What is a boundary between geologic units called

The boundary between geologic units is called a contact.

Explanation:

The geologic units are made up of different kinds of rocks of different ages. These geologic units are separated from each other on the basis of their formation and age. Every geologic unit, depending upon its formation and age is assigned a color for the purpose of representation on the map. These geologic units share boundaries with each other which for the convenience of map reading are called contacts.

It's lithogenous i believe of course i looked it up.

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