This paper explores two peripheral Northern Italian dialects (NIDs), namely Lamonat and Frignanese, with respect to their genealogical linguistic classification. The two NIDs exhibit morpho-phonological and morpho-syntactic features that do not fall neatly into the Gallo-Italic sub-classification of Northern Italo-Romance, but resemble some of the core characteristics of the putative Rhaeto-Romance language family. This analysis of Lamonat and Frignanese reveals that their conservative traits more closely relate to Rhaeto-Romance. The synchronic evidence from the two peripheral NIDs hence supports the argument against the unity and autonomy of Rhaeto-Romance as a language family, whereby the linguistic traits that distinguish Rhaeto-Romance within Northern Italo-Romance consist of shared retentions rather than shared innovations, which were once common to virtually all NIDs. In this light, Rhaeto-Romance can be regarded as an array of conservative Gallo-Italic varieties. The paper concludes with a discussion of the geo-sociolinguistic properties of the two peripheral dialect areas under investigation that lead to a conservative linguistic behaviour within the Lamonat and Frignanese speech communities. Given the relatively similar historical and geo-political background of these speech communities, we attempt the formulation of a geo-sociolinguistic model of linguistic innovation diffusion that captures the conservative behaviour of Lamonat and Frignanese. We propose that those dialect areas that, in Bartoli’s (1945) geo-spatial linguistic typology, are both “lateral” and “isolated” deflect linguistic innovations. This proposal must be interpreted within a more general “gravity” and “wave” sociolinguistic model of diffusion of linguistic innovations, whereby “lateral” and “isolated” dialect areas give rise to a mechanism that we call “the pond rock effect” and that renders such dialect areas resistant to language change.