A local feed mill grinds the feed and it is a coarse, dusty feed.
All feed gets ground into a mash. If the feed company has the machinery the mash then gets "glued" into pellets. Or it can be made into crumbles. Mash is usually less expensive to buy as there is no additional cost of processing the feed any further into another shape or texture. Hope that helps.
Until the technology of extruding and forming pellets and/or making crumbles came along, all chicken feed was "mash". That's the only way the feed came. Ground up grains and a mineral/vitamin package added, of course.
Also, while mash looks dusty, depending on how fine the mill grinds it, one can reconstitute it easily by moistening the mash. Add a bit of water and stir and you'd be surprised how nice it looks. Very much like crumbles.
This slightly wet feed, mixed in a bucket and fed to chickens (or similarly to pigs, BTW) is where the term mash came from. That it looks a bit like the mash used by the guys out in the woods running a still, or what's left from spent grains at a brewery is also how it got its name, most likely. Mush, mash. yada yada.