Chalk caves contain bentonite - not teeth cleaner

White cliffs: Signs of former chalk deposits in the bank where trees have been cleared on Weber Rd for roadworks. Photo/Christine McKay

Requests for information on the former chalk/silica cave on Weber Rd has had Dannevirke residents digging deep into their memory banks.

Les Ingram, 76, remembers playing in what he called the chalk cave as a youngster.

"We used to roll the stuff with a bit of water when we played there in the late 1940s," he said.

"It was our cave, covered by bush, but big enough to stand up and walk around."

"We'd chuck the chalk balls onto the road, not at cars, but to see them splatter the road white. We didn't think we were doing anything wrong."

Mr Ingram said whatever was in the cave was a white, very fine dust.

"I lived in McPhee St and us kids would say, 'what are we going to do today? Oh let's got to the cave'," he said. "The memories are special."

Another Dannevirke man, who only wanted to be known by his first name, also remembers playing in what were called the chalk caves above Weber Rd in the 1940s. Jim said the area was a favourite playground for children all those decades ago.

"We'd play there on our way down to the Mangatera Stream and we'd break the chalk off and throw it down on to the road," he said. "We were told not to go in there, but we did anyway."

Jim said the chalk cave looked big to him as a child and said they went back between 2.5m and 3m, with the top 3m high.

"They were part of our childhood because we used to spend all our time in the river down below," he said.

The county council decided to fill the area in as there were worries about it collapsing.

Dannevirke's Andrew Donaldson has answered the mystery of what the substance is. But it's not the stuff dentists used to clean teeth with as Bill Walker believed.
Mr Donaldson said the powdery substance was bentonite, an absorbent aluminium phyllosilicate clay.

Bentonite is widely used in the construction of large-diameter bored piles and Mr Donaldson said it had been used during oil drilling at Porangahau.

"It's a non-pugging mud," he said.

"A layer of this material comes through the back of my property and in the past when my concrete water tank was leaking, I went to the cave and took some home and mixed it into a slurry and painted it onto the inside of the tank to seal the leak."

Mr Donaldson's brother planted the trees, which have now been removed, on the bank when he was working for the Dannevirke County Council in the 1980s.

Neville Jacobsen also remembers playing in the cave.

"As kids we used to dig them deeper. They went in 3m or more," he said. "I don't know what the substance was, but it was fine, white, sandy stuff. When you think back it was quite a dangerous place to play there on the corner above the road."

Mr Jacobsen said it was lucky the cave never fell in on children playing there.