Cross-section through a giant current ripple in Pardee's gravel pit. In the fall of 1970 the gravel pit was still active, however when I visited it September 22, 2016, a locked gate blocked the entrance and it appeared inactive.

Closeup of a cross-section through a giant current ripple. The post Lake Missoula soil development lies below the yellow line. The sediment above the yellow line was dumped during the gravel pit operation.

This photo was taken in the gravel pit during the fall of 1970.

In his 1942 paper Unusual Currents in Glacial Lake Missoula, Montana, J. T. Pardee reported that no silt was deposited in Camas Prairie (p. 1580). However, as shown in this photo, ripple troughs are filled with up to a meter of Lake Missoula bottom sediment, but the deposits are too weathered to detail the internal structure. The yellow line shows the contact between the giant current ripple flood gravel (below) overlain by Lake Missoula bottom sediment. Imbricate cross bedding is still preserved in the flood gravel, indicating that these deposits were not destroyed during gravel pit operations. The preservation of the fragile lake bottom sediment suggests that the last lakes drained slowly, otherwise these deposits would have been flushed from the basin.

Although a soil has developed on both the lake sediments and flood gravel, there is no evidence of a soil between them. In 1970, Dr. R. Curry, Professor of Geology at the University of Montana and one of my thesis committee members, wrote a letter to Dr. William Bradley at the University of Colorado. In that letter, Curry believed that the soil, based on its weak A/C profile, suggested no more than about 15,000 years since the last lake draining. This date is probably too old, as Fryxell and Daughtery (1962, 1968: from Brtez, 1969) dated the last Cheney_Palouse discharge, in eastern Washington, at about 12,000 years B. P.

This photo was taken in the gravel pit during the fall of 1970.

While investigating the internal structure of the giant current ripple gravel, I found fragments of highly carbonate cemented gravel (see the closeups in the next 3 photos). The fragments in the photos appear to be what Gile, et al. (1966) call K-horizon soil. The K-horizon was proposed for carbonate horizons so strongly carbonate-impregnated that their morphology is determined by the carbonate. The enigma is that these horizons of authigenic carbonate are commonly found in soils of arid regions.

They defined four morphological stages that form in a time related sequence; the amount of carbonate accumulation increases markedly with increasing time of subaerial exposure.

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• Stage I: thin, discontinuous pebble coatings

• Stage II: continuous pebble coating with some inter pebble fillings

• Stage III: many inter pebble fillings

• Stage IV: laminar horizon overlying plugged horizon

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If these are K-soils, the big question is when did they form. Are they remnants of some of the late Tertiary K-soils or possibly the caliche mentioned by Bretz, et al. (1956), where they believe that the scabland topography recorded seven floods, with at least one additional flood occurring after formation of a heavy caliche, whose fragments are seen in the flood gravels; like I see here at Camas Prairie.

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These photos were taken in the gravel pit during the fall of 1970.

Gile, et al. (1966) define the K2 horizon as a soil horizon containing 90 per cent or more, by volume, of material with K-fabric; if indurated it is designated as a K2m soil. This photo shows what they describe as an irregular, pebble-studded, upper surface of the carbonated-plugged, indurated K2m horizon of a stage III carbonate horizon in very gravelly material

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This photo was taken in the gravel pit during  July1989.

This is an in situ fragment of a laminated, indurated K2m soil surrounded by giant current ripple flood gravel. The smooth upper surface of the laminar horizon of a stage IV carbonate horizon in very gravelly sediment. The laminar horizon forms at the top of the K2m, filling the irregularities between pebbles of the preexisting, stage III surface and coats over most of the pebbles (Gile, et al., 1966).

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This photo was taken in the gravel pit during July 1989.

This black & white photo was taken during the fall of 1970.