The gnarl pattern is created by the interplay of a high-weight waves transform with a low-weight linear one, often with added blur. Always available using the original waves variation, this only became popularised with the advent of the parameterised waves2 - waves is actually one of the few 'dependent' variations in that its parameters are dependent on its position: at the origin, it nullifies the entire fractal. A very versatile style, as the gallery shows.
An alternative expression of the pattern may be obtained using the PDJ variation.