In the yards of my neighborhood there is about 1 meter of a light brown clay over meters of sand. I'm curious what causes this geology. This is in the USA in the 88005 area code. I speculate that clay is caused by floods of the Rio Grande river, but I don't know what explains the sand. There are hardly any rocks in the sand. There aren't even many pebbles in it. I've dug several meters into it without hitting any other type of material. How deep is the sand likely to be?