suggesting that the sequence was deposited in a bajada environment, by a series of smaller fans merging to form a braided river system. Cailleach Head Formation This formation is similar to the underlying Aultbea Formation, the main difference being in grain size, with this formation being noticeably finer-grained. The sequence is made up of 22 m thick cycles, each with a basal erosion surface followed by dark grey shales with desiccation cracks, planar cross-bedded sandstones with wave rippled tops, overlain by trough cross-bedded micaceous sandstones. These cycles are thought to represent repeated progradation of deltas into a lake. A lack of evaporite minerals suggest that the lakes had through drainage. Acritarch microfossils were described from here by Teall in 1907, the first Precambrian fossils described in Britain. Age The upper age limit for the deposition of this sequence is constrained by the age of the last tectonic and metamorphic event to affect the Lewisian complex on which it was deposited, for which ages cluster between about 1200–1100 Ma. The lower limit is provided by the age of the lower Cambrian quartzite that lies above it, about 544 Ma. Radiometric ages from the Torridonian sequence itself give ages of about 1200 Ma for the Stoer Group and 1000–950 for the basal part of the Torridon Group. This implies an age gap of at least 200 Ma between the deposition of these two groups, consistent with the paleomagnetic evidence. Ages of detrital zircons also provide some constraints on the sequence age. The Stoer group and the lower part of the Sleat Group show ages consistent with derivation from Scourian and to a lesser extent Laxfordian rocks, with no dates after 1700 Ma. The upper part of the Sleat Group includes a large component of broadly Laxfordian age with almost no Archaean ages, with a lower limit of about 1200 Ma. In contrast the Diabaig Formation shows a small group clustered around 1100 Ma, the age of the Grenville Orogeny. In the Applecross and Aultbea Formations there are many more zircons giving ages around 1100 Ma and even below 1000 Ma. This evidence suggest that the Stour and Sleat Groups were deposited before the Grenvillian event, pre-1200 Ma, while the Diabaig Formation was deposited after the orogeny, after about 1090 Ma, and the remaining parts of the Torridon Group after about 1060 Ma. A more precise age has been obtained for authigenic feldspars