Shale

"...the moderate slopes, covered with scrub, loose shale, and jungle, and deceitful little trickling watercourses." (Croker, "To Let", Annotated Version https://editions.covecollective.org/edition/haunting-imperial-nonsense-what-cannot-be-captured-victorian-ghost-story)

Shale is formed when clay absorbs water and becomes compacted into a solid thin rock.

USGS, "This image is in the public domain in the United States because it only contains materials that originally came from the United States Geological Survey, an agency of the United States Department of the interior."   Reproduced in accordance with the Doctrine of Fair Use, as per 17 USC 107, https://commons.wikimedia.org/wiki/File:Geology_of_the_Richmond_Basin_18...