The Watchman Dike and Flow

Next to the Devil's Backbone, the most conspicuous dike on the cliffs of Crater Lake is the one immediately beneath the Watchman. It stands vertically, forming a high wall cutting athwart the bedded lavas and ashes. Where it disappears beneath the talus, more than two-thirds of the way down the cliffs, its thickness is only about 6 feet, but upward it widens steadily to more than 50 feet and merges into the Watchman flow. There is no difference between the material of the dike and that of the surface lava, both consisting of massive, pale-gray porphyritic andesite. Within the dike, flow banding is obscure, but horizontal and vertical joints are well developed. This is one of the few dikes on the walls of the caldera which do not have black, glassy selvages, the margins being of the same pale-gray color as the interior and no less crystalline. At first sight, the dike seems to represent the actual feeder of the Watchman flow, and it has generally been so interpreted, but a study of the banding in the flow, though not entirely disproving this idea, casts considerable doubt on it. With better reason, the dike may be considered as an offshoot from the feeder common to both.

The Watchman flow has a distinct topographic expression, as may be seen in figure 9. Its length is almost 1 1/4 miles and its average width 2000 feet. The average thickness is between 400 and 500 feet. The flow must have been unusually viscous, for the snout and sides are extremely steep.

Of particular interest is the attitude of the flow planes (figure 9). In general they strike parallel to the margins, and, except in a peripheral belt less than 100 yards wide, they stand either vertically or at steep angles. Even at the snout of the flow the planes still stand steeply and bulge outward in the direction of movement, Along the sides and toward the base, the dip of the planes rapidly diminishes to horizontality. Cross sections therefore show a fan-shaped structure. Under the Watchman itself, the planes are disposed in the form of inverted concentric cones, steeper in the middle and flattening outward. This arrangement suggests an underlying, funnel-shaped feeder. Probably the Watchman, which rises 600 feet above the adjacent rim of the caldera, represents a domical protrusion over the vent.

Although, as we have said, the banding throughout most of the visible part of the flow is vertical and parallel to the margins, it must be assumed that in the lower, unexposed parts it lies at low angles, for otherwise it is impossible to see how the lava could have advanced. Support is lent to this view by study of cross sections of dacite flows on the caldera walls. Apparently the lavas moved somewhat after the manner of glaciers, the upper layers shearing over the lower and turning upward sharply at their distal ends.

Just below and a little to the north of the summit of the Watchman, the lava surfaces are crossed by flutings and scratches which seem at first glance to be of glacial origin. They trend downhill and westward, in the general direction of lava movement. That they are not actually glacial is clear, however, since they may be found in the lee of vertical crags and on the under surface of certain ledges, where ice can have had no erosive power. They must have been caused by frictional drag when the lava had chilled to a solid or semisolid crust.

On many of these fluted faces another curious feature may be observed, namely, streaks of minute hematite crystals. Locally the hematite is concentrated in bands on the crests of the furrows, from which smoke-like trails of hematite dust branch at right angles. Apparently when the separate blocks of lava were pulled apart, fumarole gases made their way along open joints, depositing iron oxide in the initial cavities and then spreading upward as the cracks gaped wider. Elsewhere on the Watchman, hexagonal plates of hematite, up to 1/4 inch across, may be found coating joint planes, and much of the brown staining seen on the lavas results from the hydration of this mineral.

The Watchman flow is one of the youngest of the pre-caldera andesites and perhaps coeval with the andesite of Sentinel Rock. Probably it is younger than any of the lavas of the Hillman Peak cone, with the exception of the flow which escaped from Forgotten Crater. It is, however, older than the dacites erupted from the Northern Arc of Vents, for whereas only the lower margins of the dacite flows have been glaciated, all but the highest crags of the Watchman flow have been overridden by ice.