That's Grass Track hobot, very popular in a couple of different forms in UK and Europe. The tracks are, generally about 1/4 mile, or 1 mile. The longer track, known startlingly as "Long Track", uses a circuit about 1 mile long, is very popular in Europe, I saw one of the first "long track" meetings in the UK (at Blackmore Vale??) in the late 60s. Very spectacular, especially the long track. The track is essentially a fallow paddock/field, which is more or less flat, and preferably grassy. The bikes used to be similar to speedway bikes, but not the same, they had rear suspension for instance.
VERY hard men ride Grass Track.
cheers
wakeup