The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Climax: Culminating Explosions of Pumice and Scoria
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.