Incidentally, the best official text description I know of is in Sorcerer, when you actually become a grue and visit a grue colony. IIRC, even that description is vague, but does cannonize that they are large four-legged reptiles.

We discuss bicomplex convexivity, hyperbolic-valued seminorms and hyperbolic-valued Minkowski functionals in bicomplex modules.

It was a whole day's journey from I-ch'eng to Chü-wo which, in turn, is about 60 li east of Chiang Chou — one of the most important cities in southern Shansi and a center for curio-dealers. At this city, the Fêng River turns westward. While we were in Chü-wo, we decided to pay a visit to Chiang Chou to have a look at the various curio ships in that city.…Having gone to Chiang Chou from Chü-wo by the northern route through Hou-ma, we returned over a bypath through mountains in the south which has been gradually elevated from the Fêng River valley by loess deposit.…After we returned to Chü-wo, our next trip was to dash across Chung-t'iao-shan.]

Each was marked by lighted dan buoys spaced at one-mile intervals, laid by Fairmile motor launches of the Royal Navy, which followed immediately behind the minecraft.