the dark thrust of peace and a ponderous, primordial tenderness

Hi,
Here are some words from the novel Lady Chatterley's Lover(the second paragraph, page 255, chapter 12) by Lawrence (planetebook):

She quivered again at the potent inexorable entry inside her, so strange and terrible. It might come with the thrust of a sword in her softly-opened body, and that would be death. She clung in a sudden anguish of terror. But it came with a strange slow thrust of peace, the dark thrust of peace and a ponderous, primordial tenderness, such as made the world in the beginning. And her terror subsided in her breast, her breast dared to be gone in peace, she held nothing.

The sentence in blue is a little confusing for me. I guess ponderous means awkward, but what are dark thrust and primordial tenderness please
Thank you in advance

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the dark thrust of peace and a ponderous, primordial tenderness