These are two amazing images that we dug up on the Internet. The first shows the construction of a concrete foundation for the structure. The second shows the team digging down to make the foundation.
Source: Digital History Project
A Glimpse Into the Past: Captured Images of the Construction of the Washington Monument
Interesting that the view in the distance is so empty that it might as well be the Texas panhandle.
There’s a scene in Thomas Mallon’s novel “Two Moons” that describes the base of the monument as it is exposed. You can read the passage here: http://dcbythebook.org/listing/two-moons/#sthash.JgF6Xv0q
Sounds like it was dug out at two different points in time.