It's from Charlotte's Web by E.B. White.
The grass was wet and the earth smelled of springtime. Fern's sneakers were sopping by the time she caught up with her father.
Why author used 'were sopping'? Is 'sopping' a present participle or an adjective in the sentence? And is it correct to say:
Fern's sneakers had sopped by the time she caught up with her father.