Rock Flour Research Articles

Mollusks of Sodon Lake, Oakland County, Michigan. I. Stratigraphic Occurrence of Shells in Peat and Marl Sediments

In connection with palynological studies of Sodon Lake sediments (Cain, '48; Cain and Cain, '48; Cain and Slater, '48a) it was noticed that samples often included numerous shells of small mollusks. Since the Cranbrook group is interested in several aspects of the present and past biological features of this small glacial lake basin, it was decided to resample the sediments for the purpose of ascertaining whether any correlation could be discerned between the occurrence of mollusks and the past history of the lake basin as indicated by pollen analysis. The samples were obtained at footlevel intervals, by means of a peat-sampling tool, in the same location as those used for pollen work (Cain and Slater, '48a). In several instances additional samples were taken between the foot-level intervals. Altogether the shells were washed from 38 samples of 30 cc. volume each, scattered through a 25-ft. section. Thin layers of clay and sand mark the bottom of the section (over the glacial gravels), but the principal sediment is marl which extends upward to the 8-ft. level with only minor peaty-marl layers interspersed at the 21-ft. and the 14-15-ft. levels. From the 8-ft. level upward the sediments are composed of fen peat with no evidence that either Sphagnum or ericaceous plants have ever played at any time a predominant role in the vegetation of the basin. Changes in the character of the sediments are not reflected strongly in the mollusks. The number of shells drops in the peaty layer at 21 ft. It is also relatively low at 15 ft., but it rises rapidly in the upper layers of this zone to a considerable number at 14 ft. The shells are predominantly of aquatic species. Shells of terrestrial species, however, are also present in small numbers in the water-deposited marl. Their numbers are, naturally, somewhat greater in the peat-forming period when the local vegetation was of the fen type (Cain and Slater, '48b). Table I gives the number of shells of each species. None was recovered from the clay (rock flour) sample at the bottom of the section at 24 ft. 10 in., whereas 18 shells of 4 species were taken from the sandy layers immediately. above. With the commencement of marl sedimentation, the variety of aquatic forms increased rapidly at first: 6 at 24 ft., 8 at 23 ft. 9 in., 12 at 23 ft. 2 in., 15 at 22 ft. 6 in., 16 at 21 ft. 10 in. Only three additional species were added later, making a total of 19 aquatic species and varieties. The inwash of terrestrial forms was first encountered at the 19-ft. level, with 5 species. Five more species were added from there up to the 10-ft. level, and 4 more species after fen conditions had developed. The total molluscan fauna removed from comparatively small samples of this section is, then, 33 forms, all of specific rank except for 4 varieties of Valvatta tricarinata and Carychium exile var. canadense.1 This is a moderately rich fauna, the majority of the forms of which entered the waters of Sodon Lake comparatively rapidly in early post-glacial time. The number of different entities in any one sample ranged from 0 to 20; the average was about 10. The maximum

Notes on the mineral assemblage of the "white silt" terraces in the Okanagan Valley, British Columbia

Mechanical screen and pipette analyses and study of the heavy mineral assemblages of samples of the white silt deposits of the Okanagan Valley, British Columbia, support the conclusions of Duly and Flint that these sediments represent accumulations of rock flour at or near the mouths of streams emptying into the ice-dammed lakes which occupied the valley at the close of the Pleistocene.

The Flynn Creek Disturbance, Jackson County, Tennessee

A small, intensely disturbed area in Jackson County, Tennessee, has been mapped and studied in detail. All the data accumulated indicate a crypto-volcanic origin of the structure. A magnetometer survey showed a well-developed magnetic high offset 4 miles to the south-southwest. The history of this disturbed area is interpreted as follows: Shortly preceding Chattanooga deposition an explosion took place near the surface, blowing out a crater 2 miles in diameter and 300 feet deep. The Ordovician limestones forming the floor and walls of this crater were shattered into breccia composed of angular fragments of varying sizes imbedded in a matrix of smaller fragments and "rock flour." The deeper parts of the crater were filled with redeposited breccia, either as talus breccia or as bedded breccia deposited in a fresh-water lake that occupied the crater at one time. The Chattanooga sea invaded central Tennessee and filled the crater with black mud, now represented by about 250 feet of black shale. Fort Payne chert was later deposited upon the relatively smooth surface of the black shale.

Purification of Water Supplies

The water supplies west of the Cascades are principally soft, low in alkalinity and with a pH ranging from slightly acid at high river stages, to slightly alkaline at low stages. The color of these waters varies with the rainfall and the amount of tributary swampy area. The turbidities are not high, as a rule, and are usually coarse except where glacial flour is present. After heavy rainfall particularly, turbidities may be present in objectionable amounts. East of the Cascades the waters the waters of the Pacific Northwest are much more alkaline, except those coming from the mountains themselves. They have a greater range of turbidities, and carry considerably more mineral matter. Though the purification of these waters is more necessary, the problem is a simpler one, since the water is more easily coagulated. This paper, therefore, will be limited to a discussion of the problems involved in the purification of the softer waters of the western slope. These are more like the waters of New England than of any other section of the United States, but are less colored and have higher alkalinities. The average water characteristics for a number of streams in Washington, with one analysis from Oregon, are shown in table 1. From this table the general character of the water on both sides of the Cascades can be seen.

"Rock Glaciers" or Chrystocrenes

In number of Journal of Geology for June, 1910, there is an interesting article by Stephen R. Capps, Jr., on Rock Glaciers in which must be of especial interest to all those who have lived in Alaska or in Yukon Territory, and more particularly to those who have lived in city of Dawson, for these latter will at once recognize similarity of rockslides described and illustrated by Mr. Capps to the on face of Moosehide Mountain at north end of town of Dawson, since Slide is one of outstanding and ever-present features of landscape to every resident of that northern city. The city of Dawson is situated on a swampy alluvial flat on east bank of Yukon River, just below confluence of Klondike River. About a mile to northeast of it Moosehide Mountain rises to a height of 2,000 feet or more above city, a spur of mountain extending down to Yukon River, and terminating Dawson flat toward north. The mountain is composed of massive basic eruptive rock or diabase largely altered to serpentine, and cut by numerous jointage planes which allow rock to break readily into angular fragments. Over larger portions of surface of mountain rock has been weathered and decomposed to a considerable depth, and has broken down into sand or rock flour, so that natural slopes are consequently gradual and gentle. But on southwest side of mountain, directly overlooking town of Dawson, there is a steep scarped face of bare rock several hundred feet in height, at foot of which a talus of broken rock-fragments extends outward and downward toward river. This talus extends so far outward from foot of scarp, and its lower portion has such a relatively gentle slope, that it has somewhat appearance of having broken away suddenly from side of mountain; and consequently early