Geological Survey Circular 838 Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California

The Terrain

Today the country in and adjacent to Captain Jack's Stronghold consists of four kinds of topographic surfaces (see map).

1. The Tulelake plain, which in 1872-1873 lay beneath the waters of Tule Lake. Since 1906 over 3/4 of the area of this large but shallow lake has been reclaimed for farmland.

2. Lowlands, of low relief, underlain by lobes and tongues of solidified lava, border the shoreline both to the east and west of the Stronghold. The lowlands are rough surfaced in places, but the variation in height is 2 meters or less. The lava tongues grade at the shoreline into pillow lavas interspersed with sandy beaches of hyaloclastic debris (black glass sand and pillow-like ellipsoids of basaltic lava, which formed where the molten lava tongues were quenched and granulated by entry into the waters of former Tule Lake).

3. The Plateau, that part of the Stronghold where the Modocs had their living quarters (see map) is the northernmost remnant of a plateau, whose surface rises gradually from 10 meters above the former shoreline at the Stronghold to 30-40 meters a few miles farther south. This plateau (shown with a zip pattern on the map) is the surface of an older and much thicker lava flow than the ones that form tongues and lobes across the lowlands. As will be described in more detail later, before this thick flow had completely solidified, renewed eruptions swelled its still molten interior until lava broke out from the flow front, and also drained out from beneath its solidified crust in a plexus of underground lava tubes. Therefore the plateau surface in the Stronghold is dimpled with small sag basins and by vertical-walled collapse pits (see map). Moreover, the plateau edges which were close to the original flow front have "turn-down margins" produced as the plateau surface sagged outward and was riven with deep cracks and fissures as it was rafted forward and let down to a lower elevation while molten material was escaping from beneath it.

4. Schollendomes. A marginal area of large schollendomes is present on three sides of the plateau remnant in the Stronghold area, and it continues to border other plateau remnants for many miles to the east and southwest. These schollendomes are detached parts of the former plateau crust which have broken into elongate domes characterized by a deep central cleft along their top. Many deep fissures and minor cracks branch from the central cleft. (For description and sketch of a schollendome see the Explanation accompanying the map).

We now describe in more detail the geologic nature and origin of each of these four topographic units as they occur within the area of the accompanying map. This will give the background and perspective necessary to understand how the topography was effectively used by the Modoc defenders of the Stronghold.

Tulelake Plain, Contrasted With the Tule Lake of 1872
In 1872 the area north of Captain Jack's Stronghold looked very different than it does today. Instead of the grain fields laced with irrigation and drainage ditches, the waters of Tule Lake lapped against a steep slope of crazily tilted lava blocks that formed a schollendome field between the shoreline and a flat-topped plateau 12 to 20 meters above the lake. The change from lake to farms was brought about by diversion of Lost River to irrigate arid lands farther west. Over a period of about 12 years (1906-1918) the lake shrank to less than 1/4 of its former area, and the hyaloclastic silts and sands on the lake bed, fortified with organic matter from the tule swamps that flourish in shallow water, became rich farm land. The position of the original shoreline in 1873, and the shape of the present land surface immediately adjacent to it, are shown by 5-meter contours on the accompanying map.

Only over a stretch of about 500 meters (1/3 mile) did the shoreline of Tule Lake come against the belt of large schollendomes that rim the plateau remnant of the Stronghold, both east and west the shoreline is against lava lowlands that rise only 0 to 3 meters above the level of the former lake. The site of a non-permanent Indian village that occupied a rocky ledge just above lake level on the eastern lowland one kilometer (0.6 mile) northeast of the center of the Stronghold (see map) can be identified by the numerous grinding holes for seeds abraded into the surface of the lava rock, and by the litter of bird bones and shells of fresh-water clams. Today a 10-meter bluff drops from the site of the Indian village to the former lake floor, which is now cultivated farm land. Here the shore of Tule Lake dropped into deep water, but other parts of the shore, especially at the head of bays, were bordered by wide sandy beaches and bayhead bars formed of hyaloclastic gravel, silt and sand. Sand and silt blown from these beaches by the wind also smoothed over the rough lava surface of the adjacent lowlands. Travel along or close to the shoreline was relatively easy, but the bay where the schollendomed margin of the plateau reached the lake was a place where passage could be denied by a small group of determined outpost snipers.

The Broken Surface of the Lava Plateau. In describing with more detail the area above the level of Tule Lake we start with the plateau and then proceed to areas of lower elevation. This is the order in which the three topographic units (plateau surface, schollendomed margin, and lowlands), were developed geologically. Each represents a phase during the climax and dying out of a major episode of volcanic activity. Tracing them in order of development makes it easier to understand how the natural fortress used by the Modocs was formed.

The surface of the plateau (zippitone area on the map) is the top of an unusually thick lava flow which spread into this area from the north. Numerous cracks along the margin of the plateau penetrate 10 to 12 meters below its top; this depth is perhaps 1/3 of the total thickness of the lava flow at the time of its greatest inflation during the climax of the eruption. The flow is one of a great number of lava flows which spread north and east from vents in and near Mammoth Crater, located 15 kilometers (9.4 miles) airline to the south. The molten lava, however, did not travel all this distance on the surface of the ground. Instead it was transmitted most of the way through underground lava tubes. (For an analysis of various parts of the lava-tube systems see the maps and reports on display at the visitor Center in Lava Beds National Monument). Small distributary tubes fanning out from one major branch of this intricate system of large lava tubes became active within the flow that forms the plateau at Captain Jack's Stronghold during the last stages of eruption, as we show later.

The distribution of remnants of the plateau surface indicates that this thick flow reached its farthest north extent within the area of Captain Jack's Stronghold. From here the irregularly lobate flow front trended about South 65° East to and beyond Fern Cave; and in the opposite direction it runs approximately South 30° West for about 3 kilometers. At most points, however, the exact position of the flow front can be inferred only within a width of about 500 meters, because in almost all places the flow's margin was broken up and partly inundated by late movements of molten lava within the interior of the flow after its front and crust had partly solidified.

After the flow front stopped moving and a 1- to 15-meter thick crust had congealed on top of the flow, a resurgence of volcanism sent large volumes of molten lava through the long system of tubes. Lava from distal branches of these tubes entered the still molten interior of the flow, lifting it and putting so much hydraulic stress against the flow front that molten lava broke through in numerous places. During this maximum phase of eruptive activity the still molten lava beneath the plateau crust began to creep toward these new breaches, setting up skeins of flow toward the points of lowest stress. During the waning stages large parts of the flow front and parts of the crust of the flow began to break up under these stresses and were rafted slowly forward on the back of the moving currents below. At the base of the flow front small lobes and tongues of molten lava escaped to the surface and flowed on to the north. After eruptive activity at the distant vents had ceased the molten interior of the flow and the lava filling the underground tubes continued to drain out, leaving many parts of the plateau crust unsupported. Such parts sagged downward, replacing the space left by the departing lava from the interior. Large masses of the solidified flow front, and finger-like tongues of the crust extending hundreds of meters upstream from the flow front, sagged down and were dragged forward as the lava beneath them leaked out. (Note on the map the many small oval collapse basins, and the long finger-like basins with schollendomes which indent the plateau).

Wherever the edge of the plateau "turned down" to replace the escaping lava beneath, deep tensional cracks and fissures formed in the turned down flap. In places parts of these fissured margins were rafted away, forming schollendomes. Nearly all edges of the plateau remnants became an intricate maze of deep fissures which greatly impede travel across this terrain. Where shallow distributary lava tubes drained out, post-lava collapse of the plateau surface into the tubes has left numerous vertical-walled collapse pits and collapse trenches.

Inspect the area of the map at the end of the plateau remnant where the Modocs took up residence. Each of the faint lines represents a crack or fissure too wide to jump over without extreme caution, and for each line on the map there are many more cracks in the ground (omitted from the map). These fissures, especially the big ones at or near the top of the turndown flaps, are the "natural trenches" used by the Modocs. Note that they are nearly continuous along the edge of the plateau on three sides of the Modoc encampment. These plateau-edge fissures, however, were only the last line of defense; the Modocs also used rock clefts on the tops of schollendomes beyond the plateau margin as sentry outposts.

The Schollendomed Margin of the Lava Plateau. The map shows much better than words how this frontal part of the thick flow that formed the plateau was broken up and transformed into a broad marginal area of schollendomes. Schollendomes form when a thick crust of lava congeals over the top of a lava flow, and then is jostled, undermined, and partly carried away by draining out of the still molten lava beneath the crust. Thus the schollendome field at Captain Jack's Stronghold grades upslope into the "turndown" edge of the plateau remnants. Downslope the frontal end of the schollendome field is engulfed by the thin lobes and tongues of lava that escaped to the surface through the ruptured flow front. In areas east and west of the central part of the Stronghold this escaping lava aided in the building of the lowlands along the former shore of Tule Lake.

Only the largest of the schollendomes are indicated on our map. The basins between them, and also the "turndown basins" that project like fingers into unbroken parts of the plateau, are cluttered with smaller schollendomes which range down in size to schollen no larger than a small room and only a meter or two in height. Time did not permit us to map the smaller schollen. Moreover, the clutter of lines that would result from plotting all these cracks and fissures would have made the map unreadable. The unbroken expanses of white paper in this part of the map, however, do give a false impression that some of the wider basins in the schollendomed area might be easy to cross. From a distance they also look easily traversible, but actually they are deceptively cruel to a person who tries to cross them in a hurry. We have used the word hummocky in places on the map to indicate that such surfaces are quite irregular, rough with various sized schollendomes. and riven with hundreds of cracks and fissures.

Several high schollendomes outside the plateau remnants provide excellent viewpoints over the shoreline and lowland areas. The Modocs developed them into well-camouflaged sniper positions by piling loose fretworks of rock in or around parts of the central crack.

Lowlands Built of Pillow Lavas and Hyaloclastic Debris. Numerous outcrops where the lowlands meet the former shoreline of Tule Lake show that lava advancing from the south flowed into the lake. Molten lava changes on contact with a water body into pillow lavas and hyaloclastic deposits. These materials filled in the edges of the original Tule Lake, pushing the shoreline northward much like a delta grows at the mouth of a large river. For details of the mechanics of formation of pillow lavas and various kinds of hyaloclastic deposits consult Fuller (1931) and Waters and Fisher (1971).

In a quarry developed for road building materials near the site of the former Indian village (see map), we can examine typical examples of pillow lavas with chilled glass rinds, pillow breccias, and granulated slag-like bits of hyaloclastic material (basalt glass) - the three kinds of deposits characteristically formed where molten basalt advances into a water body. In nearby areas molten lava also escaped directly into the lake through lava tubes. Good examples are on the Hovey Point peninsula west of the Stronghold, and there are several additional vent areas and low maar craters to the east between the Stronghold and Hospital Rock. In places molten lava also erupted directly into the lake, building larger underwater volcanoes at The Peninsula, Juniper Butte, and Prisoners Rock - localities several miles to the east and south of the Stronghold.

Most of the lowland area probably was built into the lake by the quenching of lavas that are slightly older than the flow that formed the plateau. The late tongues of lava that broke through the schollendomed front of the plateau flow, however, have also assisted in pushing the lake front farther to the north.

The lowland areas are easily traversed. Although the lava surface is minutely rough, and is in places diversified with small schollendomes about a meter high, the area shows none of the deep fissuring, large schollendomes, or broken areas of talus that make the adjacent schollendomed area so difficult to traverse. Moreover, waves and wind have scattered hyaloclastic sands and silts over the surface of many lowland areas, assuring excellent mobility for travel. Soldiers in training on the lowland areas near Gillem's Camp, or on the hyaloclastic flats near Hospital Rock, probably had no concept of the kind of terrain they would meet in their assaults on the Stronghold.