arlisgion , 'the place of reeds ' ( p. 153 ) above the mouths of sirion , survived in lisgardh 'the land of reeds at the mouths of sirion ' in the later tuor ( p. 34 ) ; and the feature that the great river passed underground for a part of its course goes back to the earliest period , as does that of the meres of twilight , aelin-uial ( 'the pools of twilight ' , p. 195 ) .