Chris: I have to say that my gut reaction to the Washington Monument might go a little something like this, but when we visited this Spring I was fascinated to see that on closer inspection the colour of the building stone clearly becomes more tan about a third of the way up. In old stone buildings, this is a clue that the original builders had to switch source quarries during construction. Obviously, at the time an attempt is made to match the colours, but subtle geochemical differences will lead to different weathering, and hence different colours, as the decades and centuries wear on.
As it turns out, this was indeed the case for the Washington monument: there was a gap of more than 2 decades in construction and when work resumed, the stone was taken from a different quarry nearby (after a brief flirtation with yet another quarry in Massachusetts: if you squint, the darker layers just before the more obvious colour change might be evidence of this).
So we have a time gap, and a change in rock type and source: sound familiar? I suppose if you’re being technical, this is more of a disconformity than an unconformity, but it’s an interesting counterpoint to yesterday’s more standard depositional hiatus.