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pinkish-gray clay full of rolled clay pellets was laid down in layers, apparently at times when the plain was inundated by successive, perhaps very shallow and temporary, floods, a conclusion similar to that reached years ago by Hatcher. The spreading of such floods would account for the distribution of rolled clay pellets, the overturning of the shells of dead land turtles, the drowning of others in vast numbers and perhaps the suspension of their gas-distended bodies long enough for decomposition to separate the heads and limbs, for the covering of sun-dried shrunken carcasses and such bones as may have lain on the surface and for the filling of all the cavities in the skulls and turtle shells with fine mud. That such flooding was temporary may be inferred from the widely distributed carnivore excrements, perhaps coated with a film of clay, but still bearing sharp impress of the sphincter ani muscles and certainly not voided in water.

The brown rusty nodules have, I think, been produced by the rise surfaceward, through capillarity in time of drought, of subsurface waters charged with carbonate of lime and the deposition of this limey content through evaporation, locally cementing the clays along definite planes parallel to the surface of the ground, forming either solid sheets or bands of nodules, depending, perhaps, on the amount of lime present, the quantity of water being evaporated, or both. Occasionally bones acted as centers of accretion. The rusty stain is a phenomenon confined to the surface of the nodules and was probably produced subsequent to their exposure by weathering. It is not always developed, and the nodules may be similar in color to the investing clays.

The origin of the sediments of the "turtle-oreodon layer" is part of a larger problem involving the source of the materials of the White River Oligocene as a whole, consideration of which is deferred until a later occasion. The harshness of feel which characterizes both clays and sandstones is largely due to calcareous impregnation, for, strange to say, while the fossils are in part silicified and there is abundant chalcedony in the veins which traverse the clays, the cementing substance of the rusty fossiliferous nodules

J. B. Hatcher, "Origin of the Oligocene and Miocene Deposits of the Great Plains," PROC. AM. PHIL. Soc., Vol. XLI., pp. 113-131, April, 1902.

and of the soft channel sandstones is carbonate of lime and not silica. Only rarely does the matrix of a fossil or the immediate vicinity of a large chalcedony vein show siliceous impregnation. When the calcareous cement of the channel sandstones associated with the "turtle-oreodon layer" is dissolved in hydrochloric acid, the insoluble residues are found to abound in quartz grains, foils of white mica, clay pellets and fine clay dust, and very finely divided pinkish clay with minute quartz fragments constitutes the matrix of the "red layer" itself. While I have found abundant volcanic ash in one of the higher horizons of the Oligocene, I have not established its presence in the "turtle-oreodon layer," although it is perfectly possible that it occurs mixed with the pinkish clays and perhaps may have supplied the silica which replaces fossils and freshwater limestones and occurs in such abundance in the chalcedony veins. The latter are posterior in origin to the caliche nodules in the upper part of the "red layer," since fossils and nodules are frequently cut by them. Rosettes and geodes are other forms assumed by the same material.

I am of the opinion that a climatic factor is involved in the problem of the origin of the White River sediments, but, at the present stage of the investigation, am not prepared to demonstrate it fully. Certain facts observed in connection with the horizon we have been discussing seem to bear on the subject. In the valley of Indian Creek the Titanotherium beds rest with marked unconformity on the eroded surface of the Pierre shale, with a basal conglomerate at the contact containing pebbles of quartz, chert and other hard rocks. The top of the substratum has considerable topographic relief, so that the thickness of the Titanotherium beds varies from place to place. Throughout this formation cross-bedding is common and there are frequent large channels filled with coarse quartzose sediments. The impression conveyed is that of fluviatile deposition under conditions of generous rainfall, abundant run-off and rapid sedimentation. Accumulation of detritus eventually ceased, temporarily, and erosion began, producing the base-leveled surface which separates the Titanotherium from the Oreodon beds, an event which I would associate with a climatic rhythm, perhaps a swing

toward dryer conditions, bringing about the extinction of Titanotherium.

With the return of fluviatile deposition, a slower accumulation of silt by sheet-flood action over a level plain may be indicated by the predominance of approximate horizontality in the stratification and uniformity in the thickness of the color-banding. The slight southeasterly dip which the beds show (Plate VII, Fig. 2), inclining them at a greater angle than the present high-prairie top, is probably initial dip, possibly intensified by subsequent warping. Seasons or cycles of less abundant precipitation, during which ground water was drawn surfaceward by capillarity, are indicated by the zones of coliche nodules. Perhaps the pink color of the basal clays of the Oreodon beds, the "turtle-oreodon layer" or "red layer" we have been discussing, has a climatic significance, but considering how little we know about the chemical changes involved in the iron compounds responsible for the color or the conditions under which they take place, it would be unsafe to conclude that it indicated aridity, for our modern western-desert sediments are gray and not red. Certain pink clay zones farther up in the Oreodon beds, shown as darker bands in the distance in Fig. 2, perhaps owe their color to climate, but what kind of climate must remain, for the present, doubtful. The testimony of mouse-nibbled bones, carnivore excrenents and carcasses in death pose fits in with the climatic rhythm here suggested.

The zone of green channel-sandstone lenses weathering into spherical concretions, which occurs above the basal pinkish-gray clays of the Oreodon beds in the Big Badlands, may represent a swing-back toward the pluvial cycle. Minor rhythms are, I think, indicated by certain of the larger channel fillings, such as the Metamynodon channels, filled with coarse sediments and affording remains of an aquatic rhinoceros. These may have been deposited during cycles of greater precipitation about the headwaters of the streams, which sent them, charged with coarse sediments, far out over the clay-covered plain to the east.


The recurrence of Metamynodon in channels at different levels, noted by Wortman, fits in with the interpretation just suggested, Bull. Am. Mus., Vol. V., Article IX., p. 101, 1893.

for with every spreading eastward of the streams, due to climatic causes, the fauna living along the water courses would recur in the channels cutting the clays in which we find remains of the animals of the plain. So far as my observations have gone, these channels are posterior to the formation of the caliche nodules and not contemporary with them, since they cut out the nodular zones.

That these changes did not involve a lowering of temperature is suggested by the presence of alligator-like forms, few in numbers, but occurring, nevertheless, in the Titanotherium beds. They persist in the higher plains area until Pliocene time, as shown by the discovery of a single crocodilian vertebra in the Snake Creek Pliocene gravels in Sioux County, northwestern Nebraska, by our 1914 expedition. I have not yet identified any volcanic ash in the "red layer," nor have I noted therein any sediments which might be regarded as of æolian origin.


FIG. 1. Rusty concretions and turtles weathering out of the "turtle-oreodon layer" which covers the flat in the foreground. Princeton collecting locality 1015D2a on the west side of the Reservation road, 5.1 miles south of Scenic, Pennington County, South Dakota. Looking north. This locality is near the divide between Cheyenne R. and White R. drainage in the Bear Creek basin. Typical exposure at a highly fossiliferous locality.

FIG. 2. Contact, indicated by the asterisk, between the Titanotherium beds (foreground) and the banded Oreodon beds (background) on the west side of Hart Table, valley of Indian Creek, Pennington County, South Dakota. Looking east. The slight southeasterly dip of the Oreodon beds is also shown. Princeton collecting locality 1015A 1.

6 F. B. Loomis, "Two New River Reptiles from the Titanothere Beds," Am. Jour. Sci., 4th Series, Vol. 18, pp. 427–432, 1904. M. G. Mehl, “Caimanoidea Visheri, a New Crocodilian from the Oligocene of South Dakota," Jour. of Geol., Vol. XXIV., No. 1, Jan.-Feb., 1916, pp. 47-56.

7 PROC. AM. PHIL. SOC., Vol. LIV., No. 217, May-June, 1915, p. 77.




(Read April 22, 1921.)

I had always assumed that entelodonts were rare fossils in the White River Oligocene until last summer when the Princeton Expedition to South Dakota, within an area of about two square miles in the valley of Indian Creek, in Pennington County, between July 14 and 27, collected five skulls and two lower jaws belonging to three species of Archæotherium and noted, but refrained from collecting, perhaps as many more fragmentary specimens within the same area. This mass of new material made it desirable to restudy the Princeton entelodont collection as a whole, as much of it had never been adequately determined. Fortunately, the timely appearance of Mr. Troxell's1 excellent paper on the entelodonts in the Marsh collection at Yale greatly facilitated these studies. Whether certain of the characters used in the classification of these animals are of specific importance or of the nature of secondary sexual structures can not yet be determined. For the present it is safer to give a separate specific name to each well-defined variant, based on adequate material, than to group together forms which may have been rapidly mutating and thereby developing differences of the first degree of importance for detailed faunal and stratigraphic studies. A review of the genera and species represented follows.

1 Am. Jour. Sci., Vol. L., Nov.-Dec., 1920, pp. 243-255, 361-386, 431-445.


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