great big graded gradient

in the park around the corner, great machines have been toiling away for the last few days. I thought they were being used to lay a foundation or put in irrigation. Today I finally got around to finding out. There's a 10m deep, 50m long hole shaped lovingly like a big pond. So I'm guessing we're getting a wetland water catchment pond or similar. I should have read the local council flier obviously. Had no idea. 
The soil was so freshly gouged it was smooth, damp and shiny. The shiny dirt caught my eye.