Detailed Description of the Individual Flows

Having enumerated the general features of the glowing avalanches, we may now pass to an account of the individual flows.

     The Annie Creek Flows

The avalanches that swept down Munson Valley into Annie Creek left no traces of their passage until they reached a point below the present Government Headquarters, approximately a mile south of the caldera rim. In this upper part of the valley, the moraines of the Munson glacier are entirely free from any cover of pumice, the presumption being that ice still occupied this region at the time of the eruptions.

Not until the pumice flows had descended approximately 5 miles from their source did they begin to deposit their load in large amount. From the Government Headquarters southward to where Munson Valley suddenly widens, the valley floor is heavily littered with hummocky glacial moraines and outwash, veneered with a broken cover of pumice and crystalline scoria. From a point due east of Annie Spring, the cover of pumice and scoria becomes continuous. At first, where the pumice is thin, the surface reflects the buried kame-and-kettle topography of the underlying moraines, but within 1/2 mile the hummocky surface gives place to a broad, flat plain.

There is no better or more convenient place to examine the deposits of the Adie Creek flows than at Godfrey's Glen, a short distance below Annie Spring. In the glen, three layers may easily be distinguished:

a. A top layer, averaging 20 feet in thickness, composed mainly of crystal-lithic ash with pumice lumps up to 6 inches across. This deposit fell from the air and is crudely bedded. Its origin is discussed on a later page.

b. A layer of smoke-gray scoria heavily charged with large bombs and carrying abundant crystals, particularly of hornblende.

c. A bottom layer of buff dacite pumice also rich in large bombs.

At the top of the smoke-gray scoria or close to the bottom of the overlying bedded ejecta, there is a discontinuous pink zone caused by oxidation of iron-bearing gases from fumaroles. In this zone, the deposits are commonly bleached and partly converted to opal and kaolin or are cemented by brown and red oxides of iron.

Generally the dark scoria layer forms between half and two-thirds of the canyon walls at this point, but its thickness varies greatly over short distances. This variation seems to imply that the scoria flow had the power of cutting its own channel in the pumice as it swept along. Some support is lent to this opinion by the fact that the dark scoria layer is nowhere found beyond the rim of the canyon, either of Annie or of Sun and Sand creeks.

There are places in Godfrey's Glen where half the scoria layer is made up of crystal-rich bombs up to 2 feet in diameter, and for each old lava fragment more than an inch across there are at least fifty of these large bombs of scoria. At such places, the total percentage of lithic fragments by volume is not more than 5. Pieces of old lava more than a foot in maximum dimension are exceedingly rare. In the finer matrix of the scoria, the proportion of lithic detritus may be considerably higher, as the diagram, figure 26, shows.

At the head of the glen, near Dewey Falls, the scoria layer descends to the valley floor, the pumice flow being absent. Farther downstream, the pumice appears beneath the scoria and both continue to thicken. Nowhere is there a clean-cut contact between the two types of flow, yet the transition zone in all places is quite thin, ranging generally between 10 and 20 feet. This sudden change in the character of the deposits, from dacite to basic scoria, deserves to be borne in mind.

In the pale pumice layer there are short and irregular brown streaks, running almost horizontally. These may signify brief periods of fumarolic activity between successive flows. There are also small lenses of reworked glacial debris within the pumice. It does not follow, however, that these were laid down during long intervals of erosion; they are more likely to have been caused by floods accompanying the pumice flows as they melted snow and ice in their path.

In many short stretches, Annie Creek has already cut through the pumice and scoria deposits to the underlying glacial moraines and bedrock lavas. Wherever the contact is exposed, it is emphasized by copious springs, the seepage line clearly defining the hummocky surface of the recessional moraines. Occasionally small pieces of carbonized wood may be collected close to the base of the pumice in Godfrey's Glen, proof that at least a few trees were growing this far up the valley before the eruptions began.

At the margin of the pumice flow, west of the main falls of Annie Creek, and at a few other places, the original level of the flow may have been slightly higher than at present. In certain parts it may have been as much as 30 feet higher. Some of these high strand lines were produced by the surge of material up slopes opposed to the direction of movement, but they also occur where upsurging would not be expected. In such instances, the explanation must be that the whole surface of the pumice flow settled as a result of compaction. Similar strand lines were observed bordering the tuff flow in the Valley of Ten Thousand Smokes.

At the falls of Annie Creek, the succession of deposits is:

a. 20 feet of crystal-lithic ash with a few pumice and scoria bombs up to 18 inches. Downstream this topmost layer thickens to 50 feet.

b. 100 feet of smoke-gray, compacted scoria.

c. 100 feet of pale-buff dacite pumice.

In both the scoria and the pumice, bombs more than 2 feet across are common, but fragments of old lava exceeding a few inches in diameter are rare. The nature of the finer matrix is adequately shown by the histograms, figure 26.

Below the falls, the walls of Annie Creek are cut almost entirely through massive, columnar scoria, rich in crystalline bombs (plate 17, figure 1). No clear signs of fumarolic action have been observed in this stretch of the canyon, presumably because the flows had lost much of their gas by the time they had traveled this far.

Close to the south boundary of the park, the Annie Creek flows joined those of Sun Creek, and together they swept toward Fort Klamath. Just where they came to an end cannot be determined, but probably they continued to the shores of Upper Klamath Lake, which were then many miles nearer Fort Klamath than now. Perhaps the flourlike pumice in the alluvial flats about Fort Klamath was produced by settling of finer material from the flows as they entered water.

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