I don't have the guidebook in front of me; this is from memory.
Not sure about the grades, really; I don't pay that much attention to stuff harder than 7a!
This is not a big area, but is right across the bridge from the town of Sisteron. Despite the easy access, it wasn't terribly crowded when we were there, nor were the easy routes as polished as you might expect.
The river cuts at a right angle though a ridge of vertical limestone strata, exposing dramatic fins of rock on which most of the climbs are found. Some of the climbs are deep withing the recesses between the fins, which offers some protection from the howling mistral, which rips though the cleft in the ridge cut by the river.
Given the proximity of world class provencale climbing spots, Sisteron is probably not a destination in itself, but an easy way to sneak in a few hours of climbing if you're passing through.