Well the answer is can vary…
For older hills, like the appalachians, the tops will be made up of harder rocks (hard sandstones, or igneous rocks)usually, because those are the rocks that will not erode as quick, therefore over the years the slower rate of erosion consequences in the hills remaining. Usually these harder rocks are just the caps of the hills, protecting softer rocks below, then a water source, such as a stream or river eventually get through these, or finds a gap in them, they cut down through the softer rocks, such as shales and mudstones, making valleys.
In younger mountains, such as the rockies, what make up the mountains are igneous rocks usually, which are very resistant to weathering, and the valleys are just a result of the rocks cracking, and some riding up higher than others. In these valleys, sediments that have eroded from the higher regions collects here.
It is usually in the older mountains that the distinctions are found, since they have had a chance to become weathered, and the different rock layers renovate and become exposed.
Well the answer is can vary…
For older hills, like the appalachians, the tops will be made up of harder rocks (hard sandstones, or igneous rocks)usually, because those are the rocks that will not erode as quick, therefore over the years the slower rate of erosion consequences in the hills remaining. Usually these harder rocks are just the caps of the hills, protecting softer rocks below, then a water source, such as a stream or river eventually get through these, or finds a gap in them, they cut down through the softer rocks, such as shales and mudstones, making valleys.
In younger mountains, such as the rockies, what make up the mountains are igneous rocks usually, which are very resistant to weathering, and the valleys are just a result of the rocks cracking, and some riding up higher than others. In these valleys, sediments that have eroded from the higher regions collects here.
It is usually in the older mountains that the distinctions are found, since they have had a chance to become weathered, and the different rock layers renovate and become exposed.