>>>The deposits are flat and stuck to the pavements and roads. They are so numerous it is quite impossible that they could be chewing gum
I used to run a railway station and, as well as the work of our own cleaning staff, we had to have a specialist firm (with specialist equipment) spend an entire night, once per month, cleaning the ruddy stuff off our platforms. Where councils don't use special cleaning machines to get rid of the stuff (as some do), most busy streets will have many thousands of deposits dropped on them within a very short time.
It's definitely chewing gum!