Fig. 2.12. The placer mining exposure at Pearl Creek. The right, or uphill, face of the exposure contains a thick peat layer halfway up the section. This peat tilts toward the present stream, but does not appear in the left section wall. At the head wall one can see where this peat layer thins and disappears near the center of the cut. Blue Babe was found in silts interbedded in this layer of peat. We can presume from Blue Babe’s radiocarbon date that the peat is interstadial in age. The Pearl Creek exposure is typical of late Pleistocene sections seen at placer operations in the Fairbanks area.
Stratigraphy in the Blue Babe site is characteristic of later Pleistocene-aged deposits in the Fairbanks area (Péwé 1975a, 1975b; Guthrie 1968). Mining operations created an exposure along both sides of a Pleistocene creek bed, ending in an abrupt cul-de-sac on the upstream end where the miners were working. This U-shaped exposure provided a three-dimensional sample from which we could reconstruct the nature of the deposit (fig. 2.12). Silt deposits overlie Pleistocene gravels in the old streambed, but they angle from west to east, almost at right angles to the earlier streambed and to the present stream farther east. These old gravels, just beneath the fossil bison carcass, are thus a remnant Pleistocene bench of the present stream (fig. 2.13). Sediments that covered the bison are aligned more with the present angle of slope to the west, not with the present valley draining to the south. This accounts for the differences in height of the exposure on the eastern part of the U-shaped section, which bisects the toe of the old slope climbing to the west.
An overall view of the section reveals four striking stratigraphic features. First is the upper peat. From studies on other creeks, we know that this peat represents Holocene deposits after loess deposition and redeposition had ceased or greatly slowed. This peat marks the end of the Pleistocene and recolonization of the interior by hydrophytic vegetation of the present boreal forest from refugia south of the ice sheet. The base of this unit normally yields dates between 8,000 and 10,000 yr. B.P. (Péwé 1975b; Guthrie 1968). The peat consists mainly of compacted moss and tree roots. Its thickness varies, but normally ranges between one and two meters. Péwé (1975b) calls this zone the Ready Boullion Formation (fig. 2.14). Although he includes a portion of his Goldstream Formation in this time unit, the lower part of this peaty zone is roughly equivalent to Hopkins’ (1982) Birch Zone interval.
Fig. 2.13. Diagrammatic Pearl Creek section drawing. Gravels exposed at the Pearl Creek mine are portrayed as the former bench of a stream that has migrated and is now stabilized against the opposite valley wall. The fossil side “stream” which brought bedrock material down from the hill crests is also shown. Transport of schist downslope indicates rapid water runoff also capable of carrying enough silt to cover Blue Babe.
Fig. 2.14. Major vegetation and glaciation patterns over the last 100,000 years. This chart positions Blue Babe in relation to the glacial-interglacial events as well as the vegetational changes that occurred in Alaska over the last 100,000 years.
Below this upper peat is a thick bed of ice-rich silt, gray in color when freshly exposed and black when wet. Ice wedges occur throughout this zone. Their upper parts are truncated one to two meters from the base of the upper peat, and their bottoms feather out near the base. Elsewhere in the Fairbanks area this silt unit represents deposits from the final phase of the last glaciation, and in some cases it may be even older (Péwé 1975b). Péwé (1975b) calls these deposits the Goldstream Formation. Hopkins (1982) identifies them as the Duvanny Yar Interval (which corresponds to isotope stage 2 of the deep-sea cores).
On the right or west wall, another peat unit marks the middle of the section. It is composed of woody material lying in a matrix of compressed nonwoody plant parts that appear to be mostly moss. This unit is of particular interest because it is near or within this zone that the bison was found. Along the west wall of the section, the peat is continuous, thick, and not penetrated by fingers of silt. As the peat layer dips into the gravels, as seen at the head wall of the section, it changes and is marbled not only with silt but with gravel. The bison was found among this interfingering of different materials. The bison skin collagen was dated at 36,425 +2575/ − 1974 B.P. (QC-891). A piece of wood taken from above the bison was dated at 30,890 +890/ − 1000 B.P. (DIC-2417). This peat unit and the bison date from the latter part of isotope stage 3 of deep-sea cores or what Hopkins (1982) calls the Boutellier Interval, roughly corresponding to Péwé’s (1975a) Eva Formation in the Fairbanks area.
Beneath this lower peat is another thick bed of reworked loess. Péwé (1975b) calls this the Gold Hill Loess and Hopkins (1982) the Happy Interval; both are more or less equivalent to the isotope stage 4 from deep-sea cores. There were traces of an even lower peat seen only on the western side of the exposure. This peat was on top of the auriferous gravel (fig. 2.12).
These are the overall features of the section, but to understand the preservation of Blue Babe’s carcass we have to look more closely at geological features in the immediate area of the bison. The carcass lay within a horizontally bedded silt zone (2a in fig. 2.15) (with a slight tilt toward the east, as with all the beds discussed above) and about 10–20 cm above a layer of compacted peat and silt (1 in fig. 2.15). There was a slight wave-dip irregularity in the peaty material just beneath the bison. Both this peat and the peat above the bison coalesced in a single unit farther north in the section.
Within the silt around the bison (2a) there were thin lenses of relatively clear segregation ice, some gravel, hair, bone fragments, and pieces of skin. These were almost restricted to a single horizon. Farther above the bison, at a little over a meter, 2a grades into what I categorized as another zone, 2b. This zone contains larger distinguishable lenses of gravel (no stone larger than 70 mm in diameter) and then lenses of peat. This latter peat is unlike the lower peat (1) but more like the Boutellier Interval peat in the west wall of the section—very mossy, rooty, and quite uncompacted. There is also “ice-wedge” ice, as much as 1 m thick.
Another more homogeneous silt zone (3 in fig. 2.15), which unlike 2a and 2b was cross-bedded, contained no ice lenses or peat. Above zone 3 was a similar silty zone (4 in fig. 2.15), except that it was horizontally bedded. And then above that was a thick peat bed (80 cm), like the smaller lenses of peat in zone 2b in texture and content. Immediately above the bison carcass a layer of silt had been removed by the miners but could be followed horizontally to where the silt (6 in fig. 2.15) was still in place. I found it bedded horizontally, with a slight dip to the east toward the middle of the valley and not down valley.
Fig. 2.15. Aerial and section views of Blue Babe excavation. (Above) Aerial views of the exposure (left) at the end of the 1979 mining season in which Blue Babe was excavated and (right) a year later. In 1979 the miners worked around the area in which the bison was located, leaving Blue Babe in a “thumblike” projection. (Below) A stratigraphic section is pictured showing the location of Blue Babe. I had the additional benefit of watching this wall of silt as it retreated under the miner’s jet of water for the next three years. Numbers represent strata described in detail in the
The bison’s location above the gravel and the three-dimensional angling peat bed which passes beneath the body, intercepting gravel farther east, tell us the carcass was deposited slightly upslope from the streambed location of Blue Babe’s time.
Within the silt where the bison lay were lenses of vegetation and broken parts of tree or shrub limbs (fig. 2.16). These were identified to species and will be discussed later. The presence of peat beds above and below the bison carcass suggests intermittent silt deposition and revegetation. The mossy peat indicates that this was near the valley floor, where there was woody vegetation and more mesic conditions.
Excavation of the carcass proceeded through July, as each day brought some thaw (fig. 2.17) and revealed more stratigraphic context. Lenses of angular gravel were found in the silt around the bison. These lenses are unusual for silt deposits in the Fairbanks area and indicate rapid flow
of water. As mining resumed after the bison was removed, we found that these lenses increased in size and eventually became enlarged uphill to the west, running at a right angle to the old streambed. This gravel pointed toward the nearest ridge crest, indicating that a surface-flushing feeder stream had brought down angular gravel from exposed bedrock on the hill crests above. By surface-flushing stream I mean that, unlike streams today, it had not cut down through silt and peat to bedrock or basal gold-bearing gravels. Instead, the stream was carrying much water in short bursts on top of the thick and probably frozen silt.
Fig. 2.16. Detail map of Blue Babe exposure. Blue Babe was surrounded by reworked organic silt and thin ice lenses; these, in turn, were surrounded by gravel lenses and peat layers. The gravel lenses continued at right angles up the side slope. They originated from the upper right side of the valley and not from the stream or valley floor. The peats indicate irregular stabilization of the soil in a slightly wooded, near-stream environment with interstadial woody vegetation. This was episodically flushed with water-borne silt from the valley side wall. Blue Babe was buried by these flushes.
Thus the bison was covered with silt from valley slopes and not from the valley bottom stream. In fact, the entire deposit at the site originated upslope to the west, eventually crowding the creek bottom farther to the east after the last interglacial (marine isotope stage 5). During this last interglacial and earlier, the creek bottom was on the old gold-bearing gravels. The area above the stream would probably have been covered with willows and other trees as a finger of riparian vegetation that reached into the uplands.
Because we found parts of the bison, including hair, bone, fragments of skin, and incompletely scavenged bones, frozen in the deposit adjacent to the main carcass, we can say that there was apparently no significant retransport of the bison and its parts. Blue Babe died in this habitat and was covered, not by the main stream many meters to the east, but by silt moving downslope from the west. The orientation of gravel lenses and the bedding of silt upslope to the west seem to require this interpretation.
Fig. 2.17. Blue Babe excavation. Based on my photographs, this figure shows the line of thaw as mapped from above. The location of hair and bones around the carcass can be clearly seen. The carcass was removed in two parts because the torso and legs were exposed first and began to tear loose from the bank. The head and neck thawed a few days later.
It is common in these kinds of deposits to find concentrations of bones at the juncture of such side feeder systems, or “pups,” as they are called locally. I have found that extant wild sheep (Ovis dalli) bones occur in similar pup streams in areas that today serve as winter ranges for sheep. Wounded or sick animals can go downhill more easily than uphill, so carcasses are often found in these lower drainage funnels. Also, runoff flush may be strong enough to agitate some bones downslope and concentrate them at those apices. The same process covers bones with silt removed from the slopes. But Blue Babe was clearly too large to wash down the gentle gradient at the Pearl Creek site. I think we found him where he died.
The area immediately around the carcass does not show signs of thermokarst deposits in which the bison might have been trapped, nor are there other signs of anomalous preservation contexts. Thin veins of clear ice, 1–3 cm thick, were found in the silt surrounding the bison. These features were apparently segregation ice, the result of moisture freezing out of the soils after deposition.
There is no indication that the side “stream” that covered Blue Babe with silt continued in its bed over a long period because the hill shows neither gullying nor markedly irregular relief. Instead, one sees evidence along the section of sheetwash events of silt removal from upslope and redeposition of thin silt fans several centimeters deep. These horizontally bedded sheets are most apparent during mining operations when a powerful jet of water is sprayed over the frozen silt exposure. Seen in cross section, the thin silt beds fan down the valley sidewalls like book pages. This sheet wash was responsible for preserving many thousands of individual bones of Alaskan mammals during the Pleistocene. A complete carcass, like Blue Babe, could not be covered by such thin sheets, but several centimeters of silt would have been sufficient to cover most bones.
Placer Gold Mining in Interior Alaska
Since Blue Babe as well as other Alaskan and Soviet mummies, and most Pleistocene bones, have been found as a by-product of placer gold mining, it is appropriate to explain the process.
Minute gold particles are so diffusely distributed in quartz intrusions that “hard-rock” mining—going after bedrock gold—has never been very profitable in the interior of Alaska. Instead, most miners have sought placer concentrations of gold beneath old (Pleistocene) stream gravels in headwaters draining the gold-bearing bedrock ridges. The problem in most placer deposits is that the gravels are covered by a thick deposit of reworked silt which froze in place as it accumulated.
Miners in the Alaskan Gold Rush worked throughout the winter, using steam to thaw a shaft down into this frozen silt. So much wood was needed that forests near the mines were often denuded. Once the shaft reached gravel, steam was again employed as miners tunneled in drifts along the top of the underlying bedrock, moving as much gravel as possible to the surface during the winter season. Large piles of gravels were piled outside the shaft until summer, when they could be worked in a sluice box to extract the gold. Such mining was labor intensive, and progress was slow.
Soon much Alaskan mining changed to a method requiring fewer men and more equipment. By completely removing overlying frozen silt, exposed gravels could be scraped away with a dragline or other heavy equipment until a thin gravel zone above bedrock was left. These auriferous gravels were scooped, again with heavy equipment, into large mechanical sluice boxes that sorted the gold from the gravel.
Moving thick deposits of frozen silt was made easier by the use of water. Holocene humus and peat were bladed off, and water was run over the exposed silt. The physical behavior of silt-sized particles is such that they rapidly enter suspension in water, unlike smaller-sized clay particles or larger-sized sand particles. And as long as the water keeps moving, these silt particles remain in suspension. Miners take advantage of this behavior of silt in water by resuspending and flushing it downstream (fig. 2.18).
Alaskans are familiar with this phenomenon in the natural form of milky talc, called glacial flour, which clouds our glacial streams. The Tanana River, running by Fairbanks, is fed by many streams that drain the still-glaciated Alaska Range; its waters are gray and opaque. The Chena River, draining the Tanana Hills north of Fairbanks, is relatively clear because there are no significant glaciers in those mountains.
At a mining site, water running over the frozen silt cuts down toward gravels, leaving exposed sidewalk. To accelerate the thaw, these sidewalls are washed by powerful jets of water. Miners dam uphill streams to get the large quantities of water they need and pipe water downhill to the mine under pressure of gravity. The piped water is sprayed onto thawing silt with large mounted hoses called guns or monitors (fig. 2.19). Even with waterguns in use the silt thaws slowly, but by removing 4–12 inches (100–300 mm) a day across a wide front of exposure, a large volume of overburden can be moved in a short Alaska summer (fig. 2.20).
Fig. 2.18. Thawed silt flushed downstream. The flowers growing on the rich mud are “mastodon flowers,” Senecio, which gold miners argue are from fossil seeds. In fact, this species is an aggressive colonizer and is the first plant to take advantage of the newly exposed soil.
As microscopic silt particles thaw and move downstream, they leave behind material too large to enter suspension and too heavy to float. Walking along an active cutbank one can see Pleistocene bones high-graded at the foot or protruding from the freshly exposed bank above. Hundreds of thousands of Pleistocene mammal bones from interior Alaska were found in such a way. And that was how Blue Babe was found. Its feet were seen protruding from the black, wet, muck face of a freshly hosed sidewall. The rest of the body wa
s still frozen in place, deep in the bank. Walter Roman moved his monitor aside, and Blue Babe’s second life had begun.
Taphonomy of Arctic Pleistocene Mummies
Taphonomy is the subdiscipline of paleontology that studies the way organisms are preserved as fossils. When animals die, their soft tissue is usually eaten by carnivores or broken down by microbial decomposers. Many carnivores are adapted to eating skeleton parts when other food is scarce. Uneaten bones remaining on the surface are usually destroyed by physical and organic processes. Insect scavengers eat bone marrow. Collagen fibers in the bone matrix are eventually consumed by decomposers, and the mineral part of the bone is flaked off by drying and cracking. Shallowly buried bones are usually dissolved by mild acids secreted by roots as they extract calcarious minerals from the soil. Very few bones are preserved as fossils (fig. 2.21).
Fig. 2.19. Basic plan of typical placer mine in interior Alaska. After all the silt is stripped away, the upper layers of gravel are pushed aside, gaining access to gravel just above bedrock. This gold-bearing gravel is then moved into a large sluice box at the end of the mining season, not long before freeze-up. Federal and state laws now require settling ponds to remove some of the water-suspended silt and reduce down-stream silt pollution.
Fig. 2.20. A brief mining season. When freeze-up comes, most placer mines close for the winter because the water which is their lifeblood stops flowing.
Frozen Fauna of the Mammoth Steppe: The Story of Blue Babe Page 7