I had found several nice, though informal, bits of information regarding the Deep Creek pillow basalts while hunting around on the internet last fall, but haven't found the one or two really good ones this time around.
However, I did find this link Major Rock Types of the Spokane Area.
The author discusses the Latah Formation, which underlies the basalts at the northern end of the canyon in the state park. The aforementioned slide (in the last post) occurred at the contact between the two. Below is my best shot of that contact.