Yorkshire Dales

© Craven & Pendle Geological Society

Angular Unconformity

Combs Quarry, Foredale Grid Reference [SD 800 701]

Combs Quarry exposes one of the regions best angular unconformities situated in the Lower Palaeozoic Craven inlier. Basement rocks belong to the Horton Formation (Ludlow Series, Silurian). The overlying limestone is of Lower Carboniferous age.  Devonian rocks are missing altogether. The time gap is about 60 million years.

Description of the Sub-Carboniferous unconformity at Combs Quarry
In Foredale Quarry the Horton Formation is represented by a fine-grained turbidite sandstone; the Studfold Sandstone Member. On close inspection you may find thin interbeds of bentonite (off-white volcanic clay from a large, but distant subaerial volcanic eruption) in the basal Studfold Sandstone Member. Resting on these ancient rocks lies the subhorizontally bedded Carboniferous Great Scar Limestone (Kilnsey Formation).

The Angular Unconformity

The strata A to E was deposited first, tilted, subjected to erosion (peneplained) and submerged.

Strata X to Z were laid down horizontally on top of the older beds. The plane or junction, marked by the thick green line, between both sets of strata is the unconformity.

At Thornton Force the junction is marked by a basal conglomerate.

It can get more complicated!