Matsya Purana — Cosmography of Jambūdvīpa: Varṣas
प्रपातविषमैस्तैस्तु पर्वतैरावृतानि तु सप्त तानि नदीभेदैर् अगम्यानि परस्परम् //
prapātaviṣamaistaistu parvatairāvṛtāni tu sapta tāni nadībhedair agamyāni parasparam //
Those seven (regions) are enclosed by mountains made perilous with steep precipices; and, divided by distinct river-systems, they are mutually difficult to reach from one another.
This verse is not describing Pralaya directly; it presents a cosmographic model where natural barriers—cliff-like mountains and river-divisions—separate the seven regions, emphasizing ordered structure in the world’s layout.
By highlighting mountains and river-systems as boundaries, it supports a practical dharmic idea relevant to kingship: governance, travel, and protection depend on understanding terrain and natural frontiers that limit access between regions.
Indirectly, it underscores a Vastu-relevant principle: mountains, cliffs, and river courses strongly condition accessibility and site planning—useful when selecting settlement or temple locations with natural defenses and defined water boundaries.