By Alan Cooper
Natural Trap Cave (NTC) has an impressive entrance pitch – which is concealed immediately below a large innocuous looking slab of limestone, in the middle of a gently sloping ridge overlooking Bighorn Lake on the border between Wyoming and Montana. As you walk across the slab you can see why anything with hooves, or running at speed, would have had no time to react when the cave entrance came into view – suddenly all the edges slope inwards steeply, and you’re looking straight down a hole, about 30ft wide, 100ft onto rocks at the bottom of a large chamber. As a result, over the last 100,000 years or so, a very large accumulation of skeletons has built up in the sediments below – standard herbivores such as bison, horse, and mountain sheep but also large numbers of carnivores including American Lion and cheetah-like cat (really a puma on steroids), many wolves, and even giant short-faced bears and mammoths. NTC represents possibly the biggest and best preserved collection of Ice Age megafauna in the Lower 48 States and southern Canada. It is a truly valuable scientific resource – a genetic museum from the time before humans first entered the Americas, and one of our best opportunities to compare the middle of the US, to events up in Arctic Beringia, Siberia and Europe.
Dr Julie Meachen and myself are co-PIs on a 3-year project to excavate fossils in the cave, funded by National Geographic and the US National Science Foundation, and supported by the Bureau of Land Management (BLM). 2014 was our first and exploratory year, as we set out to examine the state of the cave after the excavations led by U Kansas between 1975-1984, and also the logistics of getting a scientific team safely into and out of the pit, using Single Rope Techniques (SRT) – abseiling and ascending.
I had two jobs on the expedition. The first was to excavate mammal bones and use them to build a genetic record of Ice Age mammals, and how they responded to climate change and the extinctions at the end of the Pleistocene (around 11,600 years ago). The second, and probably more important, was to oversee the logistics and safety of the caving operations. I was to use my experience as a vertical caver in the mountains of New Zealand to rig the cave with ropes to allow the large science team to safely descend, and much more trickily – ascend, the vertical pitch into the ‘drop zone’. For much of my youth I was a mad-keen caver, spending every available weekend and holiday exploring caves in New Zealand, including the alpine caves of NW Nelson. Along with friends, I’d rig and drop numerous pitches to explore deep shafts for bones, and explore and map cave passages deep into the mountains. We were trying to connect these shafts to the massive Nettlebed system which ran under Mt Arthur, and which has finally been done some 20 years later! It was the best fun ever, and as a by-product, I have a large amount of experience with SRT, hauling systems, and Cave Search and Rescue – which I hadn’t suspected would someday be useful for recovering the bodies of Ice Age mammals!
Professor Cooper ascending out of the cave after the first descent into the cave on Tuesday, July 29th. Credit L. Weyrich
A view into the cave through the opening in the grate, revealing Xiaoming Wang from the Natural History Museum of Los Angeles County, visible nearly 100 feet below. Credit: L Weyrich
The BLM has covered the entrance to Natural Trap Cave with a large bespoke grid of I beams and angle iron to prevent animals, humans – or cars – from dropping in. While rigging the drop, I was walking out over a 100ft drop balancing on the angle iron straps (placed about 6 inches apart) which was quite a surreal experience, as you don’t normally get to be so ‘exposed’ over a giant cave entrance. It certainly meant that you held onto all the rigging gear much more tightly than normal, as one slip and a crab (carabiner) would be history after the 100ft fall. The iron grid must have weighed a ton and have been quite an exercise to install, but did provide a very convenient means to tie-off the static (non-stretch) ropes we’d use for dropping in and climbing out on, and for hauling out buckets of sediments for sieving.
About 20’ below the entrance is a ledge about a metre wide which runs around one side of the shaft, providing an ideal place to enter on to and exit off the vertical ropes, and to remove buckets that have been hauled up. SRT involves abseiling and ascending a single, normally 11-12mm, static (non-stretchy) rope. They’re quite different from climbing ropes (termed dynamic), which are designed to have an enormous amount of stretch (e.g. 30%) to absorb shock if you fall. Caving ropes don’t stretch, and therefore you definitely don’t want to fall on them – a broken back will result. Hence, we use dynamic ropes for the safety, and do all the hard work (abseiling and ascending) on the statics. All the gear has to be brand new to prevent the movement of the dreaded fungal whitenose syndrome between caves, which is killing millions of bats – the best airborne insecticide known to man, and one of the least appreciated ecological assets we have.
Professor Cooper reviewing all of the safety procedures during a training exercise. Credit: L. Weyrich
Since there were a number of novices on the trip, I first rigged up a practice pitch off the side of a cliff nearby, so that everyone could practice going up a static rope, stopping halfway, switching over to a descending device and coming back down again. This exercise involves a precise series of weight transfer manoeuvres, to allow different mechanical devices such as brakebars and jumars to be swapped on and off the rope. It’s always a surprise to see how counter-intuitive this can be to beginners, and how many different ways it can be done wrong with a bit of imagination and stress while hanging up in the air. It becomes totally second nature once you’re an experienced caver, and you hardly think about it – but while learning it is definitely best to practice out in the sunshine and in a place where you can get lots of advice.
Justin Sipla from the University of Iowa repelling into the cave for the first day of excavation. Credit: L. Weyrich
To ensure that no-one could make a serious mistake in the cave, we used a safety line on both descent and ascent, belayed through a pulley and Gibbs-ascending device (which uses an assymetric cam to capture the upward motion of the rope, and prevent it returning). The safety is a dynamic 11mm climbing rope that is attached to each person completely independently of the main static line, and is actively operated by one of the experienced cavers. It means that all other systems could fail, and the caver would still be safely held in position. (Actually, the safety line is quite a pain during ascent, as it tends to get tangled and entwined around the main rope, and jam into bits of gear). Still, BLM were keen on this safety step, and you can understand the logic.
Throughout the excavation I had some great help from the fantastic local cavers who had volunteered to come along to
Natural Trap Cave illuminated from above during the first full day of digging in the cave. Credit: L. Weyrich
support the expedition, and who were magnificent throughout. The camaraderie of cavers around the world is a truly remarkable phenomenon, and the Northern Rocky Mountain Grotto is a great example. With a minimum of notification, they provided a constant team of volunteers who came out just to see if they could help. A huge thanks to Juan, Steve, Ian, Doug and all the others who gave up their own time simply to come along and support the expedition! This was a massive help, as once I had the SRT and safety systems rigged up, I could let the cavers oversee the logistics of checking people’s SRT rigs and getting them on and off the rope at the top of the pitch. This freed me up to concentrate on the excavation itself, and then at the end of the long days digging, getting people rigged up and onto the bottom of the rope for the ascent. (It’s a long evening waiting at the bottom of the pitch, checking everyone is geared up properly and watching them ascend one by one up and out of the darkness. Cold and hungry are two words that dominate – and the cave seems a big empty place without human occupants. I’m sure the pack rats were happily bidding us goodbye).
And so, to the evening. After a 5-15 minute intense workout climbing the rope depending on how fit you are – I don’t think anyone did the pitch without one or more breathers – it’s time to step off the rope and onto the ledge, where Steve or Juan was waiting to help. The first step is to clip in to a safety loop hanging next to the main rope, and haul yourself in to the ledge, and then slowly remove all the pieces of climbing gear so that you’re standing free of the main rope. Then, it’s a climb up a short ladder into the to the bright warm evening sunset over the majestic Wyoming hills, and the huge horizon and evening breeze. A beautiful counterpoint to the cave, which has it’s own magnificence. A cumbersome striptease later (to remove all the caving gear) and it’s a 10 min trudge back up to camp carrying gear and buckets, where Jon and Co have made a wonderful dinner, and cold beer and New Zealand wines await. The evening is spent in friendly discussion, scientific ideas, and anticipation of which bones are going to be unearthed the next day. What a great way to do science….
Big thanks to the BLM crew (esp. Brent Breithaupt), cavers, and National Geographic and NSF for funding. Also to the rest of the paleontology crew and Julie Meachen for patiently putting up with all of my caving safety requirements – and handling their first big cave with such aplomb and enthusiasm.
Technical details: Rigging Natural Trap Cave
For those of you interested in technical details, I rigged the main static 11mm line with an Alpine Butterfly and locking D crab, tied into a 3m tape loop wrapped over two adjacent sections of angle iron (to minimise tape contact with the right angle bends). The main line was then backed up to a figure 8 and locking D crab tied to the next two angle iron supports via another tape loop. This allows for a failure on the primary anchor point, with minimum rope drop before the secondary kicks in.
The main 11mm static haul line was also anchored with tape loops, and used 4 inch pulleys at the top, and at the sediment bucket below, to create a 2:1 mechanical advantage. (The rope does a Z between the grid, the sediment bucket pulley, and back to the grid, where it is redirected with a pulley outside the cave entrance where the local cavers provided the hauling manpower. For every haul on this tail, the bucket moves upwards 1/2 of the distance). To make this easier, Juan attached an additional 3:1 Z system onto the hauling tail itself, giving an overall 6:1 advantage – making it very easy to lift even full sediment buckets, and also providing an easy system to haul out a person should we need to (if someone got injured of one of the novices got their SRT gear tangled up on the rope). The upward progress of the hauling line was captured using a Petzl Traxion hanging below the top redirection pulley, which was great except that it would tend to engage accidentally when buckets or gear were being dropped. We fixed a short haul line to the Traxion to bring buckets and gear into the ledge, to assist with taking them off – and the haul line could also be used to easily hold the Traxion in perfect alignment when allowing the rope to run through for descent.
The safety line was a 11mm dynamic climbing rope, run through a 4 inch pulley and belayed by an experienced caver tied in separately on the ledge. The progress capture device was a Petzl Gibbs-type removable cam system, to avoid the pointy teeth used in nearly all other ascending devices. The broad ribs/bars of the Gibbs are much easier on the rope, especially if it has to be engaged rapidly when belaying a descent. A big thanks to Gonzo Caving for helping us with the gear.
Australian Centre for Ancient DNA (ACAD) 