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There is actually very little to be said about these two. They compliment each other perfectly.  
They met when Troubleshoes and his rodeo visited the plains region of Equestria. While the crowds were small, the rodeo was well appreciated by those who did come. Because of Pinkie’s endless badgering of her sisters to get out and try more new things, both Limestone and Marble went to the last show before the rodeo left. Both had as much fun as they were able, with Limestone enjoying it much more than her sister. Marble was not a fan of the noise, and ducked out early before the final applause at the end of the show, and ran into Troubleshoes on the way. He was also avoiding the crowd, and introduced himself to the shy mare out of politeness. She introduced herself, and they exchanged a bit of polite small talk, and when Troubleshoes mentioned knowing Applejack, Marble lit up and mentioned that the apple family and her family were very close.  
It quickly became clear through this initial meeting that they enjoyed each others company, and while Troubleshoes needed to leave in the morning with his rodeo, he gave Marble his address and asked her to write to him. She did and the pair exchanged letters as friends for nearly a year until Trouble and his rodeo returned to the Plains. When they met again after so long apart, they were surprised by how different they both felt about the other. Each night while the rodeo as in town was spent together, and when it was time for Trouble to leave again, he asked Marble to be his marefriend, and to keep writing him letters. She said yes, and the continued to write back and forth for another year.  
This time, when Troubleshoes came with his troupe, he came with a proposition. On his last day in town, he asked Marble to marry him. Marble cried out with joy and said yes, and with the blessing of her parents, she packed up her things and followed Trouble out into the world, with promises to write and come home soon for a traditional plains pony wedding. After several years of travel following their wedding, Marble told Trouble she wanted to have a baby.  
Troubleshoes decided that his time traveling with the rodeo was over, and they settled down in Appleloosa, where there was plenty of rodeo work to be done without needing to go far. They built a home on the edge of town, and soon after Marble gave birth to their darling son, Wishbone. Theyve lived happily in Appleloosa ever since.