The Godley Valley is a huge glacial valley with towering mountains on either side and a broad braided gravel floor.
The first thing you have to do on this one or two day adventure is cross the Macaulay River and that's dangerous in Spring and after heavy rain. But in late summer the river is generally only knee deep and not swift enough to wash away your bike if you hang on tight.
A four wheel drive track goes about 27km (17 miles) up the valley before surrendering to huge glacial outwash boulders. There is a little six bed hut at the 25km mark called Red Stag Hut, popular with hunters. You can overnight in the hut if you book ($5, nzdeerstalkerssc.org.nz) and that will give you time to ride then walk the extra 7km up to Lake Godley and the Godley Hut for a taste of a hardcore glacial environment surrounded by rugged peaks. It's stunning.
The trail starts out as a reasonable gravel road as it goes through a back country farm. But after just a few kilometres the surface becomes very coarse gravel - bouldery in places. It's very juddery all the way and there are 30 creek crossings deep enough to wet your feet - and 30 more on the way back! - not counting the Macaulay River. This adventure is all about the meaning of life rather than the cycling pleasure.
A similar trail that is slightly less challenging starts at the same place and goes 19km up the Macaulay River to New Zealand's fanciest back-country hut.