"the [science fiction] writer should be able to convince the reader (and himself) that the wonders he is describing really can come true...and that gets tricky when you take a good, hard look at the world around you."
- Frederik Pohl

Parsons has moved forward in time; his hosts do their best to see to his comfort.

"What about a beer?" Helmar said. "We have several beers from your period, all ice cold."

"This is fine," Parsons said...

Loris, seating herself opposite, said, "And we've brought magazines forward. And clothing. And a variety of objects, some of which we can't identify. Chance plays quite a role, as you might guess. The time dredge scoops up more than three tons; we often get mere debris, however, especially in the earlier stages."

The phrase is of course based on the word "dredge," which refers to an excavation device used to scrape a seabed or river bed, usually to keep waterways navigable. It can also be used with the specific purpose of obtaining material, which can be used for land reclamation. Dredging has also played an important role in gold mining.

Perhaps the earliest use of this general idea can be found in a 1939 story by Edmond Hamilton published in Startling Stories:

('The Space Visitors' by Edmond Hamilton)