The injustice of Fred's condemnation stirred her to action.

She got hurriedly into her khaki skirt and tramping shoes, slung a canteen over her shoulder, tied her green veil over her hat and under her chin, put on her amber sun-glasses, and took her stout walking stick.
She was careful not to wake Fred or the professor, though that would have been more difficult than she imagined.

She did not want them to know where she was going.

If they missed her and were worried it would serve them both right; for now she remembered that the professor had also been very unsympathetic.

Neither of them had seemed to realize what a terrible night she had spent there alone, with that terrible fire raging through the forest and with Marion gone, without saying one word to Kate about where she was going or when she expected to return.
She meant to climb Mount Hough in spite of the altitude, and find out for herself what sort of a fellow that lookout man was.