The word Maam is derived from the Gaelic word Mam meaning mountain pass. The Maam Valley, which extends from Maam Bridge north-westwards to Leenane, lies at the centre of a deeply dissected mountainous region. It is flanked on the south-west by the steep rocky ridge of the Maum Turk Mountains which rise to well over 2,000 feet at several points. To the north-east the valley is bounded by a group of lower, less hostile hills known collectively as Joyce's Country. The valley floor is relatively flat and covered in most parts with blanket peat or thin podzolic soils overlying glacial deposits. The valley is drained by two river systems, the major one being that of Joyce's, Bealnabrack and Failmore Rivers which flow into Lough Corrib east of Maam Bridge. A smaller unnamed river drains the northern end of the valley, flowing from the watershed at Culliagh Beg down to Killary Harbour at Leenane.
The whole area is underlain by Pre-Cambrian (at least 600 million years old) Connemara schist which is exposed around the base of the Maum Turks, on the hills to the north of Maam Bridge and occasionally in the valley itself. The steep, bare, rocky slopes and peaks of the Maum Turks are composed of quartzite which is also of Pre-Cambrian Age. Both the schist and the quartzite, which are known technically as metamorphic rocks, were initially deposited under the sea as clay and sand respectively and later transformed by high temperatures and pressures into their present state.
The greener hills at the northern end of the Maum Turks (e.g. Tonalee, 1606 feet) are composed mainly of grits, sandstones, mudstones and shales which were deposited during the Silurian period, over 400 million years ago. These rocks, like those forming the hills on the north-east side of the valley rest on top of Connemara schist. The hills in the north-east corner of the valley are composed mainly of slates, tuffs, breccias, grits, agglomerates and conglomerates which were formed some 500 million years ago during the Ordovician period.
The hills north of Joyce's River in the Kilmilkin area are of Silurian age and somewhat similar to those of the Maum Turks described earlier. The basic geology of the Maam Valley outlined above has been greatly complicated by geological process such as faulting and glaciation but detailed description of changes caused by these activities must be left to the professional geologist. However, one prominent feature resulting from glacial action is worthy of mention. This is the sand and gravel ridge running parallel to the road south-east of Leenane. This ridge is thought to have been deposited in water held back by ice dams which blocked off the valley in the vicinity of Leenane.