Sunset at Summer Solstice

At dawn the next morning, I found a five dollar bill in the sand that had filled a four-foot deep by 6' wide hole dug in the dark by youth.  I wish I had taken a picture of the hole.  Here's the five.

The sand at low tide at dawn the next morning looked like this in some places. I don't understand the hydraulics that make this pattern.  Is it the gradient?