The geology of Yorkshire in northern England shows a very close relationship between the major topographical areas and the geological period in which their rocks were formed. The rocks of the Pennine chain of hills in the west are of Carboniferous origin whilst those of the central vale are Permo-Triassic. The North York Moors in the north-east of the county are Jurassic in age while the Yorkshire Wolds to the south east are Cretaceous chalk uplands. The plain of Holderness and the Humberhead levels both owe their present form to the Quaternary ice ages. The strata become gradually younger from west to east.
Much of Yorkshire presents heavily glaciated scenery as few places escaped the direct or indirect impact of the great ice sheets as they first advanced and then retreated during the last ice age.
The oldest rocks in Yorkshire are represented by a number of small inliers of Palaeozoic areas along the southern margin of the Askrigg Block to the north of the Craven faults. This Ingletonian group of folded and cleaved mudstones and sandstones is of disputed age but fossils equate them with the Lower Skiddaw Group of the Lake District which are Ordovician. These rocks were laid down when the area was part of the Avalonia land mass and was positioned about 30° south of the equator.
By the end of the Ordovician period the Avalonian land mass had collided with Baltica and this event caused a marine regression which was exacerbated by a worldwide drop in sea level caused by a period of glaciation.
During the Silurian period Avalonia and Baltica moved rapidly towards Laurentia at a position about 20°south of the equator. The Iapetus Ocean which lay between them was closed. Inliers of the Silurian rocks which were formed at this time occur at Cross Fell, adjacent to the Pennine Fault, and at Horton in Ribblesdale and Austwick, north of the Craven Fault System.