Wavelength of a wall

We found Andy Goldsworthy's "Taking a Wall for a Walk" in Grizedale Forest. The wavelength is not constant but is about 8 metres. So I was wondering about the frequency. But the wall is not moving so there is no group velocity with which to calculate the frequency. We could resort to De Broglie's equation and get the momentum, which would be Planck's constant / wavelength but that won't get us frequency. See https://physics.stackexchange.com/questions/62522/de-broglie-wavelength-frequency-and-velocity-interpretation for further discussion.