"Monoliths" are triangles that are significantly larger than the average triangle (given a particular cellular automaton rule). The elementary rule 146 displays monoliths, given certain initial conditions. Typically, the signal that a monolith is present is a density that dips below 15%. Generate new initial conditions by clicking on the button with that name; you can then view the evolution of rule 146 under those initial conditions for 10000 steps. The density plot changes accordingly. It is interesting to see that most initial conditions do not, in fact, produce monoliths.