This sedimentary deposition was followed by a period of volcanic eruptions which produced a series of basaltic lavas, some of which are now found near Kelso. The summit rocks of Rubers Law were formed within a volcanic vent. There is a small area of volcanic agglomerate, formed in the vent by an explosive eruption of magma from the volcano. The rest of the summit rocks are microgabbro (previously known as dolerite), the remains of a volcanic plug formed from magma which solidified within the vent. Another microgabbro plug is located among the trees of Denholmhill Wood, 2 km north of the summit.