Offa's Dyke Path 1 Marian Ffrith
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Marian Ffrith is a small hill in an undulating landscape carved by
ice about 20,000 years ago. This hill and the hills to the north
and west are the remains of barrier reef, which formed about 330
million years ago. Car park at N 53° 17.264 W 003° 23.353
This EarthCache is one of a series of EarthCaches along the
Denbighshire section of the Offa's Dyke Path. Together these caches
explore the geological significance of the rocks and landscape,
which underpin the heritage, culture and biodiversity along the
path. The limestone of Marian Ffrith is up to 1000 metres thick. It
was formed about 330 million years ago, during the Carboniferous
Period of geological time, when Wales lay south of the equator
(Fig. 1 to follow). Wales lay under part of a large tropical sea
similar to the Great Barrier Reef on the Australian coast. This
reef formed at the edge of the shallows, not far from the shore
(Fig. 2 to follow). Reef creatures, such as corals, like warm,
clear water and grow in large colonies (Fig. 3 to follow). (Fossils
of the coral colonies can be seen at ODP 6 Trevor Quarry). As the
reefs grew, it spread out along the edge of the shallows or inner
reef, forming a reef crest. Over millions of years the reefs formed
limestone, the fossilised remains of creatures, plants and algae.
This limestone is harder than the surrounding rock and has survived
erosion by weather, water and ice and is now a distinctive
landscape. The prominent hills you can see to the west, Moel
Hiraddug, Dyserth and north-west, Graig Fawr, Prestatyn, were once
part of the reef crest and Marian Ffrith was probably part of the
inner reef (Fig. 3 to follow). The original sea floor has been
pushed up over the last 300 million years to form the mountains you
can see around you. As a result, the limestone has been folded and
faulted causing cavities in the rock. Over time, mineral-rich water
flowed through the rock forming metal ore veins of lead, copper,
cobalt, nickel, manganese and iron as haematite and pyrite. These
minerals have been mined and quarried since ancient times. Graig
Fawr was extensively mined for lead and there are many remains of
mines, shafts and adits in the hillside. Moel Hiraddug was mined
for iron, copper and cobalt and more recently spar, used in the
chemical industry and limestone. The pinnacle at the northern end
of Moel Hiraddug reminds us of the days when mining and limestone
quarrying were major industries in North Wales. The colour of the
cliff is due to the iron in the limestone. These prominent hills
have been used by ancient peoples. Marian Ffrith has substantial
earthworks and evidence of shafts and open surface workings remains
from the distant past. The top of Moel Hiraddug, the long hill to
the west, is an Iron Age Hillfort with a series of ramparts built
of the local limestone. The hillfort was partially excavated
between the mid 1950s and 1980 and any artifacts were found
including a shield, brooches, parts of a sword and sheath. Further
reading;The Archaeology of Clwyd; edited by John Manley, Stephen
Grenter, Fiona Gale, published by Clwyd Archaeology Service Clwyd
County Council 1991. To log this Earthcache: 1. What colour is the
rock at Marian Ffrith? 2. Are there any obvious fossils? 3. What is
the shape of Moel Hiraddug, the hill to the west? Is it small or
large, angular or rounded, elongated or short 4. Take a picture of
your GPS with Moel Hiraddug in the background (optional)
Additional Hints
(No hints available.)