What kind of snake is this?
Great find!
I'm pretty certain that is actually a Prairie Kingsnake (Lampropeltis calligaster - likely the subspecies L. c. calligaster).  The biggest indicator to me are the two alternating rows of dark spots on sides, with some blotches fusing.  They are distributed all over Missouri, although pretty rarely seen due to their microhabitat:

They tend to be fossorial, spending much of their time underground and
out of sight. As a result, there is little known about the biology and
status of these secretive snakes. Most [prairie] kingsnakes are observed
hiding under boards or other debris or crossing roads on warm
evenings.

I actually used to study herpetology in St. Louis, and we caught a few of these.  The juveniles tend to have well defined spots that fade in time, although there is some degree of polymorphism here.  Looks like yours is somewhere in between.  See pictures below for reference.
Juvenile:

Adult:

Worth mentioning these snakes are nonvenomous, and typically docile.
Photo cred: Edward Prenzler