There is something quietly ancient about Porthdinllaen that becomes more obvious the longer you stand there. The place has the atmosphere of a landscape that has been worked on for an impossibly long time — battered by ice, fractured by tectonic violence, and endlessly worn down by the Irish Sea. Even without knowing the geology, you can feel it in the shape of the headland and the strange mixture of smooth bays, broken rock shelves, and stubborn cliffs.

The rocks beneath Porthdinllaen are extraordinarily old. Much of the local bedrock dates back to the Ordovician period, nearly half a billion years ago, when this part of the world looked nothing like modern Wales. Instead of green fields and fishing villages, the region lay somewhere near the edge of an ancient ocean basin, in a landscape shaped by volcanic islands, submarine eruptions, drifting crustal fragments, and deep marine sediments. Ash from volcanoes settled into the sea and slowly hardened into rock. Lava cooled and fractured. Sediments accumulated, were buried, compressed, and later transformed by heat and pressure. What survives today around the coast are the remnants of those vanished environments — volcanic tuffs, altered sedimentary rocks, and harder igneous intrusions that resisted erosion long after softer surrounding rocks disappeared.

One of the reasons the coastline around Porthdinllaen feels so irregular and rugged is that the area sits within the influence of the great Llŷn Shear Zone, a major tectonic fracture system that once subjected the rocks here to immense stress. Over geological time the crust was squeezed, folded, twisted, and faulted. Layers that were once horizontal became tilted or shattered. Different rock types were pushed together in chaotic arrangements. If you examine the coastal exposures closely, there is often a sense that the landscape has been physically crumpled. The peninsula is, geologically speaking, something of a patchwork assembled under pressure.

Then came the ice.

During the Ice Ages, enormous glaciers moved across this part of Wales and out into the Irish Sea basin. The ice reshaped almost everything. It scoured the bedrock smooth in places, gouged hollows, dumped thick spreads of clay and stones, and altered the drainage and contours of the coast. Many of the rounded forms in the landscape around Nefyn Bay and Porthdinllaen owe as much to glacial abrasion as they do to the sea itself. Even today, after storms or very low tides, you can sometimes notice pebbles and boulders on the shore that originated many miles away, carried here inside the moving ice and abandoned when the glaciers melted.

What makes Porthdinllaen itself such a distinctive natural harbour is the stubborn durability of its rocks. The headland projects into the sea because the bedrock there resisted erosion more successfully than neighbouring areas. Over thousands of years the sea preferentially wore away weaker zones, leaving behind this protective arm of tougher stone. Waves bending around the coastline created calmer water behind it, allowing a sheltered anchorage to develop naturally. In many ways, the village exists where it does because geology quietly dictated the shape of the coast long before humans arrived.

Walking there at low tide reveals the story beautifully. Broad wave-cut rock platforms stretch out toward the water, their surfaces cracked into geometric joints and polished by centuries of surf. Some rocks still bear the smoothed contours left by glacial ice. In places the cliffs expose layers and textures that speak of violent volcanic origins, while elsewhere the shoreline breaks into loose gravels and erratic stones deposited during the retreat of the glaciers. The whole coast feels transitional — part mountain remnant, part drowned glacial landscape, part living marine environment still being reshaped every winter.

And perhaps that is what gives Porthdinllaen its peculiar emotional atmosphere. The village appears peaceful and almost timeless, yet it rests upon the remains of catastrophic processes operating over unimaginable spans of time: oceans opening and closing, volcanoes erupting beneath ancient seas, continents colliding, glaciers advancing, coastlines collapsing. The beauty of the place is inseparable from that violence. The quiet harbour, the low cliffs, and the rocky curves of the shore are all the surviving surface expression of a geological history stretching back hundreds of millions of years

Here you will see an excellent example of the early stages of cross bedding in a love sand deposit

To make sense of the slanted internal layers in Porthdinllaen—and in many sandstones worldwide—it helps to imagine how sediment moves when pushed by wind or water. Cross‑bedding forms when ripples, dunes or larger sedimentary bedforms migrate across the landscape. As sand is driven up the gentle stoss (up‑current) side of the dune, it cascades down the steeper lee side, building inclined layers known as foresets. When the dune moves on, these layers remain behind, recording both the geometry of the original bedform and the direction of ancient flow. 

Cross‑bedding comes in several forms, each telling a slightly different story about the currents or winds that shaped them:

Tabular (or Planar) Cross‑Bedding

Tabular cross‑beds appear broad and sheet‑like, their bounding surfaces fairly planar. These structures typically arise from straight‑crested dunes or bars—features that migrate steadily without much lateral shifting. Their foresets are usually planar or gently concave and can record large-scale, unidirectional flows whether in rivers, shallow seas, or desert dune fields. Such cross‑beds suggest a relatively stable flow direction and energy regime, allowing geologists to confidently infer ancient current pathways. 

Trough Cross‑Bedding

Trough cross‑beds, on the other hand, have a curved, scoop‑shaped geometry that hints at more dynamic conditions. These structures form where dunes have a more sinuous or linguoid shape and migrate both forward and sideways. The result is a set of concave-upward foresets that often point to shifting river channels or other environments where flow direction varied considerably over short distances. They are key indicators for reconstructing ancient river patterns and channel migration. 

Large-Scale Aeolian Cross‑Bedding

In desert settings, dunes can reach impressive heights, and the foresets they leave behind can be several metres thick. These cross-beds form at angles close to the natural angle of repose of dry sand (around 34°), producing towering, sweeping sandstone structures like those preserved in the Navajo Sandstone of the American Southwest. When found in ancient rock, they speak of vast dune seas, arid climates and powerful prevailing winds.

4.2: Structures Formed by Unidirectional Currents - Geosciences LibreTexts

Taken together, these varieties of cross‑bedding allow geologists to decode ancient environments with remarkable precision. The direction in which foresets dip provides a window into paleocurrents—the flow of water or wind long vanished. The thickness, steepness and scale of cross‑beds help determine whether they formed in towering dunes or gentle ripples, in desert sands or river channels, in calm coastal shallows or energetic tidal channels. In the case of Porthdinllaen, the large-scale cross‑bedding within the Dunaskin Rock fits comfortably with the interpretation of a vigorous fluvial system, pushing coarse sand across the landscape in migrating dunes and bars. 

References 

1. Wikipedia – Cross‑Bedding
   https://en.wikipedia.org/wiki/Cross-bedding
   [en.wikipedia.org]

2. Geology Is the Way – Cross‑Bedding
   https://geologyistheway.com/sedimentary/cross-bedding/
   [geologyistheway.com]

3. Geological Digressions – Tabular and Trough Cross‑Bedding
   https://www.geological-digressions.com/tabular-and-trough-crossbed-lithofacies/
   [geological...ssions.com]

Please note that unless otherwise stated, images in this listing have been taken from the public domain in a search engine search. I do not accept ownership of any images unless otherwise stated. The images are to enhance the Earth Science lesson visually.

In order to log a find against this EarthCache you will need to visit GZ and answer some questions and then send your answers to me via my Geocaching profile or through the Messenger Centre

1 please describe the cross bedding you can see forming in the sand deposit at GZ.

2 please explain which kind of cross bedding is occurring here