50 DR T. J. JEHU ON 



the boulder gravel. As a rule the sands underlie the gravels. The pebbles and 

 boulders of the gravels have not the flattened, ovoid form which is so characteristic of 

 the shingle on modern beaches. At some places along the margin of the north-west 

 coast, notably at Nevin, there occur laminated silty clays usually high up in the series. 

 These silty clays are stoneless and devoid of shell-remains and appear to have been laid 

 down tranquilly in water of some depth. Along this part of the coast the Drift consists 

 chiefly of gravel and sand and silty clays, and the upper surface of the Drift has a 

 terraced aspect. Some traces of disturbance and contortions are occasionally seen in 

 the sandy beds. 



The sandy series at some places lies immediately on the bed-rock, but at others and 

 especially in the deeper depressions, it rests on the Lower Boulder Clay. The line of 

 demarcation between the Lower Clay and the Sands and Gravels is often well marked, 

 but at places there appears to be a gradual passage from the one to the other. The 

 pebbles and boulders in the Gravels include distant erratics of like origin to those 

 found in the Lower Boulder Clay. Chalk-flints are not uncommon, and specimens of 

 the Ailsa Craig riebeckite granite were obtained at several places. Shell-remains occur 

 especially in the coarse sands and beds of gritty material. They are commonest in the 

 sands on the north-west coast. Neither shell-remains nor distant erratics were noted in 

 the Sands and Gravels on the south coast east of Llanbedrog. The shells found in the sands 

 are very broken, usually occurring as mere crumbs. A few of those obtained from the 

 sands at Nevin and Porth-dinlleyn have been identified, but the list is too limited to 

 yield definite conclusions as to climate. But the sands in this region are very similar 

 to the shell-bearing sands of Moel Try fan, and further south in Pembrokeshire,* the writer 

 collected in similar sands a much larger number of shell-remains which were capable of 

 identification. At both places the shells pointed to a mixture of species belonging to 

 different climates. This, together with the fact that everywhere the shells are very 

 broken and much rolled, indicates that they are not now in or near the positions in 

 which they lived. In no case were Lamellibranch shells found with the two valves in 

 apposition such as one finds in a recent and undisturbed alluvium like that which occurs 

 to the north of Dinas Dinlle. Marine-like sands of a similar nature with gravels above 

 are found inland in patches all over the peninsula. They lie on the slopes of, and 

 sometimes spread over, the lower hills. Eastwards, as we approach the Snowdonian 

 Mountains, the sands tend to become coarser and more greyish in colour and they are less 

 marine-like. No shell-remains were noted in the sands of the interior, but the exposures 

 met with are very poor and afford but little opportunity for a thorough search for shell- 

 fragments. Such fragments are known to occur in the sands on Moel Tryfan and again 

 away from the coast in Pembrokeshire, and so it is probable that this is the case also in Lleyn. 

 Stratified sands and gravels were found at Llanaelhaiarn at a height of 450 feet above sea- 

 level. The well-known shell-bearing sands of Moel Tryfan occur at a height of 1350 feet. 

 In Pembrokeshire marine-like sands and gravels were found at elevations of 640 feet. 



* Trans. Roy. Soc. Edin., vol. xli., part i. (No. 4), 1904. 



