Dig in France - Paul Mackrill

Introduction and survey

by Paul Mackrill, 17 February 2005

Lans En Vercors, France, not far from the Berger...

Re logbook entries. I'm not sure it really counts but I do have a major dig going, but it is in France, not far from where I live [Also hence no entries in the log book]. I have given the place a bit of a scientific study as I've been going along and if nothing else I'd be interested in some other opinions. What I need is encouragement as I'm working on my own.

The hole is in a mountain in the Vercors. The potential is only -300m, but more importantly it could & "should" fall into the master cave with a river of a similar size to that in the Gouffre Berger. In fact I am in the same type of limestone as the Berger and on the next mountain range to the East of it.

I'm at -8 metres in what is a boulder filled "shaft". I only have one solid wall on the down hill side of me. The place has a draft [the reason I started] and is located in a semi valley feature on the syncline that runs through the mountain. The draft behaves like a bottom entrance [out in summer / in during the winter], which is hardly surprising as the ground rises another 400m behind me [I am at 1300m].

Now it gets complicated. In winter it drafts in and the air goes all the way to the bottom point I have reached at -7.6 metres without trying to escape to the sides [uphill]. At present the draft is sucked into the boulder floor. The temperature is now below 0C where it drafts in but is slightly positive in the boulders on the uphill side. The draft could just be a recirculation of the air in the boulders as there is an outlet higher up, although the dig still drafts in when I filled this upper outlet with snow a couple of nights ago. All the rest of the boulders are now covered with half a metre of snow.

There are periods when the draft almost stops when it should pull in, but I always notice some sort of air movement. In summer the air blows out and reaches a steady state temperature of 6 C [Rising from 4.5C measured last May 2004].

I have found a lot of what I call "popcorn" on the walls. It is only present in the line of where I am digging and where the draft is going. This "popcorn" takes the form of calcite mushrooms up to 1 centimetre in diameter. I believe this to be air deposited calcite from the calcium carbonate rich air that blows out of the cave in the spring and condenses and dries on the walls [and it certainly gets soaking wet as soon as the air reverses]. I don't think the calcite is drawn out of the rocks as I have some stones with more deposit than rock and when the calcite deposit flakes off the rock underneath is in a pretty virgin state.

So I am looking for comments on my project from the point of view of my draft indicators and calcite deposits. I have a full set of temperature readings from September 2004 to now for the connoisseurs!

I attach a section of the dig in 3D (eprt format). If you want to look at it you should get a prompt to download a reader so you can view it. I also attach a .pdf version. The area I've entered to the left [with a peaked "roof"] is a rift 25cm wide which gets too tight. The draft goes down the pocket off to the right at -7.63metres.

See larger survey