The oldest rocks in the area are found in the core of the Harrogate anticline (Craven Group) and in the north-west. By 330 million years ago mud and sand, from huge river deltas, was periodically being washed into a tropical sea. Over millions of years changes in sea level produced repeated cycles (cyclothems) of alternating beds of limestones, mudstones, sandstones and coals (the Yoredale Group).
As time progressed, the extent of the river delta systems became greater and thick beds of sandstones and mudstones (the Millstone Grit Group) were laid down. North west of Ripon in the Winksley area there are small outcrops of Pennine Coal Measures Group sandstones with thin coals. The rocks were then subjected to erosion, and during the Variscan mountain building event, were faulted, folded and injected with mineral veins.
|PERIOD||AGE||VALE OF YORK GROUPS||EVENT|
|QUATERNARY||Holocene||Glacial and post glacial deposits||Regional uplift and eastward tilting|
|Miocene||Basin inversion||Alpine Orogeny|
|Palaeocene||Intrusion of Cleveland Dyke|
|CRETACEOUS||Upper||Regional shelf subsidence|
|JURASSIC||Upper||Corallian Group||Episodes of regional crustal extension|
|TRIASSIC||Upper||Penarth Group||Dominantly extensional faulting in west|
|Middle||Mercia Mudstone Group|
|Lower||Sherwood Sandstone Group|
|Upper||Upper||Zechstein Group||Dominantly extensional faulting in east|
|Stephanian||Basin inversion, regional uplift and erosion||Variscan Orogeny|
|Westphalian||Pennine Coal Measures Group||Regional subsidence with minor extensional faulting|
|Namurian||Millstone Grit Group|
|Visean||Craven Group||Major periods of extensional faulting and basin formation|