  We present an effective theory describing the low-energy properties of an
interacting 2D electron gas at large non-integer filling factors $\nu\gg 1$.
Assuming that the interaction is sufficiently weak, $r_s < 1$, we integrate out
all the fast degrees of freedom, and derive the effective Hamiltonian acting in
the Fock space of the partially filled Landau level only. This theory enables
us to find two energy scales controlling the electron dynamics at energies less
than $\hbar\omega_c$. The first energy scale, $(\hbar\omega_c/\nu)\ln\left(\nu
r_s\right)$, appears in the one electron spectral density as the width of a
pseudogap. The second scale, $r_s\hbar\omega_c$, is parametrically larger; it
characterizes the exchange-enhanced spin splitting and the thermodynamic
density of states.
