Today's goal was a rather ambitious one: to find the boundary between the Jurassic and Cretaceous periods in the area. Both the Jurassic (138 million to 200 million years ago) and Cretaceous (65 million to 138 million years ago) periods are represented in the Morrison Formation, but our Chief Geologist Kyoichi Sasazawa wanted to find a missing link, so to speak: a layer representing the 50 million years between the late Jurassic period (138 million years ago) and the first part of the Cretaceous period (up to about 90 million years ago).

Our method was to follow waypoints established by earlier crews, correlating them with our topographic and geologic maps and keeping track of our location and route with our GPS units.

What we were looking for was right under our feet. The problem was that it was way, way, WAY under our feet. If we could find an outcrop or face where the geologic history was exposed, we hoped to see the evidence of the lost 50 million years. We didn't find it, but we did learn a lot that will help us in future geological EVAs.

What we did find were what we called Martian mushrooms, which Kyoichi identified for us as a hard sandstone. (See the image in today's Photo Diary.) And what we learned about the scientific method as applied to geology was illuminating.

Developing a theory to explain an observation: We first saw a bunch of white bowling-ball-size rocks scattered over a hillside like so many lost marbles. In the area was a lot of evidence of water movement, so we thought perhaps the rocks had been transported by flowing water and dropped down randomly where we saw them.

Making more observations that might confirm or refute our theory: When we saw more of the white rocks in a line along the top of another hill, we thought perhaps they belonged there and had simply been left in place while the softer sandstone around them was eroded away.

Making still more observations: What could confirm or refute these two possibilities? As it happened, we walked around a big bluff and came upon our 'Martian mushrooms,' with the hard, round sandstone rocks still attached in some places to the softer sandstone under them, which showed that in fact the white rocks were left there during the erosion process.

And more observations: As we trudged up a nearby hill, we saw the same process repeated with another, even harder, form of sandstone, which left jagged-edged rocks on top of the eroding sandstone beneath.

Future plans are to:

• Continue our search for the Jurassic-Cretaceous boundary;
• Find some fossils; and
• Complete a geophysical survey including all the waypoints where we make geological observations, hoping again to use this scientific process to refine our deductions about the geological record.