Here's why:

* Concordant: Sills are concordant because they intrude parallel to the bedding planes of the existing rock layers.

* Nearly horizontal: Sills are typically tabular and nearly horizontal, though they can have some minor undulations.

* Exploit weaknesses: Magma, being less dense than surrounding rock, seeks out pathways of least resistance. Weaknesses between sedimentary beds provide an ideal route for magma to intrude.

Let me know if you have any other geological features you'd like to identify!