Strange Exodus

Gigantic, mindless, the Monsters had come out of interstellar space to devour Earth. They gnawed at her soil, drank deep of her seas. Where, on this gutted cosmic carcass, could humanity flee?
Westover got a shock when he stumbled onto the monster, for all that he knew one had been through here.
He had been following the high ground toward the hills, alternately splashing through waist-deep water and climbing onto comparatively dry knolls. To right and left of him was the sullen noise of the river in flood, and behind him, too, the rising water he had barely escaped. The night was overcast, the moon a faint disk of glow that left river and hills and even the mud underfoot invisible.
He had not sought in his mind for the flood's cause, but had merely taken it numbly as part of the fury and confusion of a world in ruin. Anyway, he was dead tired out on his feet.
He sensed more than saw the looming wall before him, but he thought it the bare ledge-rock of a stripped hillside until he stepped into a small pot-hole and lurched forward, and his outflung hands sank into the slime that covered a surface faintly, horrifyingly resilient.
He recoiled as if seared, and retreated, slithering in the muck. For moments his mind was full of dark formless panic; then he took a firm hold on himself and tried to comprehend the situation.
Nothing was distinguishable beyond a few yards, but his mind's eye could see the rest—the immense slug-like shape that extended in ponderous repose across the river valley, its head and tail spilling over the hills on either side, five miles apart. The beast was quiescent until morning—sleeping, if such things slept.
And that explained the flood; the monster's body had formed an unbreakable dam behind which the river had been steadily piling up in those first hours of night; if it did not move until dawn, the level would be far higher then.
Westover stood motionless in the blackness; how long, he did not know. He was hardly aware of the water that covered his feet, crept over his ankles, and swirled halfway to his knees. Only the emergence of the moon through a rift of the cloud blanket brought him awake; its dim light gleamed all around on a great sheet of water, unbroken save for scattered black hummocks—crests of knolls like that on which he stood, all soon to be hidden by the rising flood.