Complex sentence: It is thought that this end of the building was built on mediaeval foundations, certainly this would explain the installation of a single spoke wheel as this form is the only one narrow enough to fit in either of the two channels separated by a sandstone pier.
Split into simpler sentences: It is thought that this end of the building was built on medieval foundations. This would explain the installation of a single spoke wheel as this form is the only one narrow enough to fit in either of the two channels separated by a sandstone pier.