Jackson, with his back to the mountain, was waiting for Lee.

Lee was approaching rapidly, to unite the two halves of his army.
General Pope, meanwhile, was marching and countermarching, apparently ignorant of the whereabouts of Jackson,[1] General Lee, in personal command of Longstreet's corps, reached the western end of Thoroughfare Gap about sunset, on the 28th, and the sound of artillery from the direction of Groveton indicated that Jackson and General Pope had come in collision.

Jackson had himself brought on this engagement by attacking the flank of one of General Pope's various columns, as it marched across his front, over the Warrenton road, and this was the origin of the sound wafted to General Lee's ears as he came in sight of Thoroughfare.

It was certainly calculated to excite his nerves if they were capable of being excited.
Jackson was evidently engaged, and the disproportion between his forces and those of General Pope rendered such an engagement extremely critical.

Lee accordingly pressed forward, reached the Gap, and the advance force suddenly halted: the Gap was defended.