Half and Half

To me, every rock tells a story, and I find this one particularly interesting. It is clearly a piece of what geologists call a "contact," a point where two different rock types abut one another. This can be two layers of sedimentary rock, reflecting a change in the depositional environment (e.g. a layer of mud, followed by a layer of silt) or...

(NOTE: if these were sedimentary layers, they would have been laid down horizontally, not vertically as displayed..)

...or they could represent a molten rock intruding into a crack in an existing rock. (in which the orientation could be in any direction) For example, the black rock may be part of a basalt dike that intruded into whatever the gray-green stuff is.

Or maybe there is something else going on. But whatever the story, it was a striking find on the beach this morning...