What are "x-cutting relationships"? This tweet by Daryl Blanks asks:


  Day 1 VTMR trip @GeologyLeics -First stop Laxford bridge! gneiss with amphibolite and granitic pegmatite intrusions - can u spot all x-cutting relationships?!


What are cross-cutting relationships in this context, and how many are there?
Cross cutting relationships.



Here's a very rough sketch. You had the black stuff, which was then cross-cut by the red intrusion (grey in the original photo), and finally intruded by the blue intrusion (pink the in the original photo).

This is the law of superposition. For more fun follow #SuperpositionPuzzle on Twitter!