Gravels can be broken down into different kinds of substances by physical processes because they are heterogeneous mixtures.
Gravels are generally made from different rock fragments including sandstone, limestone, and basalt. The rock fragments can be sorted out by physical means.
In other words, gravels are mixtures of different rocks but the individual rocks still maintain their identities in the mixture. Such mixtures are known as heterogeneous mixtures.
More on heterogeneous mixtures can be found here: https://brainly.com/question/24898889
