This article originally appeared on Ryan Anderson's "The Martian Chronicles" blog and is reposted here with permission.
Ok, so remember the weird rock I showed in my Glacier Park geology post? No? Here it is again:
This texture is called "molar-tooth" texture, because apparently someone thought it looked like the teeth of elephants. They must have been studying some weird elephants. It's a very bizarre texture. It cuts across the layers of the rock as if it is related to fractures, but it is often deformed and squished as if it formed in wet sediment. In some places the minerals filling the fractures are broken up and collected as hard clasts, but it other places they clearly formed after the sediment was deformed.
The two leading explanations for this texture are bubbles and waves*. In the bubble model, the fractures form when gas is evolved in the gooey mud, and builds up enough pressure to propagate through the layers. Then once those voids are formed, fine-grained crystals fill them in. Sometimes, the overlying pressure of newer sediment compressed the voids before they are filled, causing them to accordion up into the contorted shapes we see today. In the wave scenario, pressure changes from powerful storm waves cause the sea floor to undulate, forming the fractures which are then filled in with minerals.
This texture has no modern-day analog because macroscopic life forms disrupt the mud before the mineralization can take place. It's very unlikely that we would find this on Mars, but it is good practice for us martians to try to explain rocks we've never seen before!
*I'm sure I am completely oversimplifying, and likely mangling, these explanations.