  Consider the Fulton-MacPherson configuration space of $n$ points on $\P^1$,
which is isomorphic to a certain moduli space of stable maps to $\P^1$. We
compute the cone of effective ${\mathfrak S}_n$-invariant divisors on this
space. This yields a geometric interpretation of known asymptotic formulas for
the number of integral points of bounded height on compactifications of $\SL_2$
in the space of binary forms of degree $n\ge 3$.
