Fractal: This fractal determines the shape of the canyons, you can modify the settings to change the overall shape. A noteable setting is the colour contrast and colour roughness settings, these determine the slope of the canyon walls and the degree to which the canyon wall is broken up.
Luminance to scalar: This is the function I was missing, it takes the degree of whiteness of the greyscale fractal and maps it to a scalar between 0 and 1. if we were to multiply this scalar now the white bits would get whiter, and so will the dark bits. The only bits that would not get lighter would be those that are absolute 0, so we have to manipulate the values a bit.
X2: This node multiplies the scalar by 2, so the range is between 0 and 2.
-1: This node subtracts 1 from the scalar, so the range is now between -1 and 1. When we multiply now, the darker bits get darker, and the white bits get whiter.
X50: This node multiplies the scalar by 50. This means that if we were to take a section of the scalar (say between 0 and -20) it would be a vertical wall. You can play with this to change the sheerness of the canyon walls.
Clamp -20,0: This node clamps the scalar between the values of -20 and 0. Think of this as being a canyon 20 meters high with valleys at an altitude of -20. Changing this value changes the height of the walls and the range of fractal detail captured. If you increase the displacement magnitude and decrease the sample range, your canyon walls will be shearer.
Attached below is the new shader. Plug it in on the end of your terrain, (you can also swap the internal fractal for your terrain fractal to have canyons only in the lowlands).
If you have any problems post here, and i'll see what I can do.
Edit> Added a quick render for you.