Mount Kenya is a stratovolcano that was active in the Plio-Pleistocene. The original crater was probably over high; higher than Kilimanjaro. Since it became extinct there have been two major periods of glaciation, which are shown by two main rings of moraines below the glaciers. The lowest moraine is found at around . Today the glaciers reach no lower than . After studying the moraines, Gregory put forward the theory that at one time the whole summit of the mountain was covered with an ice cap, and it was this that eroded the peaks to how they are today.