What are the images of the two similes used in Lines 21 and 22 to create the picture of the fountain bursting forth from the chasm?

These are lines 19,20,21

Amid whose swift half-intermitted burst
Huge fragments vaulted like rebounding hail,
Or chaffy grain beneath the thresher’s flail:

I think the speaker is referring to a geyser that expldes through the ground spitting everything in its way out.