Welsh Journals

during the long time of subareal erosion following the late Carboni- ferous mountain-building movements, which buckled and fractured the South Wales area before the Triassic deposits were laid down. The surface on which the first Triassic sedimentary sheets were deposited would have been uneven and there would have been many widened cracks. Down these cracks went the first infillings by the Triassic sands and fine dusts, accumulating in a desert setting. We are indeed lucky to be able to still see these first Triassic fillings today (about 200 million years later). Another fact must not be forgotten here. The fact that the Triassic marl fills cracks in the Carboniferous Limestone means that a one-time cover of Millstone Grit (some 1500 feet thick) and of Coal Measures (probably over 15000 feet thick) had already been removed by intense erosion before the gashes were filled. Yet to the immediate north of these Gower localities, the Millstone Grit and Coal Measures are still preserved today. In other words, the surface geology by Triassic times was not so very different from that of today, with limestone exposed over southern Gower and Millstone Grit and Coal Measures still preserved to the north. How far upwards these open gashes extended until they met the main Triassic land surface, is not easy to assess. The writer thinks they lay very close indeed to the original surfaces. For our second clue, we must go to Fall Bay, the bay between the Rhossili coastguard station and Mewslade. In the part of Fall Bay where the narrow coastal path falls steeply down to high tide level, the cliff is seen to be in one place the remnant of an almost completely eroded cave. Its back wall is just about visible. On the rock floor at high tide level, at the foot of the rocky path, there can be seen several very large boulders of limestone. At first glance one would assume they are blocks which have fallen recently. A closer look however shows them to be underlain by and in places partly smeared with, more of this same red marl. The marl in fact wraps into crevasses and gaps between some of the boulders. Oh no! these boulders did not fall yesterday. They fell in Triassic times, 200 million years ago! They represent masses of limestone which fell down, either from the open top of a cleft on the limestone surface, or from the roof of a cave system (but a cave system which soon after such collapse would be sufficiently open to allow red marl to infiltrate and wrap around the fallen blocks). Our third clue (or pair of clues) occurs in the caves known as Bacon Hole (near Pennard) and Red Chamber (near Mewslade). Bacon Hole is so called because of the wide red bands, almost horizontal and roughly parallel, which occur on the walls. These red oxide bands were once thought to be the work of man. They are now looked upon as being of natural origin, and probably represent various levels of Triassic filling. This cave must have been a cave in Triassic times. It may well have opened southwards, just as it does today. But instead of opening on a sea cliff, as it does today, it