High Angle Hell

[The Walking Dead]

Are there examples in the Civil War were trenches were dug so carriages could be lowered to a steeper degree giving the projectile a higher angle of trajectory?

 

[drezac]

I do remember reading an account where a battery had dug holes under the trail of the guns to get a higher trajectory, but right now I can't recall where it was done or the source.

 

[John Winn]

E.P. Alexander noted doing that with howitzers in Fighting For The Confederacy but right now I don't remember if he said which fights.

 

[Story]

variations on the theme

https://c8.alamy.com/comp/2CGJ87F/t...he-savannah-river-and-covering-th-2CGJ87F.jpg

https://media.gettyimages.com/photo...-and-traverse-at-picture-id99167248?s=612x612

 

[American87]

Shelby Foote mentions that at the Battle of McDowell, General Milroy dug a trench of sorts to dip his cannon into, so they could fire at a higher trajectory.

Not sure if this meets your definition of "trench," since it was a field engagement, and not "trench warfare."