Sandwich is a very small town located near the Cape Cod water front. The old glass factory used to sit on the edge of the salt marsh. Here is a photo of the dump in the marsh. Although this dump was heavily dug almost 100 years ago, it was still loaded with cool, colorful pieces of slag glass from the factory.
If you are not familiar with hunting "Tide Flats" or "Salt Marshes", I recommend that you learn about the dangers of such things before wading out.