One of the more interesting things is that a whole series of Lava Dams formed and were later worn away in the Grand Canyon. At least 13 dams have been Identified IIRC and were formed between 10,000 and 1.8 million years ago.
Yup. Dalrymple's paper on dating them is interesting, too:
K-Ar ages of Pleistocene lava dams in the Grand Canyon in Arizona. A particularly ignorant creationist once claimed that Dalrymple admittd in that paper that K-Ar dating seldom works.