The image above shows a pattern of weathering erosion  in the stucco surface on the easterly facing vertical wall of a masonry building on Bonaire, WI. 

Stucco is applied to the wall with a trowel from a mortar hod - the mortar is loaded into a hod after the stucco is mixed in a rotary mixer or mixed in a wheelbarrow or on at flat surface.

What explains the erosion pattern? More portland cement in the veins that remain, and less portland cement in the eroded areas?   Is the mortar mixed by folding it over and over like the phylo dough in baklava?   

 Here's the building -  a defunct beach bar called Politie Societeit (Dutch for Polite Society) - where the stucco was weathered.