Peach and Horne, who worked together for forty years, first went to the Northwest Highlands, the scene of their most famous work, in 1883. Peach was then forty years old. They were sent by Archibald Geikie to resolve a long standing controversy about the structure of the area. Murchison had believed that the fossiliferous Cambro-Ordovician Durness Limestone passed conformably upwards into the Eastern schists of which a large part of the Northern Highlands are formed. Nicol was the main exponent of the opposition and pointed out that the metamorphosed schists must be older than the unmetamorphosed limestones and that the junction was a steep fault.
Since it could easily be demonstrated in the field that the junction is almost horizontal, Murchisons views were held to be correct. In 1883 both Calloway and Lapworth suggested that the junction was a low-angle tectonic thrust, and this idea was now being given serious consideration by Geikie. It was during their first season of field mapping in the region round Durness and Eriboll that Peach recorded the true situation. Instead of the simple conformity which Murchison had suggested, there were gigantic structures of a kind never before encountered in the British Isles. The Eastern (Moine) Schists had been thrust westwards by a series of large-scale low-angled faults over the unmoved foreland rocks of ancient Lewisian gneiss and their cover of Late Precambrian Torridonian sandstone and Cambro-Ordovician limestones. During this process a series of smaller faults (imbricate structures) had been produced en-echelon in the underlying foreland and cover rocks. The thrust zone was eventually traced in the field from Eriboll to Skye. These well exposed structures now seem easily recognizable, but it was perhaps the most spectacular discovery of all time in British geology. By 1884 Murchisons views on the succession had to be abandoned in view of the rapidly accumulating evidence against them. Peach was somewhat reluctant to overturn Murchisons theory, for he felt a debt of gratitude to Murchison and greatly respected the old man.
A further discovery during these years was the existence of large numbers of trilobites of the genus Olenellus in the basal Cambrian rocks of the foreland. These fossils not only gave a Lower Cambrian age to the basal sequence, but confirmed the American affinities of the faunal assemblages there. In these days when rifting apart of the former Euro-American continent is readily accepted by most geologists, one wonders what Peach and Horne thought of 'American' trilobites in Scotland. Peach described the trilobites in two papers in 1892 and 1894 and he himself drew the very fine illustrations contained in them.