As for the jointing of the andesites, columnar structures are rarely well developed. On the other hand, platy or slabby jointing both parallel and perpendicular to the flow planes is seldom absent, and may be so well marked and closely spaced as to give the lavas the appearance of stratified sediments when viewed from a distance. Elsewhere, particularly in Grotto Cove, the andesites are traversed by dose-set curved joints at high angles to the banding. The origin of these aberrant structures is nor understood.
True scoriaceous (aa), ropy (pahoehoe), or block lavas have not been observed on the walls of Crater fake. It seems unlikely that original surfaces were destroyed by weathering and erosion prior to burial by later flows and ashes; much more probably the crusts of the flows have always been as smooth as they are today.
In a general way, the lavas of Mount Mazama became thicker as activity progressed. Among the earlier lavas on the caldera walls, few reach 100 feet in thickness. The thickest sheets are practically confined to the upper part of the walls, as, for example, near Discovery Point and along the northern wall. These were erupted at the close of the andesitic period, not long before the main dacitic eruptions began.
The thickest andesites are those of the Palisades and Roundtop. These rest on a thick deposit of glacial debris and were erupted after a long period of quiescence. Presumably they did not issue from the central, summit vents of Mount Mazama, but from fissures far down the northern slope. If this be true, then the eruption of these flows added nothing to the height of Mount Mazama; they were “parasitic” or subterminal flows. We may go even farther and suggest that only the thinner andesites escaped from the central vents of the volcano. It was these that built Mount Mazama to its greatest height. Accordingly, we may speak of the slopes left by the youngest of the thin andesite flows as the primary slopes of Mount Mazama, and in trying to estimate how high the volcano formerly rose these are the only slopes to be considered, since the thick flows from fissures below the summit only served to widen, not to heighten the cone.
In places, the Roundtop flow is no less than 500 feet thick. Its summit is strikingly polished and scratched by glacial action. The base of the flow has the form of a broad W (plate 25), and rests oh coarse, bouldery till. Apparently the lava moved down a U-shaped valley occupied by a medial moraine. The bulk of the flaw consists of porphyritic andesite marked by pronounced slabby jointing parallel to the banding. Near the base, the lava becomes darker and increasingly glassy and finally passes into black obsidian. Much of this basal glassy layer is intensely brecciated, and locally it has been reddened by gas action. In crossing the moraines, the lava enveloped many glacial boulders, some as much as 5 feet in diameter.
