Godan verb with 'ru' ending, Intransitive verb
to join; to be put together; to combine; to unite; to fit; to close (e.g. of a wound)
しかもなお、石の大きいブロックが非常にぴったりと合わさっているので、ブロックの間にナイフの先を差し込むことができない。And yet the large blocks of stone are fitted together so closely that you cannot put in the point of a knife between them.
Also written 合さる【あわさる】、併さる【あわさる】