The glass boulders are 'River Rocks' by Colin Reid. They're one of ~12 artworks sited along the Lune Cycleway (aka Millennium Park) between Lancaster and Caton. This location, just downstream of Forge Weir and rapids popular with canoeists (immediately below this viewpoint, in fact), may have been chosen as slightly sheltered from the main flow during floods, but I'm still a little surprised the installation is still here after ~6 years.