''Something just walked by - 200 million years ago,'' Delgalvis says as he points out a three-dimensional track on a car-size slab of stone shed by the sheer red cliffs of Lake Powell.
The track, perhaps 17 inches long, is a naturally formed cast - light stone has filled in a footprint caught in darker stone.
''Once a track is exposed, it's like a ticking clock,'' says Delgalvis, an avid explorer of Lake Powell from Grand Junction, Colo.
The emerging tracks, 170 million to 200 million years old, will add important new chapters to the history of dinosaurs in the American West, says paleontologist Martin Lockley of the University of Colorado at Denver.
''When we went to look at some of these, they really were quite spectacular,'' he says. ''We have so many significant firsts.''
Lockley says that, in addition to tracks of carnivorous dinosaurs, there are tracks from two kinds of large herbivorous dinosaurs - an Otozoum and what appears to be a relative of Anomoepus - that are virtually unknown in what is now the western United States. There is also evidence for the first time of a large concentration of crocodilians, all indicating the area was more lush than once widely believed.
Lake Powell has yielded a track of a creature called a Grallator, ''only the second example in the West and the first ever from the Kayenta formation,'' Lockley says. ''Andre found it. I interpreted it.''
But to study many of the dinosaur tracks, Lockley must race against rising water at Lake Powell. The lake - after hitting record low-water in early April on the heels of a five-year drought - is expected to gain 45 feet by midsummer with this season's relatively abundant snowmelt.
David Gillette, a curator at the Museum of Northern Arizona, says that a line of tracks offers great stores of information.
''A skeleton belongs to a dead animal. But a track allows us to look at what a creature did when it was alive,'' Gillette says.
Tracks describe a dinosaur's stride, poses, speed, power, agility and habits, such as whether it was gregarious.
The Lake Powell resource is vast and has vast potential, Lockley says, but the window of opportunity is small, just over a month. His funding from a financially strapped National Park Service is a $15,000 grant, when $50,000 to $75,000 is needed.
Lockley says he will document or retrieve as many fossil finds as possible before they are resubmerged.
''We are working with nature,'' Lockley says. ''With the water going up and down, it's sped up the erosion process. We need to take advantage of the fact that these rocks have been underwater.''
Although the first reports of tracks were made at nearby Lee's Ferry on the Colorado River early in the 20th century, scientists knew of relatively few sites around Lake Powell as recently as the late 1990s.
Lockley helped write a research paper in 1998 summarizing 36 known sites.
In 2004, with a small grant from the National Park Service, Lockley renewed efforts to document tracks on view at low water. Independently, Delgalvis had been finding site after site for a couple of years.
They recently toured sites together. The number of known dinosaur trails has now more than doubled to 75.