In geology, this is a particular type of line drawn between two facies, or rock types where a gradational boundary exists. These are conformable boundaries, meaning each layer was deposited right after the other, with no gaps.

For example, let's say the rock type we start with is muddy, limey rock with lots of calcarious fragments included. This would form just beyond a reef, where the water is too deep for the reef to survive, which receives fragments of the reef when storms come along. As the sediments thicken, the water becomes shallow enough to support a reef, which would show up in the rock record as a limestone full of coral, crinoid, bryzoan ad other fossils. This would be a gradational boundary because the reef wouldn't form overnight, rather a few of each species would move in when the conditions were right, and after some time a full fledged reef would exist.

In the rock record we would draw a shazam line between these two rock types like so:

  reefy limestone

\ |\ |\ |\ |\ |\ |\ |\ |  shazam line
 \| \| \| \| \| \| \| \|

  mud and limestone
 with fossil fragments

As you can see the shazam line depicts interfingering lithologies, which is much more accurate than drawing a straight line between the two. The shazam line must always be drawn with the upper points pointing left. All the shazam lines should be drawn in the same colour, or in a different colour for each identical change.

You cannot draw a shazam between two rock types where the boundary is sharp. Instead there must be either a transgressional boundary (representing a change in sea level) or an unconformable boundary (representing an unconformity, or a gap in the rock record.)