Its base is not exposed but in its main outcrop area, it is considered to be in excess of 5,000 metres (16,000 ft) thick though less elsewhere.

Their main occurrence is within the northern and central fells of the Lake District, either side of the major ENE-WSW aligned Causey Pike Fault, but inliers are found at Black Combe in the south of the Lake District and at Cross Fell in the North Pennines.

The inlier to the south at Black Combe contains the wackes of the Knott Hill Formation.

[3] The Kirkland Formation has provided fossils of Dichograptus octobrachiatus and Heminectere rushtoni.

The resulting slaty cleavage is parallel to the axial plane of regional folds.