  Foreground emission makes it difficult to detect the highly-redshifted
cosmological 21 cm signal at any frequency. However, at low frequencies
foregrounds are likely to become optically thick, which would make it
completely impossible to see a 21 cm signal behind them. To find out which
regions of the sky might be optically thick for the highest redshifts of the 21
cm signal, we fit the measurements from LWA1 and the Haslam 408 MHz map with a
two-component spectral model and calculate the frequency-dependent foreground
optical depth point-by-point across the sky. Limitations of the current data
prevent us from making any strong conclusions at high statistical significance,
but there is suggestive evidence ($\sim1\sigma$) that as much as 25% of the sky
could be obscured for the highest redshift 21 cm signals.
