By QUINN EASTMAN – Staff Writer Aug. 14, 200512 AM
This winter, my wife, Edith, and I went hiking at Goodan Ranch,an open space preserve near Poway. We’re relatively new arrivals inSan Diego County, and it was our first visit to the ranch.
Along the trail, we noticed some rocks whose shapes intrigued meenough that I puzzled over them for months afterward. The rocksthat I remembered were mostly the size of bowling balls and lookedas if they had been neatly cleaved in two by an axe. What naturalforce could slice through a granite boulder so cleanly, Iwondered.
At the time, Goodan Ranch had an eerie feel to it. Glisteningwith moisture from a recent rain, the hills were covered with avivid green carpet, all the same thickness. I half-expected ahobbit to walk out from behind one of the few trees in thevalley.
The ground vegetation was recovering after being scorched by the2003 Cedar fire. We weren’t around for the wildfires, but theeffects on the plants seemed clear enough even a year and a halflater.
My mind burrowed through the mechanisms of erosion that I hadlearned in school. Perhaps a small amount of water trapped in therock froze one chilly evening, expanding and driving the halvesapart. But then, I reasoned, the broken surfaces of the rock wouldbe jagged, not flat and smooth, like we had seen.
Another possibility I came up with —— rocks rolling down a hilland striking others —— I quickly rejected. Most of the brokenboulders we saw sat along the tops of ridges, rather than in thevalleys. That observation suggested that perhaps lightning,following the shortest path between cloud and ground, split therocks.
The puzzle remained unresolved until I recently e-mailed CarolCrafts, the park docent at Goodan Ranch. She and park rangerMaureen Abare-Laudy both suggested —— without seeing the rocks Idescribed —- that they had broken apart in the extreme heat of theCedar fire.
I was embarrassed. The answer had been staring us in theface.
I have written about the effects of the Cedar fire on plants,soil and people, but I had neglected how it had reshaped individualboulders.
Still favoring lightning as an explanation, I dragged my wifeout of bed early last weekend to puzzle over the rocks some more.Edith, more handy with our digital camera than I, took pictures. Ifigured that armed with some evidence, we could ask a geologist toexplain.
When we went back, we found an example that any theory of howthe rocks split would have to explain. One boulder had a chipknocked off smoothly at one angle, and a piece missing at an angleto it where the exposed surface was rough.
We also saw rocks with black smudges on their round surfaces butclean on the flat faces, hinting that the fire explanation wasright.
My colleagues sometimes tease me about my scientific background—— I used to work in a biochemistry laboratory —— but I never tooka university class in geology.
A real geologist would probably take a long rope and stretch itover the ground, making what scientists call a “transect,” andcount all the cleaved rocks. In addition, he or she would find anarea with a similar geology to Goodan Ranch, but not recentlytouched by wildfire, and count the cleaved rocks there, too.
Good experiments require several reference samples to comparewith what you’re studying, I learned from my time in the lab.
Another approach would be to take a round rock and heat it in awell-armored oven set at different temperatures. If the rock splitapart cleanly, it could explain what we saw.
My thought experiment took a final turn when I talked withgeology professor Pat Abbott at San Diego State University. Graniteboulders have cracks in them even while they’re embedded in thehillsides, he said.
“Overall, there’s a built-in mechanism for them splittingapart,” he said. “On top of that, you can superimpose fire orlightning effects.”
When the granite became a solid rock deep within the earth, itwas under great pressure, Abbott said. As erosion carries off thesoil around the boulder, the pressure slackens. The top of theridge has less pressure on it than the rest of the terrain, so therocks there may begin to split, even before they’re completelyexposed to the elements.
I was left thinking about how much cracks deep in the earthdetermine the contours of our hills, even before wind, rain andfire begin to shape them.
Contact staff writer Quinn Eastman at (760) 740-5412 firstname.lastname@example.org.