black mud in which he sank to his knees

[suprunp]

Suddenly the boy cried that he could touch bottom and Izzart put down his legs; but he could feel nothing; he swam a few more exhausted strokes, his eyes fixed on the bank, and then, trying again, felt his feet sink into thick mud. He was thankful. He floundered on and there was the bank within reach of his hands, black mud in which he sank to his knees; he scrambled up, desperate to get out of the cruel water, and when he came to the top he found a little flat with tall rank grass all about it.
(W.S. Maugham; The Yellow Streak)

Does it mean that the bank consisted of that mud in which he sank to his knees while still being in the cruel water?

Thanks.

 

[SoothingDave]

I think the bank was knee-deep mud, but it was out of the water.