An analogy is made at the opening of this stanza between stirring up dirt in a spring and the water then being all the clearer, and the present situation the speaker finds himself in.
swish of the water also alludes to the idea of displacement in the previous stanza. While initially the dirt would muddy the water and make it dark, opaque, and impossible to see through, eventually, once settled, it would be clearer.
Effectively, this image prepares us for the conclusion of the poem as the speaker is about to reveal what is now clear to him following this encounter.
dirt in this case is the murky thought of how humans evolved into what they now are.
The idea of
dirt in our origins continues with the choice of the word
slime - the primeval slime from which we and all other living organisms were created. This links once more the evolution of humanity with that of the shark.
In his mind's eye MacCaig has a surreal image of himself crawling out of this slime and returning to the initial, fundamental beginnings of human existence.
emerging in the final line of this stanza reinforces this new, almost epiphanic sense of clarity associated with coming out of the dark into light. The word
everything again reinforces our similarity with every other species at the start of this process.