less durable rocks were gradually worn away to form valleys

There are two ways of using the infinitive as an adjective, 1. attributive, 2. predicative. Naturally 2. is a subject complement.

Suspicion that this amounts to insider trading has strengthened.

Right, I'll check your work, OK? Please go ahead and inspect. I think I gave it a good go.

He is confined to his house by illness.

The sooner you get your ticket, the cheaper the fare.

Circumstances are entirely favorable to us.

The other day, a water quality inspection was carried out at our house. We had it done by a certain famous company's Environment Analysis Center or some such name.

The suspect told a lie to the inspector.

(iv) Emergency flotation equipment shall enable safe water landing.