Unlike Sisi Butte, Olallie Butte has undergone relatively little erosion. Glacial erosion exposed the central volcanic plug and excavated troughs on Olallie's northeastern side, suggesting that the volcano is at least 25,000 years old. Part of the steepness of lower slopes at Olallie can be explained by glacial erosion by an ice sheet somewhere between in thickness, which previously covered the Olallie area. Glacial till, consisting of angular blocks of basaltic andesite and andesite, forms a veneer throughout the Olallie area, covering about 50 percent of the land, so systematic geochemical samples are generally not considered informative for volcanic rocks near the Butte. The volcanic rock that can be studied is of Cenozoic age, consisting mostly of basaltic andesite and andesite lava flows and associated breccias. Most of these have a porphyritic texture, containing phenocrysts; basaltic andesite deposit phenocrysts are made of minerals like light to dark-gray hypersthene, clinopyroxene, or olivine, and the andesitic deposits have minerals such as platy pyroxene or olivine. Labradorite phenocrysts are common among both the basaltic andesite and regular andesite deposits, while magnetite is more rare. There has been very little geological alteration of these rocks other than a few instances of alteration of olivine to iddingsite. All the rocks in the area show a normal magnetic polarity, indicating ages less than one million years and corroborating that they are of Pleistocene age. The domes at Double Peaks and southwest of View Lake have phenocrysts with andesine and dark red-brown basaltic hornblende, the latter of which contained altered magnetite, iron oxide hematite, and other unspecified minerals.