By the time they have reached the mystery destination, the Doctor has taken off the tan cardigan he had been wearing and has rolled up his shirtsleeves, and River has taken off her jacket.
The Doctor once more glances up at the sky, and then he says, "We're just in time."
"In time for what, dear?" River asks, the tone of her voice mildly frustrated.
"You'll see." The Doctor answers, smiling enigmatically. "We should move over there. We'll have a much better view." He adds, leading River over to a spot immediately front and center in relation to the ninety-eight foot high steppe pyramid.
"It's impressive, but why is it considered so much more sacred than the others?" River asks.
Putting his right arm around her and pointing with his left hand, the Doctor answers, "See the carved snake heads at the base of the part of the pyramid facing us?"
River nods.
"Watch what happens when the sunlight hits the stones lined up above them, starting at the top of the pyramid."
River takes the Doctor's suggestion and looks up. For the first few minutes, she sees nothing particularly notable, but she keeps such thoughts to herself. After half an hour has passed, she receives a partial answer to her question. As the rays of the sun illuminate each stone one by one, a pattern slowly takes shape along the length of the vertical rows they create.
Without taking her gaze from the spectacle occurring before their eyes, River exclaims in wonderment, "That's fascinating! Was that section purposely designed to make the light fall on the pyramid in exactly that way?"
Nodding, the Doctor answers, "Yes. But that isn't the full extent of this phenomenon. Keep watching."
"My eyes haven't left it yet, Sweetie." River remarks in a playfully wry manner.
The Doctor chuckles briefly, then replies, "Yes, I noticed."
The couple watches silently during the remaining ten minutes it takes for the pattern of light to fully manifest. When it has, the Doctor says, "There, it's done. Now, what do you see?"
For a few moments, River's gaze lingers on the section of the pyramid that is being discussed. Then, a look of realization arises in her eyes, and she answers, "It looks like a pair of snakes are slithering down the pyramid!"
The Doctor smiles, and then he replies, "Exactly! Now do you understand why we came here, and why I refused to tell you anything about what we would be doing?"
River changes her observation spot so she is facing her husband. Then, smiling and putting her arms around him, she says, "Yes, I do. It was a wonderful surprise, dear." To further indicate how much she liked the surprise, River gives the Doctor a kiss.
"I thought you'd like it, since your field of research is archaeology." The Doctor, still smiling, replies after they have concluded the kiss.
"It's one of the most wonderful birthday presents I've ever been given." River tells him. After giving her husband a brief kiss on the cheek, she asks him, "Where to now, dear?"
"Back to the TARDIS."
"For?" River inquires quizzically.
"To pick up our lunch, dear." The Doctor answers, smiling drolly.
"Oh, that's fine then." River says pleasantly.
"Did you really think visiting this site was all I had planned, you silly woman?" The Doctor replies affectionately. "You know me better than that." He adds, smiling.
"Well you haven't exactly been forthcoming about today's plans, dear." River remarks, smiling as she fiddles with the Doctor's cardigan, which he had earlier rolled up, then tied loosely about his neck like an oversize scarf.
"But now that I know why, I'm willing to forgive you."
"I'm pleased to hear that." The Doctor tells her, putting his arms around her, then giving her another kiss. "Are you ready to go back to the TARDIS, or would you like to look around here a bit longer?"
"Yes, I am." River answers. "We have done quite a lot of walking, and I'm a bit parched." She adds somewhat flirtatiously.
"Well we can't have that." The Doctor replies, smiling. "We should return to the TARDIS immediately."
"Yes, I believe we should." River remarks, smiling back and holding out her hand. The Doctor grabs it, and then the couple starts the long walk back to the TARDIS.