In Wales and its borders the general succession of the Upper Silurian rocks may be taken to be as follows, in ascending order (fig. 57):—­

(1) The base of the Upper Silurian series is constituted by a series of arenaceous beds, to which the name of “May Hill Sandstone” was applied by Sedgwick.  These are succeeded by a series of greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes of great thickness; and these two groups of beds together form what may be termed the “May Hill Group” (Upper Llandovery of Murchison).  Though not very extensively developed in Britain, this zone is one very well marked by its fossils; and it corresponds with the “Clinton Group” of North America, in which similar fossils occur.  In South Wales this group is clearly unconformable to the highest member of the subjacent Lower Silurian (the Llandovery group); and there is reason to believe that a similar, though less conspicuous, physical break occurs very generally between the base of the Upper and the summit of the Lower Silurian.

(2) The Wenlock Group succeeds the May Hill group, and constitutes the middle member of the Upper Silurian.  At its base it may have an irregular limestone ("Woolhope Limestone"), and its summit may be formed by a similar but thicker calcareous deposit ("Wenlock Limestone"); but the bulk of the group is made up of the argillaceous and shaly strata known as the “Wenlock Shale.”  In North Wales the Wenlock group is, represented by a great accumulation of flaggy and gritty strata (the “Denbighshire Flags and Grits"), and similar beds (the “Coniston Flags” and “Coniston Grits”) take the same place in the north of England.

(3) The Ludlow Group is the highest member of the Upper Silurian, and consists typically of a lower arenaceous and shaly series (the “Lower Ludlow Rock”) a middle calcareous member (the “Aymestry Limestone"), and an upper shaly and sandy series (the “Upper Ludlow Rock” and “Downton Sandstone").  At the summit, or close to the summit, of the Upper Ludlow, is a singular stratum only a few inches thick (varying from an inch to a foot), which contains numerous remains of crustaceans and fishes, and is well known under the name of the “bone-bed.”  Finally, the Upper Ludlow rock graduates invariably into a series of red sandy deposits, which, when of a flaggy character, are known locally as the “Tile-stones.”  These beds are probably to be regarded as the highest member of the Upper Silurian; but they are sometimes looked upon as passage-beds into the Old Red Sandstone, or as the base of this formation.  It is, in fact, apparently impossible to draw any actual line of demarcation between the Upper Silurian and the overlying deposits of the Devonian or Old Red Sandstone series.  Both in Britain and in America the Lower Devonian beds repose with perfect conformity upon the highest Silurian beds, and the two formations appear to pass into one another by a gradual and imperceptible transition.