A common cause of dirt ingestion (off road) is through the exhaust. YES the exhaust!

Here is the scenario. You crash or cartwheel the car.  It stuffs the exhaust stinger into the dirt. When the engine stalls, there is always residue fuel and oil in the pipe. It mixes with the dirt, you carry the car and are not careful how you hold it. The dirt/fuel/oil runs back into the exhaust port!!!!
In addition, the pipe has a reverse pulse action [part of what make a tuned pipe work] when running, dirt in pipe ends up in engine!!

One more scenario (off and on road), you blow up your old trusty engine…… bummer you bolt in your spare or even brand new back up. DID you remember to wash the debris out of your pipe from the exploded engine before mounting and starting your new engine!

I have seen brand new engines ruined just spinning them over on the starter box the first time!