This is usually still white, but may be anywhere from a quarter of an inch to 3 or 4 inches thick. Its not quite as hard as ice, but almost. If relatively smooth, it presents no special problems, other than being slippery. The other side of that coin is that it usually comes with embedded gravel supplied, most often, by your tax dollars. This embedded gravel makes it possible to ride hardpack without studs, as it really does supply ample traction.
Hard pack can have a smooth as glass surface.
More often, you end up with a certain degree of rutting, caused by cars or bumps caused by footprints that freeze over. Hardpack on road is often bladed off by road maintenance crews making it skating rink smooth. If this gets a little water on it it can be really slippery. Hardpack will eventually melt, making for some of the most bumby riding conditions you will ever find. Uaually it will break up in chunks due to auto traffic first.