Kirkby is situated on an escarpment of magnesian limestone laid down in the more recent Permian era (250 million years ago). At that time, Ashfield lay near the equator and was on the edge of a shallow, tropical lagoon. Limestone is made up of the shells and exo-skeletons of sea-creatures and corals that lived and (more relevantly) died in the lagoon.

The surrounding sandstones represent the deeper seas away from the lagoon, and were formed from tiny grains of quartz eroded and washed down from the Pennine and Welsh mountains of the West and North-West. The Peak District in prehistoric times really were mountains. Millions and millions of years of weather and crashing seas have eroded them down from large volcanos to mere hills, and the resulting sand underlies most of the Heart of England. Evidence of the volcanos can still be seen as ridges of hard igneous rock (that is rock that was once volcanic lava) around Matlock and Northern Derbyshire.

In geological terms:
The whole of the District of Ashfield lies on part of the Nottinghamshire, Derbyshire and Yorkshire coalfield. In the western (approx. 40%) part of the District, the coalfield is exposed at the surface. Here, coal seams mainly from the Middle Coal Measures outcrop at the surface. In the eastern (approx. 60%) part of the District, the Permo-Triassic Strata that lie unconformably on top of the coal measures conceal the coalfield. The Permo-Triassic Strata consist of Lower Permian Marl, Lower Magnesian Limestone, Middle Permian Marl, Lower Mottled Sandstone and (in the northeast of the District) Bunter Pebble Beds. Generally, the strata dip gently to the east throughout the District.