Valid evidence for geology writing is usually peer-reviewed journal articles or data you collected in a lab or research project. When you cite data, it’s important that it was collected with up-to-date technology and methodology. Remember that any statement of fact you make needs to be cited! This gives credibility to your statements in addition to not being plagiarism.
Looking through the literature thoroughly is key, especially for grant proposals or lit review assignments. Search for any important original landmark papers on the topic you’re researching, even if they’re old. Then, look over all articles from the last ten years or so and all the most frequently cited articles from the last twenty years. These exact date ranges may vary for different topics, but your professor will be able to tell if you actually reviewed the literature or picked the first studies you could find at random. Additionally, if you’re going to cite a piece of information, use reference sections to find where it originally came from. Don’t cite a 2004 study for a piece of data that has been recycled 5 times and actually comes from 1955! If a fact is presented in the introduction, it usually came from somewhere else (again, you can use the references to find it).