The Correspondence of Sir Archibald Geikie: Thomas Wright to John Young sub-series includes letters from:

Gregory writes that the lower jaw of a horse has been discovered in the upper drifts filling the pre-glacial valley of the Clyde at Lanark. He suspects that it dates from around the Early Neolithic period. He asks Ewart to look at the specimen and write a short note on it for inclusion in the Hunterian Museum glacial vertebrate fossils.

Gregory enquires after Ewart's note on the horse for the Museum catalogue, as it is ready to go to print.

Gregory writes that he has provisionally inserted the bed in a tabular classification as Neolithic. He believes it must be post-glacial, as it represents the silting up of valleys which were excavated at the end of glacial times, but the horse jaw is the only fossil evidence of this.