comet droppings
Comet nuclei are loose collections of ice, dust and small rocky particles, and leave a trail of debris behind them. Comet droppings are the dirty piles of snow and ice compacted by plows that starts off white but eventually becomes grungier as it melts down to reveal dirty, gravel, cigerette butts, and other debris when the tempurature rises
Once the sun came out and thawed the compacted snow on the curbs, the streets were lined with comet droppings.