The Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the asthenosphere beneath southern New England and easternmost New York State, has been interpreted as a site of mantle upwelling. We synthesize a combination of new and previously published data that indicates the following: (1) The upwelling has eroded or delaminated the lithosphere in a localized region centered in southern Vermont that we call the “Green Mountains Anomaly.” Forty‐second period Rayleigh wave phase velocities, which have peak sensitivity at lithospheric depths, are slow in this region, and S wave receiver functions are dominated by shallow (60‐km depth) mantle structures indicating reduced velocities. Thermal springs and sites with anomalously high concentrations of mantle‐derived helium‐3 are concentrated at the borders of the Green Mountains anomaly, perhaps due to stress concentrations produced by geologically recent, uncompensated delamination of the lower lithosphere. (2) S wave receiver functions indicate intense (>10%), shallow (60‐km depth) short wavelength structures along the southern and western edges of the NAA, indicating that the lithosphere there is being intensely altered, possibly by a combination of shearing and introduction of volatiles. And (3) Notwithstanding the localized thinning and alteration, the NAA lithosphere as a whole does not appear to have experienced pervasive heating, for the compressional wave quality factor of QP = 870 inferred from the decay rate of Po waves is very significantly above the QP ≈ 60 value previously reported for the NAA asthenosphere.