Capping the lava shield are the remains of two cinder cones, aligned in a north-south direction. Little is preserved of the older cone, which forms 6889 Hill, north of the summit. There is no trace of a crater. All that can be seen beneath the pumice are a few small outcrops of bright-red scoria with vesicular bombs up to 3 feet across and patches of agglutinate.
The younger cone is complex, and only slightly sculptured by erosion. It is oval in plan, and extends for a mile in a north-south direction, rising approximately 600 feet above the lava shield. Here and there small exposures of well bedded scoria may be found, though rounded bombs are extremely rare. Most of the ejecta consist of angular blocks and lapilli of dense, black vesicular basalt, presumably already solid when erupted. Accompanying the explosive eruptions were short and sluggish flows of lava.
Within the summit cone three craters may readily be distinguished. The northernmost is indicated only by a remnant of the northern rim. The other two coalesce to form a depression between 50 and 100 feet deep, measuring about 1/4 mile from north to south and 250 yards across at the widest place. Probably these two vents were active at the same time, though the deeper, southern crater may have continued to erupt after the other became extinct.
We may conclude, therefore, that during the explosive eruptions which brought to an end the activity of the Timber Crater volcano, there was a southward migration of the vents, parallel to the general trend of the Cascade Range, from 6889 Hill to the present summit region, more than a mile away.
Activity at the Timber Crater volcano had certainly ended before the last pumice explosions of Mount Mazama. Its closing eruptions were probably contemporary with those which formed the parasitic cinder cones on the sides of Mount Mazama. A little gravelly drift on 6889 Hill indicates the former presence of a small icecap at that point, but the summit cone of Timber Crater and the sides of the lava shield do not appear to have been covered by glaciers. The final activity of this volcano may therefore be assigned to post-Pleistocene times.
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