Model Mattie Maderos is Erecting Castles in the Sand

Model Mattie Maderos ain't afraid to get her hands -- and ass -- dirty with a little bit of sand.

The Wilhelmina L.A. model was on all fours in Santa Monica ... reliving her childhood, which wasn't all that long ago ... she's 19. Of course, the great thing about getting sand everywhere is, you can always rinse it off in the ocean.