Below are explanations for the two Relative Clause Samples:

Relative Clauses from The Heart of Darkness

Forthwith a change came over the waters, and the serenity became less brilliant but more profound. The old river in its broad reach that rested unruffled at the decline of day had provided good service done to the race who peopled its banks. As it spread out in the tranquil dignity of a waterway leading to the uttermost ends of the earth, we looked at the venerable stream not in the vivid flush of a short day that comes and departs for ever, but in the august light of abiding memories. And indeed nothing is easier for a man who has, as the phrase goes, 'followed the sea' with reverence and affection, than to evoke the great spirit of the past that settled majestically upon the lower reaches of the Thames.

As the interactive exercise instructs, the first step is to highlight the Main Verb Phrases to determine the clauses for analysis:

Forthwith a change came over the waters, and the serenity became less brilliant but more profound. The old river in its broad reach that rested unruffled at the decline of day had provided good service done to the race who peopled its banks. As it spread out in the tranquil dignity of a waterway leading to the uttermost ends of the earth, we looked at the venerable stream not in the vivid flush of a short day that comes and departs for ever, but in the august light of abiding memories. And indeed nothing is easier for a man who has, as the phrase goes, 'followed the sea' with reverence and affection, than to evoke the great spirit of the past that settled majestically upon the lower reaches of the Thames.

There appears to be five relative clauses in the passage:

1. THAT RESTED UNRUFFLED: Adjectival Relative Clause - THAT is the dependent marker and serves as subject - Clause modifies REACH
2. WHO PEOPLED ITS BANKS: Adjectival Relative Clause - WHO is the dependent marker and serves as subject - Clause modifies RACE
3. THAT COMES AND DEPARTS FOR EVER: Adjectival Relative Clause - THAT is the dependent marker and serves as subject - Clause modifies DAY
4. WHO HAS . . . FOLLOWED THE SEA: Adjectival Relative Clause - WHO is the dependent marker and serves as subject - Clause modifies MAN
5. THAT SETTLED MAJESTICALLY: Adjectival Relative Clause - THAT is the dependent marker and serves as subject - Clause modifies SPIRIT

Challenge Sample

Margot's mind reeled. She could hear the listless, scraping sounds of fingertips against the door, which grated behind her eyes. It haunted her to consider the sea of faces outside. They were faces she once knew in a different light, but now they were putrid reflections of squalor and decay. There was nothing left for her to glean from their lifeless eyes except the grim reminder that she was alone. She tried to ignore the thought, but she was the last bastion of humanity. Death awaited her outside.

Again, the first step is to highlight the Main Verb Phrases to determine the clauses for analysis:

Margot's mind reeled. She could hear the listless, scraping sounds of fingertips against the door, which grated behind her eyes. It haunted her to consider the sea of faces outside. They were faces she once knew in a different light, but now they were putrid reflections of squalor and decay. There was nothing left for her to glean from their lifeless eyes except the grim reminder that she was alone. She tried to ignore the thought, but she was the last bastion of humanity. Death awaited her outside.

For this sample, there appears to be three relative clauses in the passage:

1. WHICH GRATED BEHIND HER EYES: Adjectival Relative Clause - WHICH is the dependent marker and serves as subject - Clause modifies SOUNDS
2. [THAT] SHE ONCE KNEW: Adjectival Relative Clause - Deleted THAT is the dependent marker and precedes the subject - Clause modifies FACES
3. THAT SHE WAS ALONE: Adjectival Relative Clause - THAT is the dependent marker and precedes the subject - Clause modifies REMINDER

Tips to Keep in Mind

The KEY MARKER for identifying Relative Clauses is the relative that replaces or precedes the subject of the clause. As evident in the second passage above, the relative THAT (like the nominal-THAT) when preceding the subject, can be deleted. But even though the relative is deleted, the clause is still relative and still functioning adjectivally. So whenever you see two clauses in a row without a dependent marker or punctuation (such as THEY WERE FACES SHE ONCE KNEW above), you should suspect a deleted THAT.