Of course, I cant say for certain that this stone went through an actual earthquake – or rather several earthquakes – but this specimen from the Mojave National Preserve sure went through SOMETHING. This is a tilt thunderegg, where the horizontal layers indicate that it has moved in the ground. In fact, this is the most extreme tilt thunderegg I have ever seen by a long way. Most of the time, tilts are subtle, indicating that the ground has moved a little and that’s it. The story this one has to tell though is somewhat more dramatic. Some upheavals have shifted it almost precisely 50 degrees in various stages, pretty much turning it on its side.
Here’s my own speculation about it: close examination reveal that the tilt mostly happened quite gradually, without any massive single changes in angle. Rather, the stone has ‘crept’ slowly (though sometimes faster than others and with a few jolts and sudden changes), causing the horizontal lines to be laid down in a fan shape, starting with the white opal layer and ending with the main central area of banded agate. However, some fairly dramatic moments also seem to have occurred. A certain amount of debris has built up on top of the white opal layer – even clearer in other specimens from this location (which follow more or less the same pattern). A more dramatic shock occurred later, which actually fractured the old agate, leaving a small shard loose, before the fill continued (after a period of fortification banding).