Garrard's Cavalry Raid

Marker Text:
On July 20, 1864, Union forces under Major General William T. Sherman, USA, were closing in on Atlanta Headquarters, 2nd Cavalry Division (US) Brigader General Kenner Garrard was in Decatur. His three brigades were scattered from Decatur to Roswell, guarding bridges over the Chatahoochee River and picketing the left flank. That night, Garrard was ordered to assemble his command and march to Covington (30 miles southeast) to burn bridges over the Yellow and Ulcofucheche (Alcovy) rivers and to destroy the railroad between Lithonia and the Alcovy.
Garrard marched late on the 21st. He returned to Decatur on the 24th, after marching almost 90 miles. He destroyed 4 wagon bridges, 2 railroad bridges, and more than 6 miles of track. At Covington, he burned the depot, a newly built army hospital center, 2,000 bales of cotton, and large quantities of quartermaster and commissionary supplies. Three trains were captured and destroyed.
Garrard's raid cut off all communication between Atlanta and Augusta and destroyed any hope that the Army of the Tennessee (CS)--the hard pressed defenders of Atlanta--might receive supplies or reinforcements for the Eastern Confederacy; but this success was not without its price. Because of Garrard's absence from the Union left, Hardee's Corps (CS) was able to approach undetected and to launch a smashing blow at that unguarded flank which has passed into history as the Battle of Atlanta.

044-82 Georgia Historical Commission erected 1957

Location: Decatur Square by the Old Courthouse
GPS :
Visited: June 30, 2008

So here we have confirmation of my suspicions that this little push by Bate's and Walker's division onto McPherson's line is the Battle of Atlanta. Prior to this battle was Kennessaw Mountain and Peachtree Creek. What I have been wondering through is the battle. The rest must be the siege and aftermath.
It seems as though Sherman or McPherson was relatively confident in their battle plan to send away a whole division, cavalry or not, on a guerrilla war task - cutting supply lines and communication routes. This ties into the text from one of the markers where McPherson was worried about his left flank. And from what we have seen in east Atlanta, his left side at Glenwood Avenue and Flat Shoals road was weak.
How would have Garrard's presence affected the outcome? Would Hardee have been unable to attempt this move due to the physical impossibilities of walking that far overnight and then fighting? Even now, with the length of Hardee's march as long as it was, the attack occurred later than desired. They mis-estimated the length of time for the march around the left end. I don't think that they would have been able to walk that far and then back in. Of course Garrard could have covered the line from Decatur over to Leggett's Hill and this would make this flank rather impregnable.
Also...90 miles in three days? And able to destroy so much? That seems a bit much. I know that they are mounted soldiers, but still. Even destroying six miles of railroad line seems like an arduous task that would at least be time consuming. Maybe they used explosives or just removed a couple of ties in order to bend the rails.
A couple more markers and monuments are contained here on the Decatur square, two civil war related, and one geographic.