Barcelona Field Studies Centre

The Big Dig in Barcelona, Spain

'Death mounds' mystery

In the foothills of the Spanish Pyrenees, the UK Channel 4 Big Monster Dig team was put on the trail of a very unusual puzzle: mounds of mud in a disused limestone quarry that were full of bones. To add to the mystery, most of the bones were from big, sabre tooth cats. What were these mounds and why had the remains of so many predators come to be in the same place?

The Big Dig site at Besalu
The Big Dig site at Besalu

The limestones here are very soft and, in geological terms, very young - a mere two million years old. These limestones are also rather unusual in other respects. The majority of limestones, especially in the UK, were deposited in tropical seas as reefs or in shallow water just behind them. These, however, were deposited in temperate lakes very similar to the modern day Lake Banyoles, just 30 kilometres from the dig site.

Students from Kensington School in Barcelona assisted the Big Dig team and 13 year-old Katalin Laszlo van Harreveld helps illustrate and explain the mystery.

The Mystery Explained

by Katalin Laszlo van Harreveld

To the north of Catalonia just outside Besalu there are large limestone quarries.
In these quarries are several mounds of mud held in and surrounded by limestone. In these mounds large amounts of ancient, prehistoric bones have been found. Both predators and prey were found. In fact there was an unusually large number of predators to prey. The normal predator to prey ratio is 1:20. The ratio of the bones found was 1:3. This is a big difference compared to the normal predator/prey ratio. One of the most common predators found was the sabre tooth cat. Some of the bones were crushed around the edge but some were extremely well preserved.

The question is how did all these bones get here? The three main theories of the archaeologists who studied this were all possible. Which one was correct?

First Theory: Death Trap

The death trap is a naturally formed hole in the limestone. This may have been a watering hole that fell low on water at the time (in this case it could have been a dried up or very low doline like the many dolines in the area). The prey comes along and tries to reach the water. The animal (e.g. hippopotamus) cannot reach the water and tries to reach out to get a drink.

The hippopotamus arrives for a drink
Sabre sees the nice easy meal
Sabre is trapped
Sabre dies
Bones are lying on the ground around the doline
Bones get washed in by water

The hippopotamus leans a bit too far and falls in. It struggles to get out but the sides are too high and vertical. The predator (e.g. sabre tooth cat) comes along and sees a big easy meal and as all animals do it would rather get a free meal than have to work for it. The sabre tooth cat jumps in to reach the prey. It eats the hippopotamus and fully satisfied with the free meal it tries to leap out. But it too (like the hippo) can't get out of the hole. The sabre tooth cat struggles to get out but can't because the hole is too deep and the sides are too steep. After a few days it dies of thirst or starvation and its carcass rots away or gets eaten by other animals who fall into the hole. Its skeleton would be left in good condition unless it is right at the bottom and gets crushed by the next animal.

The evidence for this theory is:

  • There are both prey and predators because the prey's skeleton would still be there after the predator had eaten the flesh.
  • All or most of the bones were found on the edge of the hole. If this theory was true the skeletons would be on the edge as the animals would be die whilst trying to get up the sides of the doline.
    However there is some evidence against this theory such as:
  • The bones were all jumbled up but this could be due to the turtles living in the dolines when there was water. The turtles ate some of the remaining flesh which could tear the bones apart from one another.

Second Theory: Washed In

This theory claims that the bones were washed into the hole.  The animals would have died outside the hole or doline and were lying on the ground. The doline would probably just be an empty or dry hole. Heavy rains come washing the bones into the hole and mud also gets washed in later.

The evidence for this is:

  • Some of the bones are jumbled up and trampled. If this theory was correct the bones would have been jumbled up by the water and some of the bones would have been trampled by other animals while they were lying outside on the ground.
  • The biggest bones were all at the edge and if they were washed in, the water would dump the heaviest bones first.
  • There was abrasion on some of the bones which means they were definitely in water at some point. If they were washed into the hole they would definitely have suffered from abrasion on the way there.
    However there is some evidence against this theory:
  • If the bones were washed in then there would be a normal ratio of predators to prey.
  • If the bones were washed in they would all have suffered from abrasion but some of the bones that were found were in perfect condition.


Third Theory: Den

A den is like a cave or a place where predators, often big cats, live. If the hole was a den it would explain why there were so many predators compared to the normal ratio. If it was a den then there would have been many sabre tooth cats' skeletons. There would also be the skeletons of the prey they had eaten.

Bones of the predators that used to live there
Bones of prey now trampled on

The evidence for this theory is:

  • Sabre tooth cats lived in caves so it would make sense if it was a den. There is evidence that caves in Mexico were once sabre tooth tiger dens.
  • There was a very high percentage of predators compared to the usual predator to prey ratio. If it was a den there would be many predators as they all lived there and the skeletons of prey were of the prey the predators had eaten.
  • Most of the bones were in fairly good condition.
    The evidence against the den theory is:
  • If it was a den there would be sabres of all different ages but though they found elderly and adult sabres they found no cubs in any of the holes.


So which theory is correct? All the theories have some evidence that they are true but they all have evidence against. I think that all of the theories are correct but they may have happened at different times. First of all, there was a cave, which was a den for the sabres. Then, after many years, the cave collapsed and it became a watering hole. It then dried up and became a death trap. Afterwards, it rained heavily and other bones were washed in. The water jumbled the bones. It could have been like this or in any order. All of them could happened but not at the same time.


Doline: A doline is a hollow formed on the surface when the roof of an underground limestone cave collapses. The hole can fill with water to form a lake.

Roof of cave thinning
Roof collapses to form a doline
The doline can fill with water to become a lake

Sabre-tooth tiger

A sabre-tooth tiger is an extinct big cat. It was one of the most dangerous predators.

Sabre tooth tiger

All images courtesy: Katalin Laszlo van Harreveld