The cliffs of the Helderberg are coarse grained quartzitic sandstone made up of rocks of the Peninsula formation of the Table Mountain Group.
The lower and middle reaches are underlain by rocks of the Malmesbury Group, probably 900 million years old, and the cliff rocks are some 400 million years old.
|Cape Granite intruded the Malmesbury Group some 600 million years ago. There are no outcrops within the reserve, but granite underlies part of the reserve and outcrops to the north west.|
|The Malmesbury group can be seen in several places in the reserve. It consists of phyllitic shales, siltstones and fine grained greywhackes. It is from these rocks that the soils of much of the lower and middle reaches of the reserve have been derived.|