Chevron Chevron put a $126.00 hold on account for $23 gas purchase, Internet

On 9/7/2012 I stopped by the Chevron on Crown Valley in Mission Viejo located across the street from the Kaleidoscope to put gas. My purchase was exactly 23.44 of gas. On 9/8/2012 I checked my account and found that I have two pending charges one for 23.44 and the other for $126.00. This sent me into panic mode thinking somehow someone had gotten a hold of my credit card info. I immediately began searching for my receipt thinking I was going to have to prove my purchase amount, then I contacted my credit card company only to be told that gas stations such as Chevron are now routinely doing this. I have pumped gas from various stations all over the county coast to coast and never had this happen before. There is no warning, indication or information posted on the pump to alert consumers of the practice of holding substantial additional monies against the consumer's account in addition to the actual purchase amount. Is this legal?