"To be sure, the bridge is a thing of its own kind; for it gathers the fourfold in such a way that it allows a site for it. But only something that is itself a location can make space for a site. The location is not already therefore the bridge is. Before the bridge stands, there are of course many spots along the stream that can be occupied by something. One of them proves to be a location, and does so because of the bridge. Thus the bridge does not first come to a location, and stand on it; rather, a location comes into existence only by virtue of the bridge. The bridge is a thing; it gathers the fourfold, but in such a way that it allows a site for the fourfold. By this site are determined the localities and ways by which a space is provided for."
A meandering river
A meandering river presenced by a bridge
If this is in fact the case, does it have to be a man made 'thing' which presences, or is it just the 'other' nature of the bridge, which allows the location to come into existence? So, could for example the old mysterious tree from Tim Burton's Sleepy Hollow serve the same purpose? This was a tree which was like all the other trees of the forest in its 'treeness', but was absolutely something other in it's character and form. So, does the 'thing' bringing locations into existence have to be a man-made 'thing' or can the 'thing' remain simply a markedly different or 'other' thing from its surroundings? I think Heidegger would contend that a tree such as the example given in 'Sleepy Hollow' would not suffice because of its direct relation to the fourfold. Maurice Merleau - Ponty may have a different paradigm on the subject.
In Maurice Merleau Ponty's book Phenomenology of Perception he describes a single red spot painted over a field of gray. Here, Ponty contends that it is the red spot itself which allows the understanding of the field. The 'presencing' of the red painted spot, provides us the ability to describe the gray field in which the spot sits. Before there was simply a field of gray with little if any differentiation. Yet, once we have painted the red spot, the spot has creates the perception that everything of the gray color is merely contextualizing the red spot. He goes on further, saying that we want to see this simple red spot as being within the field of gray when actually this red dot is painted on top of the gray placing it in another context all together. It is then, our consciousness which we impose on the simplicity of the red painted dot, resting on top of the gray field, which allows us the ability to speak about the two colors of paint in relation to the other. I believe, the presencing of a location by the building of a bridge, which Heidegger writes about is similar if not the same point Ponty is making about the relationship of two colors on a canvas. Does a natural site and a man-made structure outside the realm of 'the fourfold' differ from a canvas field and a paint spot? I believe these are two philosophers describing the same subject matter through disparate avenues.