The answer is NO. Valleys cannot be roofed...it ruins the landscape. Just havin some fun.
What do you mean by the question? There are millions of successful roof jobs with valleys. The materials used are subject to different answers. Metal, tile, clay, composite,asphalt, fiberglass, plastic and thatch(hay)roofs are all different. I'm sure I left some out...but regular asphalt/fiberglass shingles are best done in a weaved valley, with a membrane underneath.
I hope this helps you to explain more about your question. We are here to help.