Hi John, We are products of our environment. Mine is periglacial; yours is desert. This conditions our outlook when we seek to explain natural phenomenon. It is well known that both thermal expansion and freeze/thaw contribute to erosion by weakening the surface of the rock. How much each effect contributes depends on local conditions. We ended our postings with predictions based on our theories. If the effect is confined to impact carving of sedimentary rocks, I win. If the effect applies to non porous igneous rocks and incised carving as well, you win. We need input from those who have observed the phenomenon to help us here. In a discussion like this, evidence and not well argued conjecture rules.
Here are a couple of observations that have conditioned my thinking. 1. The mountains that surround me are sedimentary rock, limestone, sandstone and shale. Climbers consider it "rotten rock". It is shattered by freeze/thaw and cannot be trusted to provide firm foot and hand holds. Climbers prefer the solid granite of the Bugaboos in the mountains of British Columbia. 2. The most solid mountains around here are synclines*, downward compressive folds. The corresponding upward folded anticlines have been eroded away. As you drive from Banff to Lake Louise, you are in a low valley that used to be the highest part of the fold but it has been eroded away. You can see the syncline structure of Castle Mountain. The compressed rock of the syncline suffers much less erosion initiated by the freeze thaw cycle. Cheers, Roger Bailey N 51 W 115 where we remember fold structure with the following philosophy. "Syn and you go down." -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Carmichael Sent: November 1, 2002 8:07 AM To: [email protected] Subject: Re: Wind erosion of incised lettering. Roger: That's an interesting theory explaining why a stone's engraving weather less than the surrounding stone and it might be true, but I doubt it. You are correct about dust being created in the carving process and about it getting into the surface pores, but it has been my experience that this dust is very easily washed out of the pores with just a little water. A light rain would do that. So I don't think dust in the pores is significant. But you are also correct that erosion is principally caused by freeze/thaw periods. Here's my theory: Stones are big "Heat Sinks". They store heat well for a long time. When the weather begins to freeze, the inside of the stone stays warm and the outside of the stone freezes first. Therefore the face of the stone is expanding and contacting more than the warmer inside. Since the engraving are closer to the interior they are subjected to less stress. If my theory is correct, then this effect would show up on all types of stones. John John L. Carmichael Jr. -
