def lookup_clutter_geotype(clutter_lookup, population_density):
    """
    Return geotype based on population density

    Parameters
    ----------
    clutter_lookup : list
        A list of tuples sorted by population_density_upper_bound ascending
        (population_density_upper_bound, geotype).
    population_density : float
        The current population density requiring the lookup.

    """
    highest_popd, highest_geotype = clutter_lookup[2]
    middle_popd, middle_geotype = clutter_lookup[1]
    lowest_popd, lowest_geotype = clutter_lookup[0]

    if population_density < middle_popd:
        return lowest_geotype

    elif population_density > highest_popd:
        return highest_geotype

    else:
        return middle_geotype