now above the portals of his cavernous hall was a steep slope falling to the river , and there grew mighty beeches ; and one there was that was named hirilorn , the queen of trees , for she was very mighty , and so deeply cloven was her bole that it seemed as if three shafts sprang from the ground together and they were of like size , round and straight , and their grey rind was smooth as silk , unbroken by branch or twig for a very great height above men 's heads .