The southern and western parts of the Pentland Hills are formed from
sandstones together with some
conglomerates, all of
Devonian age and assigned to the
Old Red Sandstone. Within the
sedimentary sequence are
extrusive igneous rocks, principally of
basaltic and
andesitic composition. The sedimentary rocks are also
intruded by
dykes of
porphyrite. The oldest rocks are a sequence of
Silurian mudstones,
siltstones and sandstones collected together as the
North Esk Group. From oldest to youngest they comprise the
Llandovery age Reservoir Formation (named for North Esk Reservoir where these rocks are to be found), the Deerhope Formation, the Cock Rig Formation and the Wether Law Linn Formation overlain by the
Wenlockian age Henshaw Formation. These are in turn
unconformably overlain by the late Silurian to early Devonian age sandstones and conglomerates of the Greywacke Conglomerate and Swanshaw Sandstone formations. A further unconformity separates these from the overlying Pentland Hills Volcanic Formation though all three formations are collected together within the
Lanark Group. The Volcanic Formation forms such summits as East and West Kip. The southeastern edge of the range is defined by the Pentland Fault with a substantial
downthrow to the southeast. Running NE-SW through the middle of the range is the Cairnmuir Fault which downthrows to the northwest. West Cairn Hill and East Cairn Hill are formed by the sandstones of the Kinnesswood Formation which reach as far north as Hare Hill (though also underlie much of the southeastern part of Edinburgh) and which terminate to the southeast at the Cairnmuir Fault. The Kinnesswood sandstones together with rocks of the Ballaggan Formation, which form the lower ground immediately northwest of the Pentlands, constitute the early
Carboniferous age
Inverclyde Group. Black Hill stands out as being an intrusion of micro
granite or
felsite, as in part is nearby Harbour Hill. South Black Hill on the other hand, together with Scald Hill and Carnethy hill are formed from various
lithologies of the Pentland Hills Volcanic Formation, as too are Allermuir, Caerketton and Castlelaw hills. The Volcanic Formation gives rise to generally steeper and craggier hillsides as these rocks are more resistant to erosion than the sedimentary rocks. Much of the lower ground is covered by glacial
till from the
last ice age. Glacial
meltwater channels are identified in places as at Deer Hope and the deep cleft of Green Cleugh between Hare Hill and Black Hill. == List of peaks ==