dragon fly, that looks like what they called a termites mud/travel tubes at my mil's house a few years ago. Hope it isn't but I would get someone out there, just in case
jolana

I agree with Jolana. I have had that exact problem but not quite so big. They almost look like swarm castles, so I'm surprised you haven't had the little buggers flying around your living room. Time to call the exterminator. Ours used a product called termidor - it was the cheapest option and worked fast. Drilling holes in the slab foundation was the hardest part for me to accept, but they were really good inside the house to put them where they wouldn't show. You probably ought to take care of it pretty soon.

jolana and deb,

Thanks for the brainstorming. I went to look at some termite web sites and pictures of mud travel tubes and don't see any resemblance.

My picture may be a little deceptive. There is no mud involved. The 'web' stands off the wall and is very light,
full of what seems like wood dust. In other words, I don't see any tunnels or anything structural attached to the wood frame itself. But you're right, I should have someone out to take a look because it's clearly not a good sign. I just thought someone might have seen this before. It easily wipes away and could almost be done with a feather duster.

Upon further investigation I think I solved the mystery.
This evening I took a flashlight and watched the area carefully. There are some small holes/gaps in the wood frame between it and the fireplace wall. I saw a medium reddish ant emerge just long enough to deposit something and then go back inside. So then I googled carpenter ants and saw this description of the signs to look for their presence and it sounded like what I pictured above:

"ÂFrass, which is made up of wood shavings, bits of soil, dead ants and parts of insects. This material is often piled up outside of nest openings and found in spider webs and window sills."

I'm guessing that some moisture may be getting in there which is probably what attracted the ants. Anyway, I'll have a pest control guy come and investigate to be certain.

Wood ants can cause some pretty bad damage...get the pest control people quick.