シンと is a state mimetic for profound silence — 'dead quiet / perfectly still,' where the と is the adverbial marker characteristic of mimetics. 静まる is the intransitive 'to become quiet / settle down.' The second sentence is a cleft construction: 響くのは...だけ 'the (only) thing echoing is X,' using the のは nominalizer to foreground the echoing action. クロッキー帳 is the loanword for 'croquis pad' (quick-sketch notebook). 擦れる 'to rub / scratch / graze against' is intransitive.