The oldest rocks in Devonshire are probably not, as was once thought, the granites, but the highly altered or metamorphic formations in the extreme south; that is to say, the mica and quartz schists and the hornblende epidote schists which extend from near Start Point to Bolt Tail, a district which, owing in great measure to distortion by volcanic upheaval, includes some of the most picturesque scenery in Devon.
Next in order of age is the series called Devonian, after the name of the county, in which they were first distinguished from the Old Red Sandstone. They are, however, by no means confined to Devonshire, but are very widely distributed, covering a large part of Cornwall, and occurring on the continent of Europe, especially in Russia, and in Asia and North and South America. The Devonian beds—which are found both in the north and south, occupying two distinct areas separated by widespread deposits of culm or carboniferous measures—were, it is thought, formed in open water, and probably at the same time that the Old Red Sandstone of the adjoining county of Somerset and elsewhere, which is not found in this county at all, was being deposited in estuaries and land-locked seas.
NAMES OF SYSTEMS | SUBDIVISIONS | CHARACTERS OF ROCKS | ||
|---|---|---|---|---|
| P R I M A R Y | Recent Pleistocene | Metal Age Deposits | Superficial Deposits | |
| Neolithic„ | ||||
| Palaeolithic„ | ||||
| Glacial„ | ||||
| Pliocene | Cromer Series | Sands Chiefly | ||
| Weybourne Crag | ||||
| Chillesford and Norwich Crags | ||||
| Red and Walton Crags | ||||
| Coralline Crag | ||||
| Miocene | Absent from Britain | |||
| Eocene | Fluviomarine Beds of Hampshire | Clays and Sands Chiefly | ||
| Bagshot Beds | ||||
| London Clay | ||||
| Oldhaven Beds, Woolwich and Reading Groups | ||||
| Thanet Sands | ||||
| S E C O N D A R Y | Cretaceous | Chalk | Chalk at top Sandstones, Mud and Clays below | |
| Upper Greensand and Gault | ||||
| Lower Greensand | ||||
| Weald Clay | ||||
| Hastings Sands | ||||
| Jurassic | Purbeck Beds | Shales, Sandstones and Oolithic Limestones | ||
| Portland Beds | ||||
| Kimmeridge Clay | ||||
| Corallian Beds | ||||
| Oxford Clay and Kellaways Rock | ||||
| Cornbrash | ||||
| Forest Marble | ||||
| Great Oolite with Stonesfield Slate | ||||
| Inferior Oolite | ||||
| Lias-Upper, Middle, and Lower | ||||
| Triassic | Rhaetic | Red Sandstones and Marls, Gypsum and Salt | ||
| Keuper Marls | ||||
| Keuper Sandstone | ||||
| Upper Bunter Sandstone | ||||
| Bunter Pebble Beds | ||||
| Lower Bunter Sandstone | ||||
| T E R T I A R Y | Permian | Magnesian Limestone and Sandstone | Red Sandstones and Magnesian Limestones | |
| Marl Slate | ||||
| Lower Permian Sandstone | ||||
| Carboniferous | Coal Measures | Sandstones, Shales and Coals at top Sandsones in middle Limestones and Shales below | ||
| Millstone Grit | ||||
| Mountain Limestone | ||||
| Basal Carboniferous Rocks | ||||
| Devonian | Upper | Devonian and Old Red Sandstone | Red Sandstones, Shales, Slates and Limestones | |
| Mid | ||||
| Lower | ||||
| Silurian | Ludlow Beds | Sandstones, Shales and Thin Limestones | ||
| Wenlock Beds | ||||
| Llandovery Beds | ||||
| Ordovician | Caradoc Beds | Shales, Slates, Sandstones and Thin Limestones | ||
| Llandeilo Beds | ||||
| Arenig Beds | ||||
| Cambrian | Tremadoc Slates | Slates and Sandstones | ||
| Lingula Flags | ||||
| Menevian Beds | ||||
| Harlech Grits and Llanberis Slates | ||||
| Pre-Cambrian | No definite classification yet made | Sandstones, Slates and Volcanic Rock | ||
DIAGRAM SECTION FROM SNOWDON TO HARWICH, ABOUT 200 MILES.
This cross section shows what would be seen in a deep cutting nearly E. and W. across England and Wales. It shows also how, in consequence of the folding of the strata and the cutting off of the uplifted parts, old rocks which should be tens of thousands of feet down are found in borings in East Anglia only 1000 feet or so below the surface.]