Strange dark lines , trace fossils ?

[Denis Arcand]

I keep finding these strange black marks. At first I thought it was a break line between two plates, but now I'm not so sure. They are always exactly 5 cm wide and 1 cm thick and they usually have a thin, lighter line in the middle.

 

Is this a trace fossil of some kind? worm ? alga? 

 

Period: Ordovician

Formation: Nicolet River formation

Location: Quebec

 

Here are two examples, A and B:

 

A – In the first and second pictures, you can clearly see the dark line  it is partly hidden in the rock, and the thickness is evident in the second picture.

 

 

B – In this plate, the dark line is not flat and seems to go over some brachiopods.

 

Edited 6 hours ago by Denis Arcand