Pontcysyllte Aqueduct (Llangollen, Wales)
A stunning peice of engineering and skill to create such a massive stone bridge, and not only that but to support a watercourse (the canal) across its length. There are a number of nature paths around the area so access is quite good down to the river. Where there was ice things were a bit more interesting.
Info: Built by Thomas Telford, there are 18 peirs made of local stone, the central ones over the Dee being 126 feet hight to the railings.
The canal runs through an iron trough, 1007 feet long and just under 12ft wide. (wouldnt fancy directing a boat through it).
Total cost £47,000, they don't make them like that anymore!